258 LECTURE XXVIII. 



of the reservoir or air vessel, on the water in a second reservoir, at any 

 distance either below or 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 opera- 

 tion 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, in- 

 tended 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 ap- 

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

 the 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 reservoir of more than a few inches, or at most 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. 



The passage of the pipe being stopped by a valve, which is raised by the 

 stream, as soon as its motion becomes sufficiently rapid, the whole column 

 of fluid must necessarily concentrate its action almost instantaneously on 

 the valve ; and in this manner it loses, as we have before observed, the cha- 

 racteristic property of hydraulic pressure, and acts as if it were a single 

 solid ; so that, supposing the pipe to be perfectly elastic and inextensible, 

 the impulse must overcome any pressure, however great, that might be 

 opposed to it, and if the valve open into a pipe leading to an air vessel, a 

 certain quantity of the water will be forced in, so as to condense the air, 



* See Heronis Spiritalium Liber, Lat. & F. Commandino, 4to, Par. 1583; or 

 Veter. Math. Op. fol. 1693. 



