HYDRAULICS. 



deliver water into it, (for several pumps 

 are frequently made to open into one 

 common air-vessel,) and ought in all 

 cases to contain at least six or eight 

 volumes of the pump, in order that the 

 increasing expansive force of the air 

 may not influence the motion of the 

 piston during a single stroke : but for 

 this no precise rule can be given, as the 

 relative dimensions may vary to suit 

 the circumstances of the case. These 

 forcing-pumps with air-vessels are now 

 very generally adopted in Water-Works 

 for supplying cities or towns ; and the 

 height at "which the water is at any time 

 delivering may be very nearly estimated 

 if the air-vessel is large, and the supply 

 equable, by examining the degree of 

 condensation of the air within it. This 

 is very conveniently done by a gauge, 

 consisting: of a glass tube with a close 

 top, applied by a stop-cock to the lower 

 part of the air-vessel, or that which is 

 always filled with water: thus, rs (fig. 

 10.) "represents such a tube, and as it 

 has an open communication with the 

 air-vessel when the cock * is open, the 

 air in the top of the tube will suffer the 

 same condensation as that within the 

 vessel. The height of the space occu- 

 pied by air within the tube must be 

 measured ; and as the air, at its ordinary 

 density, will balance a column of water 

 33 feet high, so if confined air is loaded 

 with the weight of such a column, it 

 will shrink or be condensed into half its 

 former bulk. Whenever, therefore, the 

 air contained in the tube r is diminished 

 to half its original length, the conden- 

 sation within the air-vessel must be 

 equal to two atmospheres, or, what is 

 the same thins:, the water in the pipe 

 p p must stand at the elevation of 33 

 feet. If the water in pp is raised to 

 twice 33 feet, or G6 feet, then the con- 

 densation within the air-vessel must be 

 equal to three atmospheres, and the air 

 within it, as well as within the tube, will 

 be diminished to one-third of its origi- 

 nal bulk. One-fourth of the bulk will 

 indicate four atmospheres of condensa- 

 tion, and be equal to the elevation of 

 the water column to 132 feet, and so 

 on, more or less, as the barometer may 

 vary. 



That useful machine, the Fire-engine, 

 or engine for extinguishing fires, is 

 nothing more than two forcing-pumps 

 of the construction shown at Jig. 10, 

 working into one common air-vessel 

 placed between them, and from which 

 the spouting pipe for directing the water 



upon the fire proceeds. The handles 

 are so disposed, that while the piston of 

 one pump is up the other is down ; and 

 they are elongated for the purpose of 

 enabling a great number of men to work 

 them at the same time, for the purpose 

 of throwing a very large quantity of 

 water, which is rendered a continuous 

 stream by the action of the air-vessel. 

 It is curious that the most ancient pump 

 we are acquainted with, namely, that of 

 Ctesebes, at least as it is handed down 

 to us, very closely resembles the present 

 fire-engine, for it consists of two forc- 

 ing-pumps, disposed as just described ; 

 but instead of discharging their con- 

 tents into an air-vessel, they merely 

 deliver them into an intermediate close 

 cistern, from which the water ascends 

 by a perpendicular pipe, and in which 

 nothing is wanting but the condensation 

 of air*. It must, however, be observed, 

 that both the pumps shown at figs. 10 

 and 1 1 , would be forcing-pumps without 

 their respective air-vessels ; and though 

 they act much more advantageously 

 with them, they are sometimes con- 

 structed without these useful appen- 

 dages. 



A pump, the invention of M. de la 

 Hire, produces the full effect of two 

 pumps with the friction of one only, for 

 it is a lift and force pump that raises an 

 equal quantity of water by its up and 

 down stroke. " But few instances of its 

 adoption occur, and considering its ad- 

 vantages, it is surprising that it is not 

 more frequently put into practice. It 

 is shewn in section at fig. 12, 1 1 being 

 the working ban-el, in which the solid 

 piston v moves up and down, u is the 

 feeding-pipe, and w the stop-valve upon 

 it; a? is a lateral pipe proceeding up- 

 wards from the bottom of the working 

 ban-el, until it terminates in the under 

 part of the air vessel y, such termina- 

 tion being closed by a valve opening up- 

 wards into the air-vessel, from which z 

 is the discharging-pipe. So far this 

 pump precisely resembles that shewn 

 and described by Jig. 10, both in con- 

 struction and action ; but in the present 

 pump, instead of the working-ban-el 1 1 

 being open at its top, it is closed by a 

 cap, and the piston rod a a works in an 

 air-tight manner through the stuffing- 

 box b, consequently when the piston is 

 depressed to expel the water out of the 

 lower part of the working-ban-el into 



* See Ctesebes's Pump, article " Pomp," in Dr. 

 Hutton's Mathematical Dictionary, vol. ii. p. 304. 



