Part 6

If the water is emptied out, the tumbler feels much lighter than it was before; water, therefore, has =weight=.

And, finally, if you throw the water out of the tumbler at any slightly supported object, the water hitting against it would knock it over. That is to say, the water being put in motion is able to =transfer= that motion to something else.

All these =phenomena=, as things which happen in nature are often called, are effects of which water, under the conditions mentioned, is the cause, and they may therefore be said to be properties (§ 4) of water.

All things which occupy space, offer resistance, possess weight and transfer motion to other things when they strike against them, are termed =material substances= or =bodies=, or simply =matter=. Water, therefore, is a kind, or form, of matter.

15. =Water is a liquid.=

You will easily observe that, though water occupies space, it has no definite shape, but fits itself exactly to the figure of the vessel which holds it. If the tumbler is cylindrical, the contour of the surface of the water will be circular when the tumbler is held vertically, and will change, without the least break or interruption, to more and more of an oval when the tumbler is inclined, and whatever the shape of the vessel into which you pour it, the sides of the water always exactly fit against the sides of the vessel. If you put your finger into the water you can move it in all directions with scarcely any feeling of obstacle. If you pull your finger out there is no hole left, the water on all sides rushing together to fill up the space that was occupied by the finger. You cannot take up a handful of water, for it runs away between your fingers, and you cannot raise it into a permanent heap. All this shows that the parts of water move upon one another with great ease. The same fact is illustrated if the tumbler is inclined so that the level of the surface rises above the edge of the tumbler on one side, and the water is therefore to some extent unsupported by the tumbler at this point. The water then =flows= over in a stream and falls to the ground, where it spreads out and runs to the lowest accessible place, or gradually soaks up into crevices.

Nevertheless, although the parts of the water thus loosely slip and slide upon one another, yet they hold together to a certain extent. If the surface of the water is just touched with the finger, a little of it will adhere; and if the finger is then slowly and carefully raised, the adjacent water will be raised up into a slender column which acquires a noticeable length before it breaks. So, in the early morning, after heavy dew, you may see the water upon cabbage-leaves and blades of grass in spherical drops, the parts of which similarly hold together.

Material substances, the parts of which are so movable that they fit themselves exactly to the sides of any vessel which contains them, and which flow when they are not supported, are called =fluids=, and fluids the parts of which do not fly off from one another, but hold together as those of water do, are called =liquids=.

Water therefore is a liquid.

16. =Water is almost incompressible.=

It has been seen that water, like every other material substance, resists the intrusion of other matter into the place which it occupies. But many things, though they resist, can be easily squeezed or =compressed= into a smaller volume. This, however, is not the case with water, which like other liquids, is almost =incompressible=; that is to say, an immense pressure is needful to cause its volume to diminish to any appreciable extent. It may seem strange that anything so apparently yielding as water should yet be almost as difficult to squeeze as so much iron; but the apparent yieldingness of water is due to the ease with which it changes its shape; and, if water is prevented from changing its shape, it is very difficult to drive its parts closer together. It has been ascertained that if water is confined in a closed space, a pressure amounting to fifteen pounds on the square inch diminishes its volume by only 1/20000th part. Take a common syringe, and having seen that the plug or =piston= fits the =cylinder= of the syringe well, put the nozzle into water and draw the piston up. Then turn the nozzle upward and push upon the piston till a little of the water squirts out, so as to make sure that the cylinder contains nothing but water. Now put your finger on the opening of the nozzle firmly, so as to stop any water from passing out, and then try to push the piston down. You will find that you cannot make it stir without great force; and, if the piston moves appreciably, it will be because some of the water has escaped by the sides of the piston. In fact, if the piston presented a square inch of surface, and fitted accurately, and the column of water in the cylinder were one inch long, it must be pressed down by a weight of 30,000 pounds (about thirteen tons) to make it move one-tenth of an inch.

17. =The Meaning of Weight.=

Let us next consider the property of weight. We say that anything has weight when, on trying to lift it from the ground, or on holding it in the hand, we have a feeling of effort. Or again, if anything which is supported at a certain height above the ground, falls when the support is taken away, we say that it has weight. Now the ground merely means the surface of the earth; and, as all bodies which possess weight fall directly towards the surface of the earth when they are not kept away from it by some support, we may say that all bodies which have weight tend to fall in this way. And it does not matter on what part of the surface of the earth you make the experiment. Rain consists of drops of water, and it does not matter whether we watch a shower in calm weather here, or in New Zealand; the drops fall perpendicularly towards the ground. But we know that the earth is a globe, and that New Zealand is at our antipodes, or on the opposite side of the globe to England. Hence if two showers are falling at the same time, one in New Zealand and one here—the drops must be falling in opposite directions, towards one another; that is, towards the centre of the earth which lies between them. In fact, all bodies which have weight tend to fall towards the centre of the earth—that is to say they fall in this way if there is nothing to prevent them; and when we speak of weight we mean this tendency to fall. To call anything heavy, is the same as saying that we fully expect that, if there is nothing to support it, it will fall to the ground; or that if we support it ourselves we shall be conscious of effort.

18. =Gravity and Gravitation.=

The word =gravity=, when it was first used, had exactly the same meaning as weight; and a body which has weight is said to =gravitate= towards the center of the earth. But gravity has now acquired a much wider sense than weight. For an immense number of careful observations and experiments have established the general rule, or law of nature, that every material substance, tends to approach every other material substance, just in the same way as a drop of rain falls towards the earth; and, in fact, that any two portions of matter, whatever the nature of that matter may be, will move towards one another if there is nothing to prevent them from doing so.

To make this clear, let us suppose that the only material bodies in the universe were two spherical drops of water, each a tenth of an inch in diameter. Each of these drops would have the same bulk as the other, and would be a quantity of matter exactly equivalent to the other. Then, however great the distance which separated these two drops, they would begin to approach one another; and, each moving with gradually increasing swiftness, they would at length meet in a point exactly half-way between the positions which they at first occupied. But if the bulk of one drop were greater than that of the other drop, then the larger would move more slowly, and the point of meeting would be by so much nearer the larger drop. It follows that, if the one body of water were as big as the earth and the other remained of its original size, no bigger than a rain-drop—the motion of the large mass towards the small one would be an inconceivably minute fraction of the total distance travelled over. It would appear as if the large body were perfectly still and drew the small body to itself.

This is just what happens when a single drop of water falls from a cloud, say through a distance of a mile, to the earth. The earth really moves towards it, just as it moves towards the earth, on the straight line which joins the centres of the two. But the length of this line which each travels over is =inversely proportional= to the quantity of matter in each, that is to say is the less the bigger the quantity. So that we have a rule-of-three sum. As the quantity of matter in the earth is to that in a rain-drop, so is a mile to the distance travelled over by the earth. And if any one worked out this sum, he would find that the fourth term of the proportion would be an inconceivably minute fraction of an inch. For all practical purposes, therefore, we may consider the earth to be at rest in relation to all falling bodies, inasmuch as the quantity of matter in any falling body is insignificant, in comparison with that contained in the earth.

What is true of water is true, so far as we know, of all kinds of matter, and we therefore say that it is a law of nature that all kinds of matter possess gravity; that is to say, that of any two, each tends to move towards the other, at a speed which is the slower the greater the quantity of matter it contains in proportion to that which the other contains; and this speed gradually becomes quicker as the two bodies approach.

What is usually called the =law of gravitation= is a statement of the same observed facts in another and more complete fashion. (See Physics Primer.)

19. =The cause of Weight: Attraction: Force.=

We know nothing whatever of the reason why bodies possess weight. Bodies do not fall on account of the law of gravitation (§ 9); nor does their gravity explain why they fall. Gravity, as we have seen, is only a name for weight, and the law of gravitation is only a statement of =how= bodies approach one another, not =why= they do so.

It is often said that gravitation is =attraction=, and that bodies fall to the earth because the earth attracts them. But the word “attract” simply means to “draw towards,” and “attraction” means nothing but “drawing towards;” and to say, when two bodies move towards one another, that they are “drawn towards” one another, is simply to describe the fact and makes us no whit wiser than we were before. On the contrary, unless we take great care, it may make us a little less wise. For the words “drawing towards” are so closely associated with ropes and hooks and the act of pulling, that we are easily led to fancy the existence of some analogous invisible machinery in the case of mutually attractive bodies.

Again, gravitation is spoken of as a =force=; and as the word force is in very common use, let us try to make out what we mean by it. A man is said to exert force when he pushes or pulls anything so as either to exert pressure upon it or to put it in motion. A wrestler’s force is proved by his hug; a bowler’s force is shown by the swiftness of motion of the ball.

Force, then, is the name which we give to that which causes or, in the case of pressure, tends to cause, motion. The force of gravity therefore means the cause of the pressure which we feel when bodies which possess gravity are supported by our bodies, and the cause of their movement towards the centre of the earth, when they are free to move. But it is exactly about the cause of these phenomena that we know nothing whatever.

A good deal of mischief is done by the inaccurate use of such words as attraction and force, as if they were the names of things having an existence apart from natural objects, and from the series of causes and effects which are open to our observation; while they are, in reality, merely the names of the unknown causes of certain phenomena. And it is worth while to take pains to get clear ideas on this head at the outset of the study of science.

Let us remember then that, so far as we know, it is a law of nature, that any two material bodies, if they are free to move, approach one another with gradually increasing swiftness; and that the space over which each travels before the two meet, is inversely proportional to the quantity of matter which it contains. =Attraction of gravitation= is a name for this general fact; =weight= is the name for the fact in the case of terrestrial bodies; =force= is a name which we give to the unknown cause of the fact. The fact is that which it is important to know. The names are of no great consequence so long as we recollect that they are merely names and not things.

20. =The Weight of Water is Proportioned to its Bulk.=

We must next consider, not weight in general, but the weight of water. We say that a tumbler full of water is heavier than an empty tumbler, because the full tumbler gives us a greater feeling of effort when we lift it than the empty tumbler does. The more water there is in the tumbler the greater is the effort. A pail full of water requires still more effort, though the empty pail feels quite light; and, when we come to deal with a large tub full of water, we may be unable to stir it, though the empty tub could be lifted with ease. Thus it seems that the greater the bulk of water the more it weighs, and the less the bulk the less it weighs. But then a single drop of water in the palm of the hand seems to weigh nothing at all. However, this clearly cannot be, for the drop falls to the ground readily, and therefore it must have weight. Moreover, a few thousand drops would fill the tumbler, and if a thousand drops weigh something, each drop must have a thousandth of that weight. The fact is that our feeling of effort is a very rough measure of weight, and does not enable us to compare small weights, or even to perceive them if they are very small. To know anything accurately about weight we must have recourse to an instrument which is contrived for the purpose of measuring weights with precision.