HOROST1TIC3. 



THIS department of science treats of the pressure and equilibrium of liquids, 

 the most remark-able property of which is, that of equality of pressure. This 

 property arises from the extreme minuteness and independent gravitation of each 

 of the particles, and from the manner in which they act upon each other ; 

 being arranged, not perpendicularly one above the other, but obliquely, as shown 

 at fig. 1. One particle above pressing between two particles beneath, the 

 latter consequently sustain a lateral pressure, just as a wedge driven irko a 

 piece of wood separates the parts laterally. This lateral pressure is the 

 result, therefore, of the pressure downwards, or the weight of the liquid above. 



Fig. 2 illustrates the different degrees of force with which water flows from 

 apertures in a vessel at different heights. Fig. 3 represents part of a teapot, 

 which we suppose to be filled with columns of particles of water ; the particle 1, 

 at the bottom, will be pressed laterally by the particle 2, and thus be forced into 

 the spout, where, meeting with the particle 3, it presses it upwards, and this 

 pressure will be continued till the water in the spout has risen to a level with 

 that in the pot. 



Fig. 4 is another illustration of the upward pressure of water. A is a glass 

 tube, widened at the lower end, against which, by a string passing up the tube, a 

 thick piece of metal is held close by the hand. Upon immersing the glass and 

 plate thus held together in the water to a certain depth, the hand may be with- 

 drawn from the string, the upward pressure of the water being sufficient to sup- 

 port the piece of metal. 



Several interesting illustrations can be offered to prove the remarkable fact, 

 that the pressure of water on the bottom of the containing vessel does not at all 

 depend on the quantity of water, but upon the size of the base and the perpen- 

 dicular height at which the water stands. 



Figs. 5 and 6 represent two vessels of precisely similar capacities, and each 

 containing the same quantity and weight of water, but which have very different 

 pressures upon their bottoms ; that upon c D being less than the absolute 

 weight of the water, viz., the weight only of the cylindrical column, A B c D ; 

 while that upon o H is more than the absolute weight of the water, viz., the 

 weight of a cylindrical column, E F G H, for the water in the central column 

 G H I K, presses laterally with the same force, as it does on the part ou which 

 it stands ; and thus an uniformity of pressure is exerted over every part of the 

 bottom. 



Fig. 7 illustrates the latter case still more strikingly : F is a tube communi- 

 cating with the chamber, E E, and on these being filled with water, the pressure 

 upon the bottom, c D, will be precisely the same as if the whole space, A B c D, 

 were filled with water. 



Fig. 8 represents the hydrostatic bellows, which has been contrived to exem- 

 plify the great effect of a column of water. The tube A communicates with the 

 interior of the bellows, and upon these being Glled with water, the upper board, B 

 will be raised, and enabled to sustain a very considerable weight; for if the tube 

 A hold but an ounce of water, and have an area equal to the thousandth part of 

 the area of the top of the bellows, the ounce of water in the tube will sustain a 

 thousand ounces placed on the bellows. 



Another important principle in reference to liquids is their tendency to seek an 

 uniform level. If we pour water into a bent tube, as fig. 10, it will stand at as 

 equal height in both limbs. 



If there be two tubes or limbs of a tube connected together, however different 

 their width or form may be, a liquid poured into them will stand at the same 

 level in both, and thus a portion, however small, will balance a portion, however 

 large, as shown at fig. 11. 



Fig. 12 represents a number of vessels of different forms fixed in the vessel 

 A B, so as to communicate with it, and by means of it with each other. Water 

 being poured into any one of them will stand at the same level in all, as shown 

 by the line, c c. 



From these' considerations, a most important conclusion follows, namely, that 

 water will, by being confined in pipes or close channels, rise to the height from 

 whence it came ; and upon this principle depend all the useful contrivances for 

 conveying water into to.vns and houses by pipes from distant reservoirs. 

 References to figs. 1'3 and 14 will illustrate this more clearly. 

 Fig. 13 represents the water level, and tig. 16 the spirit level. 



