ON HTDRAULIC MACHINES. 33T 



above it, and forces this water to ascend through a third pipe to any height 

 less than that of the first column. The air vessel is then emptied, and the 

 second reservoir filled, and the whole operation is repeated. The air must, 

 however, acquire a density equivalent to the pressure, before it can begin to 

 act ; so that if the height of the columns were 34 feet, it must be reduced to 

 half its dimensions before any water would be raised; and thus half of the 

 force would be lost; in the same manner, if the height were 68 feet, two 

 thirds of the force would be lost. But where the height is small, the 

 force lost in this manner is not greater than that which is usually spent in 

 overcoming friction and other imperfections of the machinery employed; for 

 the quantity of water, actually raised by any machine, is not often greater 

 than half the power which is consumed. The force of the tide, or of a 

 river rising and falling with the tide, might easily be applied by a machine of 

 this kind, to the purposes of irrigation. (Plate XXIII. Fig. 320, 

 321.) 



The fountain of Hero precisely resembles in its operation the hydraulic 

 vessels of Schemnitz, which were probably suggested to their inventor 

 by the construction of this fountain. The first reservoir of the fountain is 

 lower than the orifice of the jet; a pipe descends from it to the air vessel, 

 which is at some distance below, and the pressure of the air is communi- 

 cated, by an ascending tube, to a third cavity, containing the water which 

 supplies the jet. Many other hydraulic and pneumatic instruments, intend- 

 ed for amusement only, and some of them of much more complicated 

 structure, are also described in the works of Hero. (Plate XXIII. 

 Fig. 322.) , 



The spontaneous vicissitudes of the pressure of the air, occasioned by 

 changes in the weight and temperature of the atmosphere, have been applied, 

 by means of a series of reservoirs, furnished with proper valves, to tiie purpose 

 of raising water by degrees to a moderate height. But it seldom happens that 

 such changes are capable of producing an elevation in the water of each re- 

 servoir of more than a few inches, or at mpst a foot or two, in a day: and 

 the whole quantity raised must, therefore, be very inconsiderable. 



The momentum of a stream of water, flowing through a long pipe, has also 

 been employed for raising a small quantity of water to a considerable height. 



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