A BIG SUBJECT
A formidable list of sciences—Their real relationship—Is a little learning a dangerous thing?—The day of specialists—The building of the great edifice of science—Its beginning in the long-ago—Different ways of discovering things—Man's knowledge.
 SOME time ago a noted chemist, then well advanced in years, remarked to the author that in his youth he could boast that he knew the whole of Chemistry as it was then known, but that no youth of to-day could hope to say the same. This remark referred to one branch of Science alone, so it goes without saying that the most learned scientist of to-day cannot possibly know the whole of Science. What, then, of the average person who cannot devote his life to study?
Not many of us would undertake to write down from memory even the names of all the sciences, while for the meaning of some of the names not a few people of ordinary intelligence would require to consult a dictionary. We should have no difficulty in writing down:—Astronomy, Anatomy, Anthropology, Archaeology, Biology (which includes both Botany and Zoology), Geography, Geology,  History, Meteorology, Philosophy, Physiology, Psychology, Sociology, and Theology. Then we might follow with the names of some sciences which end in the letters "ic" or "ics," such as:—Ethics, Logic, Mathematics, Mechanics, Metaphysics, Politics, and Physics. And those of us who are more interested in Science might add: Cosmology, Embryology, Epistemology, Histology, Hexicology, Morphology, Ornithology, Paleontology, and Philology. After that we might name a few of the branches or sub-divisions: Acoustics, Algebra, Arithmetic, Chemistry, Crystallography, Dermatology, Dynamics, Electricity, Ethnology, Etiology, Etymology, Geometry, Hydraulics, Ichnology, Ichthyology, Metallurgy, Mineralogy, Magnetism, Optics, Pneumatics, Statics, and many others.
No apology is required to accompany the statement that this volume does not profess to cover the whole field of Science. But there is another object in putting down this formidable list of sciences. We must not look upon the different sciences as we might picture so many separate or individual buildings—not even as so many rooms in one great building, but rather as different parts of one great room.
If one examines this list of sciences it is quite apparent that each interpenetrates another. Any person commencing the study of one particular branch of Science will find that he has entered other branches also. It has been said that to know any one branch of Science is to possess a private door into the whole realm of Nature.
If we wish to make a serious study of Science we must select one subject, or even one branch of that subject. On the other hand, there is a great interest in taking a wider view of Science. One so often hears the words of  Pope—"a little learning is a dangerous thing"—being used where they do not apply. There is no danger whatever in learning the general principles involved in any branch of Science without learning all the detail. Perhaps if this fact were more generally recognised there would be more real knowledge acquired by our young people.
The present is the day of Specialists, and there can be no doubt that in the future it will become more and more necessary to specialise. It is the natural outcome of the accumulation of knowledge which man has been building up, and with what a quickening pace is the edifice now arising! But we must not run away with the idea that all the actual building has been done in these modern times. While these islands of ours were peopled with ignorant and semi-barbarous natives, learned men were at work in the East, and had it not been for the serious study of these students of long-past ages the edifice of Science could not have reached its present proportions. As Macaulay has said:
"Every generation enjoys the use of a vast hoard bequeathed to it by antiquity, and transmits that hoard, augmented by fresh acquisitions, to future ages."
In the long-ago, Science was confined to the few "wise-men" in each generation, and even in quite recent times Science was considered "dry stuff" so long as its story was to be found only in scientific textbooks. But now that Science has been applied to such a great extent in our everyday life, it attracts in some measure the attention of all thinking persons. The lad who studies Science instead of sensational novelettes has much more real pleasure in his reading.
In his presidential address to the British Association for the Advancement of Science, in 1909, Sir J. J. Thomson  said:—"I think a famous French mathematician and physicist was guilty of only slight exaggeration when he said that no discovery was really important or properly understood by its author unless and until he could explain it to the first man he met in the street."
Our present subject is Scientific Discovery, and it is obvious that things may be discovered in several different ways. Professor Roentgen was surprised to discover the X-rays while he was experimenting with some vacuum tubes.
In one sense his discovery was accidental, but only in a qualified sense, as we shall see later.
But some great discoveries have been made in the same sense as a gold-digger discovers the gold for which he is searching. M. and Mme. Curie were searching deliberately for a radioactive substance when they discovered Radium.
The discovery of electric waves, or what some people think of as "wireless waves," was also the result of a deliberate search. Their existence had been predicted many years before they were discovered. The youthful German professor, Heinrich Hertz, had no doubt whatever of the existence of electric waves, and he succeeded in devising means by which he could discover their whereabouts.
Yet another kind of discovery is illustrated by the experience of the ancient mathematician Archimedes, who was born about three hundred years before Christ. Most of us heard the story of Archimedes while we were still in the nursery, or at least before we could understand how his plunging into a bath could enable him to discover whether the king's crown was made of pure gold, or whether the goldsmith had cheated the king by stealing some of the  gold given to him and substituting silver in its place. We knew that the discovery must have been of great importance or Archimedes would not have rushed shouting through the streets to his own home without waiting to dress. These particular discoveries, mentioned here by way of illustration, will be dealt with in later chapters. Our present purpose is to note that all discoveries are not made in the same way.
It must be quite evident that man has discovered everything he knows concerning this planet on which he lives; concerning the great Universe in which his planet is but a little thing; and concerning himself and every existing thing. We wish to consider how some of his great discoveries have been made, and the most natural point to set out from is the world in which he finds himself.
Look, the world tempts our eye,
And we would know it all,
We map the starry sky,
We mine this earthen ball,
We measure the sea-tides, we number the sea-sands,
We scrutinise the dates
Of long-past human things,
The bounds of effaced states,
The lives of deceased kings:
We search out dead men's words, and works of dead men's hands.
OUR PLANET'S MOVEMENTS IN SPACE
The true shape of the solid Earth—Early ideas—The Earth supposed to be the centre of the Universe—A revolution of ideas—Difficulty in realising that the Earth is flying through space—Astrology—Kepler's discovery—How we discovered the "year"—Why we require leap-years—The whole solar system moving en bloc
 FROM our infancy we have been taught that the world is not unlike an orange in shape. But if we draw a diagram of the Earth to scale, it is obvious that the actual flattening of the poles is very much less than that of an orange. But how did we discover that this Earth upon which we live is a great round planet floating in space?
Some people imagine that this was a modem discovery, and that all the Ancients believed the Earth to be a flat disc floating upon water. But the astronomers who lived many centuries before Christ pictured this world as a huge sphere, and some of these early astronomers even taught that the Earth was spinning round upon its axis like a top. It is true that this idea was formed before the discovery of any proof that their picture was a true one. The first argument was that the Earth must be round because a sphere is the most perfect form. But while those learned men in the East did not doubt the rotundity of the Earth, the ignorant people who inhabited the British Isles and the West of Europe continued to believe that the world was flat, right on to the time of William the Conqueror.
 Quite recently there appeared in one of our daily papers the report of a meeting of a society the members of which still maintain that the Earth is flat. These cranks are only advertising their own ignorance, for if any thoughtful person considers our present knowledge of the subject he cannot fail to be convinced of the roundness of the Earth. It was discovered that a ship on the horizon appears or disappears bit by bit, just as though it were ascending or descending a steep hill. No matter on what part of the world this observation is made, and no matter in what direction the mariner is looking, the result is always the same. Therefore there can be no doubt as to the shape of the Earth. Then we can actually see the shadow of the round earth cast upon the moon.
In these days of modern travel it seems incredible that anyone can imagine the Earth to be flat. A journey round the world is a comparatively common event. As children we were doubtless amused with the idea that the people on the other side of the world must be walking about, like flies on a-ceiling, with the Earth above them instead of beneath them. But later on when we came to consider that there is no up and down except relative to the earth, the mystery disappeared. We realised that no matter on what part of the great ball we happen to be we are held down to its surface by that force which we call gravitation, and the boundless sky is overhead.
It was a long time before man discovered the true position of his planet in the great Universe. He thought of his world as the one thing of greatest importance, and he pictured the starry vault of heaven as a great crystal dome resting upon the Earth, or encircling it. Even when the wise men studied their movements of the Sun and the other  heavenly bodies, they believed the Earth to be the centre of the Universe around which all the heavenly bodies travelled. It does look as though the Sun rose in the east, climbed up over the sky, and dropped down in the west. When in a railway station, we ourselves have sometimes been cheated as to whether it was the train we were on board or a neighbouring train that was in motion.
While the most natural picture was that of the solid Earth at rest, there were some ancient philosophers who declared that the Sun was the central body and that the Earth travelled around the Sun. But these philosophers could not bring forward any observed facts to prove their theory, and so it was that their ideas were abandoned by succeeding generations, and men continued to believe the Earth to be the central body right on to the time when Queen Elizabeth ruled over England. It happened to be during her lifetime that the great Italian astronomer Galileo Galilei forced men to believe that the Earth goes round the Sun. The actual proofs of this great fact had been established several generations earlier by the great Polish astronomer Copernicus, but it was Galileo who brought the subject into prominence and caused a complete revolution in man's ideas.
Even now, after this great discovery has been firmly fixed in man's mind for three hundred years, it is no easy matter to realise that we are on the surface of a great planet which is spinning round and at the same time is flying through space at a prodigious speed. As we go about our daily duties it is difficult to realise that we are being carried through space at the speed of one thousand miles per minute, a thousand times faster than the most reckless motor-car, and sixty times faster than a rifle bullet. No  wonder that it took man a long time to accept this great discovery!
It is true that the ancient astronomers were apt to let their science drift into "Astrology," which was practically fortune-telling, but we should remember that away back in those far-off days some of the astronomers did make a serious study of the heavenly bodies. If we are apt to think of all Science as being of very modern origin, we should keep before us the fact that when Julius Caesar besieged Alexandria, there was a great university in that town, where for centuries many learned professors had been at work. We should remember that it was there and then that Professor Euclid wrote those books on geometry which have worried many schoolboys, but which are indispensable in modern Science and Engineering.
The professors of that ancient university believed the heavenly bodies to move in circles, because the circle gave the most perfect motion, but the great German astronomer Johann Kepler discovered that the planets moved in ellipses or oval-shaped orbits. Kepler made this discovery in a rather curious way. He kept guessing different ways in which the planets might move, and then testing how each particular path would account for the observed motions of the planets. Formerly the astronomers had been forced to make a most elaborate arrangement of circles, one moving within another, in order to try and explain the motions of the planets. But Kepler's simple ellipses did away with all these intricate theories; he had discovered one of the great secrets of Nature.
Now we picture ourselves on board the Earth making a continuous journey in a great elliptical path around the Sun. We know that this journey takes us one year of  365 days; it will be of interest to consider how we discovered this great fact.
It was an easy matter for the Ancients to divide time into days, as each day is marked off clearly by the accompanying night. Then it was obvious to them that there was a regularly recurring cycle of the phases of the moon, and they noted that twenty-nine days elapsed between one "new moon" and the succeeding one. We have discovered since these early days that the month, the time of the moon's journey around the Earth, is not exactly twenty-nine days. It is almost twenty-nine and a half days, the actual time being 29 days 12 hours 44 minutes and 2.86 seconds.
The discovery of the "year" was not so easy, for the completion of our circuit around the Sun is not so apparent. However, it was obvious that there were regularly recurring seasons, and that the Sun was much higher in the heavens at noon in summer than in winter. This fact would cause the shadow of an object to be much shorter at noon in summer than in winter. By watching the shadow cast by some fixed object, it became clear which was the longest and which was the shortest day; mid-summer and midwinter. Then again, the shadows cast at sunrise and sundown would keep changing gradually their direction day by day, so that by noting carefully its positions at sunrise and sunset on any day, and counting the number of days until the shadows again reached the very same positions, it was possible to determine the length of the year. Indeed, the Egyptians discovered about five thousand years ago that the year consisted of three hundred and sixty-five days. This discovery was based upon the seasons alone. The Greeks had a year of  three hundred and fifty-four days, and other nations had a different number of days.
Some centuries later the Egyptians discovered that the actual journey around the Sun took about a quarter of a day more than three hundred and sixty-five days, and now we know that the exact time of the journey is 365 days 5 hours 48 minutes and 48i seconds.
THE UPPER PHOTOGRAPH SHOWS THE ACTUAL SIZE OR THE MICROSCOPE SLIDE AND THE GROUP OF FORAMINIFERA BELOW THE CIRCULAR COVER GLASS. THE CENTRAL PHOTOGRAPH SHOWS A FEW OF THESE OBJECTS MAGNIFIED, WHILE THE THIRD PHOTOGRAPH SHOWS A SINGLE OBJECT GREATLY MAGNIFIED.
It is generally known that a "leap year" is introduced in order to get rid of this awkward fraction of a day coming into each year. It was our old friend Julius Caesar who introduced the leap year of 366 days in every fourth year. This would have cleared away the disturbing fraction if the figures had been exactly 365 ¼ days, but as already stated, the exact fraction is a little less than a quarter. At a much later date it was agreed to omit a leap year at every hundredth excepting the four hundredth year. The reason for all this juggling is very simple.
We start at a particular point in our orbit at the beginning of the year, but at the close of the last day of the year we have not quite reached that starting-point. We require an additional 5 hours 48 minutes and 48 seconds to reach that point. We get later by that amount each year, so that in four years we are about 231 hours behind time. We put an extra day in the fourth year to give our planet time to reach its proper starting-place before the next year opens. The extra day means 24 hours, whereas the planet will reach its goal in 281 hours. The Earth has set off f hour in advance of its proper time, and it will get ahead by that amount every fourth year, so that in one hundred years it will be about 19 hours in advance. We therefore agree to omit the leap year at every hundredth year, thus taking 24 hours off that year. This not only disposes of the  hours which we had to spare but robs us of other 5 hours, making us late again to that extent. As we shall lose another 5 hours at every hundredth year, we shall be 20 hours late in four hundred years, and therefore we agree not to omit the leap year at that point. By keeping in the extra day we allow the Earth to reach its proper starting-place in good time. Indeed, we shall have a fraction to spare at every four-hundredth year, but it will take some thousands of years before this occasional addition reaches the total of one day, so that we can, leave some far distant descendants to put matters right then.
We picture ourselves on board this planet Earth, with our attendant moon, waltzing round this great elliptical path around the Sun. We picture the other planets making similar movements, some nearer and some farther from the Sun. Even then we must not picture the solar system as being fixed in space, for in these modern times we have discovered that the whole solar system is moving through space, and that even the far-distant "fixed" stars are in motion.
Having formed a picture of the Earth's movements in space, it will be of interest to consider some of the things we have discovered concerning our planet itself.
SOME DISCOVERIES CONCERNING OUR PLANET
When was the Earth created?—How we can tell the age of the Earth—How the Oceans became salt—The birth of the Moon—The inside of the Earth—Eruption of Krakatoa—What we learn from earthquakes—The thickness of the Earth's crust—The evidence of meteorites
 WE know that our planet has not existed for all time, but had a definite beginning; it was created. Is it possible to discover when it was created? Our forefathers thought they could reckon back to the time of the creation, and their estimate was that it took place in the year 4004 B.C. Unfortunately, they had such confidence in that estimate that they placed that particular date opposite the first chapter of Genesis in our printed Bibles. It is even more unfortunate that the printers continue to put this date down as the actual date of the creation, for we have discovered that the age of the Earth must be reckoned in millions of years. It would be only reasonable to withdraw the erroneous date from our Bibles, as it was put there merely on the authority of a learned archbishop who was living at the time the Bible was translated into English. From actual discoveries of the buried ruins of ancient cities we can trace the history of civilised nations as far back and beyond the date given.
Before considering what we have discovered concerning  the age of the Earth, it will be of some assistance to picture its early youth. There is no doubt that at one time it was a great mass of incandescent gas. It was not "burning," because there was no air surrounding it at that time. However, it was natural that this great fiery mass should cool down. As it cooled it became smaller and smaller; it passed into the state of a molten liquid and then a white-hot solid. By this time an envelope of water vapour had formed around the solid globe. The pressure of this envelope of steam would exert an immense pressure on the surface of the hot planet, and there is little doubt this pressure played an important part in distorting the surface of the Earth, depressing it at some places and raising it at others. When things had so far cooled down that the water vapour could condense upon the Earth, there were formed boiling and steaming oceans of water lying in the hollows.
The foregoing short description will enable us to appreciate two of the calculations made concerning the age of the Earth. Knowing that the Earth had cooled from a highly heated body to its present condition, the late Lord Kelvin set himself the task of calculating how long it had taken the planet to cool down. The final result was that at least twenty million years had elapsed since the Earth was a molten globe. Since the discovery of Radium in the Earth's crust it is contended that these twenty million years must be very substantially increased, and that now the Earth is increasing in temperature because of these radioactive substances. However, Lord Kelvin's calculation leaves no doubt that this planet upon which we reside has been in existence for many millions of years.
Professor Joly, of Dublin, has put forward a very  interesting theory, but one which some geologists do not accept. He maintains that the sea has become salt very gradually by the rivers washing down sodium compounds from the rocks. He has calculated how long it would take the rivers to bring down the necessary amount of sodium compounds to account for the present saltness of the sea. He has calculated that all the rivers of the Earth could bring down about one hundred and sixty million tons of sodium in each year. He finds that the total amount of sodium contained in the oceans of the Earth is at least ninety million times more than these figures, so that the age of the Earth must be at least ninety million years. Some of the geologists claim one thousand million years as necessary for the formation of the Earth's crust.
Yet another method of calculation has been used in trying to fix the age of our planet. We have no doubt that our faithful satellite the Moon was at one time a part of this great planet. The Earth must have been in a very plastic condition when it threw off that portion of itself, which drifted away and became the Moon. The late Sir George Darwin calculated how long ago the birth of the Moon must have occurred, and the figures come out about fifty-six million years. Again we are among the tens of millions, and all those calculations go back only to the time when this world was a molten mass of, say, five thousand degrees temperature. But when the Earth was a mass of incandescent gas, its temperature was that of the stars, the hottest of which is about thirty thousand degrees. There must have been an enormous lapse of time to allow of this great drop of temperature. But if we consider only the time required to form the crust of the Earth, even then  we have to count the age of our planet in many millions of years. This will become apparent when we consider the building up of the Earth's crust in a succeeding chapter.
Some people have held very queer ideas about the inside of the Earth. One of the strangest ideas has been that the Earth was hollow, and that the inside was the habitation of living beings. The idea was not that these inhabitants were fiery demons, such as are represented in a pantomime by some Mephistopheles being shot up from the lower regions. The recorded descriptions tell of a beautifully mild climate within the Earth, which was reached by an opening at the North and at the South poles of the Earth. Needless to say that the Arctic and Antarctic explorers did not search for any such entrances. It is quite apparent that the writers of these absurd notions were not scientists, and yet we have such writings put down in all seriousness within the last hundred years.
If one desires to know what is inside anything, the best plan is to make a direct investigation and examine the contents. In the case of the Earth this is impossible; we cannot sink a shaft to any great depth. When one is going down a deep coal-mine for the first time it seems quite a long journey, but the deepest mine ever made goes down only about one mile into the Earth. To reach down to the centre of the planet would mean a journey four thousand times as great. But it would be quite impossible to continue the journey to any great depth. Even if we were able to overcome the difficulties of the increasing temperature and the increasing pressure of air, it would take more than a lifetime to dig a depth of ten miles. Suppose for a moment that it were possible to continue at the same rate of progress, it would take thirty thousand years  to reach the centre. But the whole idea is quite ridiculous, so we may give up all hope of ever discovering the contents of the inner Earth by direct investigation. Our discoveries must be made by the study of phenomena occurring in the Earth. We can only question and cross-examine these phenomena as we should do witnesses.
Our conceptions of the inside of the Earth have changed greatly within quite recent years. Some thirty years ago there occurred a tremendous volcanic eruption in the island of Krakatoa. Although this volcanic island is situated far away in the East, the eruption threw such immense volumes of dust particles into the air that for the succeeding three years our sunsets in Great Britain were more than usually red. A great wave also swept through the oceans of the planet. In a public lecture dealing with this great eruption, and delivered some years after the occurrence, an eminent astronomer described the relation of the solid crust of the Earth to be to the molten interior no more than the solid shell is to the liquid of an egg. Since then we have had to alter this picture very materially. Not only does the solid crust extend to nearly one quarter of the four thousand miles, but we have reason to believe that the contents of the Earth are even more solid at the centre. Indeed, that within the great rock mantle there exists a solid metallic core. We have strong evidence in this direction from one witness in particular: the phenomena of earthquakes.
When an earthquake occurs at some great distance, we are able to take a record of it here, and at any other part of the globe where there is a delicate seismograph. One form of such an instrument is a very delicately poised indicator which will move will every tremor of the Earth.  The movements of the indicator are recorded by means of a ribbon of photographic paper. There are now about one hundred different seismological or earthquake stations spread over the planet. On comparing the records obtained at different stations we can see how long it has taken the tremors to travel to the different points. The tremors going along the solid crust of the Earth will take a time proportional to the distance travelled by them. But there are other tremors which go right through the Earth from the place of the eruption to the distant recording station. If the Earth were of homogeneous material throughout, then the rate of travel would be proportional to the distance of this short-cut route through the Earth from point to point. But the earthquake records show that if the short-cut, from place to place through the Earth, pass through a greater depth than eight or nine hundred miles, there is a considerable increase in the velocity of the tremors. Indeed, the tremors pass about twice as fast through the core, and the change of velocity is quite sudden at a depth of eight or nine hundred miles. The evidence of this witness is that the core of the Earth is of different materials from its crust. By supposing the centre to be a great metallic core we can get the different records of earthquakes to agree. Indeed, this seems the only satisfactory way in which we can account for the evidence, although all geologists are not agreed upon the point.
It will be of interest to inquire if there is no other witness we can call to give evidence on the question of the metallic core of the Earth. We have discovered the weight of the planet, as we shall see in the succeeding chapter, and this brings us some further evidence. We know that the weight of the whole Earth is five and a half times that  bulk of water. But the rock mantle weighs not more than three and a half times the weight of water, therefore the centre is heavier. If we allow even one thousand miles for the depth of this rock mantle, we find that the centre core must be about eight times as heavy as water. It so happens that iron is about eight times heavier than water, but is there any more reason why we should think of the Earth's core being iron in preference to copper or gold? Yes, we have several witnesses to call. The rocks thrown up from a great depth by active volcanoes have been found to be very rich in iron.
Picture the molten Earth cooling, it would be natural for the heavier material, say iron, to fall to the bottom while the lighter material which forms the rock mantle would float to the surface. We know that in the blast furnace the stones or slag float to the surface of the molten iron, and are conveniently drawn off.
Sir Isaac Newton is said to have guessed the density of the Earth to be between five and six times that of water. It was no blind guess. He argued that when the Earth was cooling from a molten to a solid state that the lighter materials would float to the surface, and he made his calculations accordingly. It is remarkable that these figures arrived at some two hundred and fifty years ago are found to be correct.
There is still another witness to give evidence. When lumps of matter have fallen on to our planet from stellar space, we find most of meteorites to be very rich in iron. Again, we shall see in a later chapter how we have discovered the contents of our far-distant Sun, but mean-time it is of interest to note that iron is one of its constituents.
 We have seen that the Moon was thrown off by the molten earth during its process of cooling. If at the time of the birth of the Moon the slag were on the surface of the molten metal, we should expect to find that the Moon is made of the rocky material alone. The weight of the Moon corroborates this, for its total density is about the same as the rock mantle of the Earth. The most recent calculations concerning the Earth give a "crustal zone" of about 30 miles in depth, with a mean density of 2.8. Then a "stony zone" or rock mantle estimated to be from 600 to 900 miles in depth, and having a density of 3.4.
It may be that there still remains a layer of molten material within the Earth, but we can no longer picture the centre of the Earth as a fiery mass; we have strong evidence in support of the rock mantle with a solid iron core.
Before considering what we have discovered concerning the crust of the Earth, it will be of interest to see how we discovered the weight of the Earth itself.
HOW WE DISCOVERED THE WEIGHT OF THE EARTH
Weighing large masses—Weighing a mountain—More exact methods—Measuring gravitation by a clock pendulum—A most interesting experiment in weighing the Earth—The difficulties which the experimenter had to overcome—The actual weight of the Earth—An imaginary demonstration showing the enormous difference between a million and a billion—Pioneer experiments by Cavendish
 TO WEIGH a very large mass of iron, which could not be handled, would not be difficult if the mass were regular in shape. The cubic contents of a square or rectangular block could be calculated by a schoolboy. Then a cubic inch of iron might be weighed, and a simple multiplication sum would give the total weight of the large mass. Even if the iron mass were in the form of a large ball, its contents and weight could be calculated without much trouble.
We know the size of the Earth, and we might set about weighing it in the same fashion; first of all weighing a block of the crust of the Earth and then using our multiplication table. This will not do, however, for, as we have seen in the preceding chapter, the Earth is not of the same density throughout. But if we consider what the weight of an object is we can appreciate another method of weighing. Without the aid of Science we know that a large mass is attracted by the Earth with a greater gravitative pull than  a smaller mass. It is easy to realise that the force of gravity between the Earth and any object on its surface depends upon the mass of the object. If we could only cut some great block from the Earth's crust, and a similar block from the core, we could test the gravitative pull of these two great masses upon some light object. We could determine not only whether the crust or the core was the heavier, but we could obtain the relative densities of the two objects. It goes without saying that this cannot be done, but a mountain is practically a great lump of the Earth's crust. But how are we to test the attractive pull of the mountain?
When a mason wishes to test if his building is quite perpendicular, he suspends a weight on the end of a string. He calls it a plumb line, but it might also be called a pendulum. The weight or bob of the pendulum is pulled straight down to the Earth, and the line supporting it is exactly perpendicular. As every particle of matter attracts every other particle, the building must have some attractive pull upon the suspended weight, but this will be so very insignificant compared with the pull of the Earth, that it will be inappreciable. But would a great massive mountain have any measurable pull? One might suppose that even in this case the difference in mass would be so very great that the comparatively feeble pull of the mountain would not be observed. But when an English scientist, Dr. Maskelyne, tested the matter with one of the great steep mountains of Scotland, he found that the mountain did pull a sensitive pendulum away from the perpendicular. Similar experiments had been attempted at Peru about a generation earlier.
The foregoing discovery was made more than one and  a half centuries ago. The task was not a suitable amusement for a summer holiday. Experiments were made on both sides of the mountain, so that the pull could be observed in directions exactly opposite to one another. The distance through which the weight was pulled was found by an astronomical method. Then the mountain had to be measured and its cubic contents calculated; not an easy task for such an irregularly shaped mass. After that work was done, it was necessary to make extensive collections of samples of the rocks of the mountain, so that its density might be known. Then by comparing the pull of this mass upon the plumb-line with the pull of the whole Earth, it became apparent that the density of the Earth was greater than that of the mountain. This was looked upon as a fair method of weighing the mountain.
When these early experiments were made, the methods were not very exact, but with the aid of more sensitive apparatus and more exact methods we have arrived at the figures given in the preceding chapter. The crust of the Earth is three and a half times as heavy as water, and the average density of the Earth is five and a half times that of water.
An ingenious way of determining the local density of the crust of the Earth has been discovered. We know that the planet is flattened at its poles and distended at its equator. Therefore the attractive pull at the poles will be greater than at the equator. This difference in the gravitative pull should affect the swing of a clock pendulum; the greater the pull the faster should the clock go. This we find to be the case. A pendulum clock set to keep perfect time at the equator would gain about half an hour  every week when transferred to the neighbourhood of the North Pole. Very sensitive apparatus has been made with pendulums swinging in air-tight boxes in a partial vacuum and at a constant temperature. The swing of the pendulums has to be compared with the beat of a chronometer, which having no pendulum is not affected by changes in gravitation. The chronometer causes an electric flash to occur at every half second so that the swing of the pendulum can be determined conveniently. If the swing of the pendulum is faster than the chronometer then it is apparent that the attractive pull of gravitation has increased, while a falling off in speed indicates a lessening in gravitation. By this means we can tell the density of the Earth at any particular place. The general finding is that the Earth beneath the oceans is most dense, while that forming mountains is not so dense.
Other methods of weighing the Earth have been discovered. One of the most interesting of these was used a few years ago by Professor Poynting, who won the "Adams prize" for an essay on the density of the Earth.
The general idea of the experiment was to suspend two weights, one at either end of a sensitive balance, so that the pull of the Earth upon each was exactly balanced. If some massive piece of matter were then placed beneath one of the balanced weights there would be a slightly greater mass attracting that end. The learned Professor knew very well that the additional pull would be so very small that it would be extremely difficult to observe and measure. That he did succeed in this experiment is remarkable. He used two fifty-pound weights on the balance, and a large mass of metal weighing three hundred and fifty pounds was placed beneath one of these. The movement  of the balance was so small that it had to be observed by a telescope. A microscope would have served the purpose if the Professor could have remained close beside the apparatus, but the smallest movement near the balance would affect it. He had to place his weighing-machine in a cellar and look down upon it through a hole in the ceiling. He found that a movement of anyone in the house disturbed his apparatus. To obviate this he had to place the instruments on large blocks of rubber.
The Professor's intention was to bring the three hundred and fifty pound mass under one of the suspended weights and measure the attraction. Then he wished to place the heavy mass under the opposite end of the balance and measure the attraction again as a check upon his first figures. But there was an unforeseen difficulty; he found that when he moved this heavy mass from one place to the other it actually altered the level of the cellar floor. Of course, the difference of level could only be detected by sensitive apparatus, yet this difficulty had to be overcome also. In passing it may be remarked that with the aid of a sensitive seismograph the level of a substantial building has been found to alter ever so slightly after an ordinary shower of rain.
It is remarkable that Professor Poynting succeeded in measuring the actual attractive pull of his three hundred and fifty pound weight. With this data he calculated the density of the Earth, and his figures worked out practically the same as had been obtained by other methods; the density of the Earth was approximately five and a half times that of water. But what is the actual weight of this planet upon the surface of which we are being whirled through space?
 As we know both the density and the size of the Earth, it is not difficult to discover its actual weight. The tonnage put down in figures reads 6,000,000,000,000,000,000 tons. The figures take us far out of our depth, and even when we try to think of the Earth as weighing six trillions of tons, we find it quite unthinkable. It is even difficult to realise the enormous weight of coal taken out of the Earth every year. Recent statistics show that the total is over one thousand million tons per annum.
To anyone who has not realised the enormous difference between a million, a billion, and a trillion, one thousand million tons of coal every year may seem an appreciable encroachment on the total of sixty trillion tons representing the weight of the whole planet. Of course, our deepest coal mines are merely as pin-pricks on the very surface of the planet, and the total weight of the coal withdrawn annually is a mere nothing when compared with the weight of the Earth.
As we shall have occasion to refer to millions and billions in connection with different subjects in succeeding chapters, it will be worth while realising the enormous difference. Suppose we were to attempt to give an actual demonstration of a million. In our imagination we can erect a large tank to hold exactly one million dried peas. Suppose we arrange a clockwork apparatus to let one pea drop from the foot of the tank at the end of each second of time. We should soon realise the rate of discharge, and after watching the regular counting of the peas for a little, we should feel that it was unnecessary to wait through the whole demonstration. A simple calculation would tell us that we could return in a little less than a fortnight to see the millionth pea leave the tank. Working continuously  night and day the tank would be emptied in eleven and a half days.
Now suppose we desired to have a similar demonstration of a billion. In our imagination we construct a larger tank to hold one billion peas, and of course we are prepared for a much longer demonstration. A simple calculation will tell us that we need not cancel any previous engagement to enable us to be present at the appearance of the billionth pea, as we shall have disappeared from the planet long before the end of the demonstration. Nor will our great-great-great-great-grandchildren be in the land of the living then. Indeed, had this demonstration of a billion peas been begun before the time of Christ, there would be little appreciable difference in the tank of peas to-day. It would take thirty thousand years to empty the tank.
It is apparent that no one need have difficulty in realising the enormous difference between a million and a billion. A million is to a billion as a fortnight is to thirty thousand years. We are reckoning a billion as a million million.
To make a similar comparison between a million and a trillion we should have to say that a million is to a trillion as one second is to thirty thousand years. Although it is not easy to realise the latter length of time, the great difference is easily appreciated. Therefore, when we say that this great globe upon which we live weighs six trillion tons, we think of something very different from six million tons.
Before leaving the subject of the density of the Earth, it should be noted that pioneer experiments were made by that wonderful genius, and eccentric millionaire, the Honourable Henry Cavendish, about one hundred and fifty years ago. Cavendish's method forms the basis of Professor Poynting's experiments, which we have con-  sidered in the present chapter. We need not trouble with the detail further than to remark that, instead of a gravity balance, Cavendish used what is known as a torsion balance, and that he measured the force of attraction between two heavy spherical masses of metal and two small metal balls forming the ends of the beam of his torsional balance.