HYDROSTATICS. 



CHAPTER I. 

 Definition* Nature of Fluids. 



HYDROSTATICS is the science which 

 treats of the pressure of watery or 

 liquid fluids ; Hydraulics treats of their 

 motion ; and Pneumatics treats of the 

 pressure and motion of the air and 

 other light elastic fluids of a similar 

 kind. These words are derived from 

 the Greek tongue, which being well 

 fitted to combine words together, and 

 thereby to express the union and the 

 difference of ideas, has been very gene- 

 rally used for scientific names. Hydro- 

 statics comes from two Greek words 

 which signify the stopping or balajicing 

 of water ; hydraulics from two words 

 signifying water and a pipe, referring 

 to the movement of water in certain 

 musical instruments used by the Greeks ; 

 and Pneumatics from a word denoting 

 breath or air : and these three branches 

 of science are intimately connected with 

 each other. The whole science of li- 

 quids, or watery fluids, comprehending 

 both Hydrostatics and Hydraulics, is 

 sometimes called Hydrodynamics, from 

 the Greek words for water and power or 

 force. 



When we make the division of fluids 

 into watery or liquid, and aeriform, or 

 air-like, we arrange them more accu- 

 rately than if we used the old distinc- 

 tion " of non- elastic and elastic ; for 

 though the aeriform fluids are much 

 more elastic than the watery, the latter 

 are by no means without elasticity. It 

 was at one time believed that they were 

 wholly without it : and could not be 

 compressed, or made to occupy a smaller 

 space by being squeezed. A society of 

 scientific men in Tuscany (the Academia 

 del Cimento of Florence) made an 

 experiment which was for a long while 

 supposed to prove this. They filled a 

 hollow ball of thin beaten gold with 

 water, and placing it in a press or vice, 

 they applied a great force to squeeze 



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it : by altering the shape of the ball, 

 the pressure made the water ooze 

 through the pores of the gold, and 

 stand in drops on its surface. But 

 although this only proved that the water 

 was not easily compressible, it did not 

 shew that no force could change its 

 bulk ; and Mr. Canton afterwards 

 proved that liquids are in some degree 

 compressible, and therefore elastic. 

 His experiment was very simple, and 

 quite decisive. He observed the height 

 at which water, previously well boiled, 

 and some other liquids, stood in a glass 

 tube, in the air ; and then, by means 

 of an air pump, he removed the air : he 

 found the liquid rose in the tube, 

 so that the weight of the ah- must 

 before have compressed the liquid, or 

 made it fill a smaller space. It was 

 found that the weight of the air com- 

 presses rain water about 1 -22000th of 

 its bulk, or makes it shrink about one 

 part in 22,000 ; olive oil, about one 

 part in 21 ,000 ; spirit of wine, one part 

 in 15,000, and mercury only one part in 

 33,000. 



By Mr. Perkins's late experiments, it 

 should seem that water is more com- 

 pressible than those older observations 

 indicate. It had always been remarked, 

 that when a bottle, filled with water, 

 and corked tight, was plunged to a great 

 depth in the sea, the water in the bottle 

 tasted salt on bringing it up, as if the 

 cork had been forced in when it was 

 under water. He therefore constructed 

 an instrument to ascertain how far the 

 cork is forced in. He made a hollow 

 cylinder of brass, water-tight, with a 

 rod moving in the top through an air- 

 tight and water-tight hole, and on this 

 rod he put a spring ring, which re- 

 mained fixed at any point at which it 

 was placed. Over the whole he screwed 

 a cover or cap, to protect the rod, but 

 drilled with holes to let in the water ; 

 he filled the cylinder with water, and 

 plunging it five hundred fathoms deep, 

 he found that the ring, when the in- 



