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above it, and forces this water to ascend through a third pipe to any heightless than that of the first column. The air vessel is then emptied, and thesecond reservoir filled, and the whole operation is repeated. The air must,however, acquire a density equivalent to the pressure, before it can begin toact; so that if the height of the columns were 34 feet, it must be reduced tohalf its dimensions before any water Avould be raised; and thus half of theforce would be lost; in the same manner, if the height were 68 feet, twothirds of the force would be lost. But where the height is small, theforce lost in this manner is not greater than that which is usually spent inovercoming friction and other imperfections of the machinery employed; forthe quantity of water, actually raised by any machine, is not often greaterthan half the power which is consumed. The force of the tide, or of ariver rising and falling with the tide, might easily be applied by a machine ofthis kind, to the purposes of irrigation. (Plate XXIII. Fig. 320 ,321.)
The fountain of Hero precisely resembles in its operation the hydraulicvessels of Schemnitz , which were probably suggested to their inventorby the construction of this fountain. The first reservoir of the fountain islower 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 whichsupplies the jet. Many other hydraulic and pneumatic instruments, intend-ed for amusement only, and some of them of much more complicatedstructure, are also described in the works of Hero. (Plate XXIII.Fig. 322.)
The spontaneous vicissitudes of the pressure of the air, occasioned bychanges in the weight and temperature of the atmosphere, have been applied,by means of a series of reservoirs, furnished with proper valves, to the purposeof raising water by degrees to a moderate height. But it seldom happens thatsuch changes are capable of producing an elevation in the water of each re-servoir of more than a few inches, or at most a foot or two, in a day: andthe whole quantity raised must, therefore, be very inconsiderable.
The momentum of a stream of water, flowing through a long pipe, has alsobeen employed for raising a small quantity of water to a considerable height.
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