264 LECTURE XXI. 



unequal diameter, a fluid must stand at tlie same height in both of them, ia 

 order to remain in equiUbrium : for if any portion be supposed to stand, in 

 either leg, above the surface of the fluid in the other leg, it is obvious that 

 its centre of gravity may be lowered, by removing so much of it as will raise 

 the fluid in the opposite leg to its own level, the situation of the fluid belo%T 

 remaining unaltered : consequently the centre of gravity of the whole fluid can 

 never acquire its lowest situation, unless both the surfaces are in the same level. 



The air, and all other elastic fluids, are equally subject with liquids to this 

 general law. Thus, a much greater force is required, in order to produce a 

 blast of a given intensity, with a large pair of bellows, than with a smaller 

 pair; and for the same reason, it is much easier to a glassblower, when he 

 uses a blowpipe, to employ the muscles of his mouth and lips, than those of 

 his chest, although these are much more powerful. If we estimate the sec- 

 tion of,the chest at a foot square, it will require a force of seventy pounds to 

 raise a column of mercury an inch high, by means of the muscles of respira- 

 tion, but the section of the mouth is scarcely more than eight or nine square 

 inches, and a pressure of the same intensity may here be produced by a force 

 of about four pounds. The glassblower obtains, besides, the advantage of 

 being able to continue to breathe during the operation, the communication 

 of the chest with the nostrils remaining open, w^hile the root of the tongue is 

 pressed against the palate. 



It is obvious that the pressure on each square inch of the side of a vessel, 

 or on each square foot of the bank of a river, continually increases in de- 

 scending towards the bottom. If we wish to know the sum of the pressures 

 on all the parts of the side or bank, we must take some mean depth by which 

 we can estimate it; and this must be the depth of the point which would be 

 the centre of gravity of the surface, if it were possessed of weight. I'hus, if 

 we had a hollow cube filled with water, the centre of gravity of each side 

 , being in its middle point, the pressure on each of the upright sides would be 

 half as great as the pressure on the bottom, that is, it would be equal to half 

 the weight of the water contained in the cube. 



If, however, we wished to su])port the side of the cube externally by a 

 force applied at a single point* that point must be at tl\e distance of one 



