Fig. 85.
132. Hydrostatic Bellows.—The instrument called the Hydrostatic Bellows is represented in Fig. 85. It consists of two circular boards, A and B, united together by strong leather, and having a tube, C, through which water can be poured into it. The amount of weight which can be sustained on the bellows without forcing the water out of the tube depends on the size of the bellows. If the area of the tube is only one thousandth of that of the top of the bellows, a pound of water in the tube will balance a thousand pounds' weight on the bellows. It is for the same reason that in Fig. 84 one pound of water in the tube c balances five pounds in a. As the weight presses upon the top as a whole, it is the same as if there was a vessel of the same size with the bellows resting upon it and containing a thousand pounds of water. The water, in that case, would stand at the same height in the vessel and the tube. This shows that the Hydrostatic Paradox is only one of the exemplifications of the great fact that a fluid, from the influence of gravitation, seeks to be on a level. It is the water in the bellows seeking to be on a level with that in the tube that causes the upward pressure sustaining the weight.
When the weight on the bellows is less than is required to balance the water in the tube, the weight can be raised continually by pouring water into the tube. But observe that although the lifting force be so strong, it is very slow in its operation. If the comparative areas of the tube and the bellows be as above supposed, the water must fall in the tube ten inches in raising the weight the one hundredth part of an inch.
Fig. 86.
133. Bramah's Hydrostatic Press.—The principles which I have elucidated have been applied by Mr. Bramah in his Hydrostatic Press. This consists of a small metallic forcing-pump, Fig. 86, in which the water, a, is pumped up by the piston, s, worked by the lever, c b d, and forced into a strong and large cylinder, A. In this cylinder is a stout piston, S, having a flat head, P, above. Between this plate and another, R, is placed the body, W, which is to be compressed. It is obvious that the pressure exerted will be in proportion to the difference between the size of the pump, a, and the cylinder, A, just as in the case of the bellows, it depended on the difference between the areas of the tube and of the top of the bellows. In the press the force of a pump is substituted for the pressure of a very high column of water, simply because it is more convenient. This press is of great service in the mechanic arts. It is used in pressing paper, cloth, hay, cotton, etc. It has also been recently used in raising enormous weights. The tubes of the celebrated bridge over the Straits of Menai were raised by a machine constructed on this principle.
[CHAPTER VIII.]
SPECIFIC GRAVITY.
134. Nature of the Subject.—We now come to a very interesting subject, which is at least intimately connected with Hydrostatics, if it may not be considered a part of it. The principles which have been developed in the chapter on Hydrostatics in relation to liquids are to be applied here to various kinds of substances. And as we proceed you will see that all the phenomena brought to view in this chapter are to be referred to the same cause with those of the previous chapter; viz., the attraction of gravitation.
135. Specific Gravity Defined.—Before proceeding with the investigation I will give you the definition of specific gravity. The specific gravity of any substance is its weight as compared with the same bulk or volume of other substances. Water is taken as a standard, and its specific gravity is for convenience called 1. Mercury, then, is said to have a specific gravity of 13.5, for it is thirteen and a half times as heavy as the same volume of water. It is easy to see how the specific gravities of different fluids may be ascertained. One mode, and the most obvious one, is to weigh in a vessel equal quantities of them. In what way the specific gravities of solids are ascertained will be explained in another part of this chapter.