MECHANICS. 349 



increasing solely with the depth, and without regard to the shape. 

 When a close vessel is filled with a liquid, a pressure applied to any 

 one part, is distributed and felt on every part alike. 



When a body floats on a liquid, it displaces a bulk of liquid of 

 equal weight with itself; and is thus supported by the upward pres- 

 sure of the liquid tending to regain its level. A floating body can be 

 in equilibrium, only when the centre of gravity is in the same vertical 

 line with that of the liquid displaced. If the body is totally immersed, 

 it is still pressed upwards ; and if thus suspended by a thread, it will 

 weigh less in the liquid than in the air, by the weight of an equal 

 bulk of the liquid ; which weight may thus be found. The specific 

 gravity of any body, denotes the number of times that it is heavier 

 than water, taking equal bulks of each. Thus as platinum is 21 times 

 as heavy as water, the number 21.000 expresses its specific gravity; 

 and cork is so much lighter than water that its specific gravity is ex- 

 pressed by the decimal 0-240. In the case of gases, air, instead of 

 water, is taken as the standard of comparison. 



Water, and all other liquids, have some viscidity, or cohesiveness ; 

 as shown by their collecting in drops, before, or while falling. A 

 similar cohesion between them and the containing tubes or vessels, 

 causes the phenomenon of capillary attraction; shown also in 

 sponges, and other porous bodies ; by which the water along the 

 edges is raised above its general level. When water is confined in a 

 bent tube, or an enclosure of any shape whatever, it tends to rise to 

 the same level, or horizontal plane, in every part of its exposed sur- 

 face. If there be any aperture or orifice by which it can flow out, 

 its velocity will depend somewhat upon the shape of the aperture, 

 but principally on its depth below the surface of the liquid : it being 

 nearly the same velocity which a heavy body would acquire in falling 

 freely through the same depth. Allowance must be made here for 

 friction, and the resistance of the air. 



4. Under the division of Pneumatics, are included all the pe- 

 culiar mechanical laws of elastic or compressible fluids, whether gases 

 or vapours. Gases, retain their aeriform state at all ordinary tem- 

 peratures and pressures ; but vapors, are substances ordinarily liquid, 

 which have taken the gaseous form, owing to heat or diminished 

 pressure. The air, or atmosphere, is a permanently gaseous fluid, 

 elastic and compressible, surrounding the earth on every side, and 

 extending at least to a height of 45 miles above its surface. The 

 lower parts of it are compressed by the weight of the upper parts, so 

 that for each three miles that we ascend, its density is reduced by 

 about one-half; or, the height increasing in arithmetical, the density 

 diminishes in geometrical progression. Its total weight is about 15 

 pounds for every square inch of the earth's surface, at or near the 

 level of the sea. This pressure would counterpoise a column of 

 water 34 feet high, as shown in the sucking pump ; or a column of 

 mercury 30 inches high, as shown in the barometer. 



In the sucking pump, as the piston rises and removes the air from 

 within, the pressure of the air on the external water forces it into, 

 and up the pump, till it is in equilibrium. Then, when the piston de- 

 scends, the fixed valve, below, closes, and prevents the descent of 



