Part 54

Mrs. B. Undoubtedly it would. You may even reverse the experiment, by pouring water into the spout, and you will find that the water will rise in the pot, to a level with that in the spout; for the pressure of the small quantity of water in the spout, will force up and support, the larger quantity in the pot. In the pressure upwards, as well as that laterally, you see that the force of pressure, depends entirely on the height, and is quite independent of the horizontal dimensions of the fluid.

As a tea-pot is not transparent, let us try the experiment by filling this large glass goblet, by means of this narrow tube, (fig. 6.)

Caroline. Look, Emily, as Mrs. B. fills it, how the water rises in the goblet, to maintain an equilibrium with that in the tube.

Now, Mrs. B., will you let me fill the tube, by pouring water into the goblet?

Mrs. B. That is impossible. However, you may try the experiment, and I doubt not that you will be able to account for its failure.

Caroline. It is very singular, that if so small a column of water as is contained in the tube, can force up and support the whole contents of the goblet; that the weight of all the water in the goblet, should not be able to force up the small quantity required to fill the tube:--oh, I see now the reason, the water in the goblet, cannot force that in the tube above its level, and as the end of the tube, is considerably higher than the goblet, it can never be filled by pouring water into the goblet.

Mrs. B. And if you continue to pour water into the goblet when it is full, the water will run over, instead of rising above its level in the tube.

I shall now explain to you the meaning of the specific gravity of bodies.

Caroline. What! is there another species of gravity, with which we are not yet acquainted?

Mrs. B. No: the specific gravity of a body, means simply its weight, compared with that of another body, of the same size. When we say, that substances, such as lead, and stones, are heavy, and that others, such as paper and feathers, are light, we speak comparatively; that is to say, that the first are heavy, and the latter light, in comparison with the generality of substances in nature. Would you call wood, and chalk, light or heavy bodies?

Caroline. Some kinds of wood are heavy, certainly, as oak and mahogany; others are light, as cedar and poplar.

Emily. I think I should call wood in general, a heavy body; for cedar and poplar, are light, only in comparison to wood of a heavier description. I am at a loss to determine whether chalk should be ranked as a heavy, or a light body; I should be inclined to say the former, if it was not that it is lighter than most other minerals. I perceive that we have but vague notions of light and heavy. I wish there was some standard of comparison, to which we could refer the weight of all other bodies.

Mrs. B. The necessity of such a standard, has been so much felt, that a body has been fixed upon for this purpose. What substance do you think would be best calculated to answer this end?

Caroline. It must be one generally known, and easily obtained; lead or iron, for instance.

Mrs. B. The metals, would not answer the purpose well, for several reasons; they are not always equally compact, and they are rarely quite pure; two pieces of iron, for instance, although of the same size, might not, from the causes mentioned, weigh exactly alike.

Caroline. But, Mrs. B., if you compare the weight, of equal quantities of different bodies, they will all be alike. You know the old saying, that a pound of feathers, is as heavy as a pound of lead?

Mrs. B. When therefore we compare the weight of different kinds of bodies, it would be absurd to take quantities of equal weight, we must take quantities of equal bulk; pints or quarts, not ounces or pounds.

Caroline. Very true; I perplexed myself by thinking that quantity referred to weight, rather than to measure. It is true, it would be as absurd to compare bodies of the same size, in order to ascertain which was largest, as to compare bodies of the same weight, in order to discover which was heaviest.

Mrs. B. In estimating the specific gravity of bodies, therefore, we must compare equal bulks, and we shall find that their specific gravity, will be proportional to their weights. The body which has been adopted as a standard of reference, is distilled, or rain water.

Emily. I am surprised that a fluid should have been chosen for this purpose, as it must necessarily be contained in some vessel, and the weight of the vessel, will require to be deducted.

Mrs. B. You will find that the comparison will be more easily made with a fluid, than with a solid; and water you know can be every where obtained. In order to learn the specific gravity of a solid body, it is not necessary to put a certain measure of it in one scale, and an equal measure of water into the other scale: but simply to weigh the body under trial, first in air, and then in water. If you weigh a piece of gold, in a glass of water, will not the gold displace just as much water, as is equal to its own bulk?

Caroline. Certainly, where one body is, another cannot be at the same time; so that a sufficient quantity of water must be removed, in order to make way for the gold.

Mrs. B. Yes, a cubic inch of water, to make room for a cubic inch of gold; remember that the bulk, alone, is to be considered; the weight, has nothing to do with the quantity of water displaced, for an inch of gold, does not occupy more space, and therefore will not displace more water, than an inch of ivory, or any other substance, that will sink in water.

Well, you will perhaps be surprised to hear that the gold will weigh less in water, than it did out of it?

Emily. And for what reason?

Mrs. B. On account of the upward pressure of the particles of water, which in some measure supports the gold, and by so doing, diminishes its weight. If the body immersed in water, was of the same weight as that fluid, it would be wholly supported by it, just as the water which it displaces, was supported, previous to its making way for the solid body. If the body is heavier than the water, it cannot be wholly supported by it; but the water will offer some resistance to its descent.

Caroline. And the resistance which water offers to the descent of heavy bodies immersed in it, (since it proceeds from the upward pressure of the particles of the fluid,) must in all cases, I suppose, be the same?

Mrs. B. Yes: the resistance of the fluid, is proportioned to the bulk, and not to the weight, of the body immersed in it; all bodies of the same size, therefore, lose the same quantity of their weight in water. Can you form any idea what this loss will be?