334 LECTURE XJtVIII. 



If, instead of forcing the water to a certain height through a pipe, we 

 cause it to form a detached jet, we convert the forcing pump into a fire engine ; 

 and in general two barrels, acting alternately, are connected, for this purpose, 

 with the same air vessel; so that the discharge is thus rendered very nearly 

 uniform. The form of the ajutage, or orifice of the pipe, is by no means in- 

 different to the eff^ect of the machine, since the height of the jet may be 

 much increased by making it moderately contracted, and a little conical 

 rather than cylindrical. When the air vessel is half filled with water, the 

 height of such a jet will be about 30 feet, when two thirds filled, about 60, 

 the height being always nearly proportional to the degree of condensation of 

 the air, or to the excess of its density above that of the surrounding atmo- 

 sphere. Sometimes a double forcing pump, or fire engine, is formed b}^ the 

 alternate rotatory motion of a flat piston within a cylindrical barrel: the 

 axis of its motion coinciding with that of the barrel, and the barrel Being 

 divided by a partition into two cavities, which are filled and emptied in the 

 same way as the separate barrels of the common fire engine. The mechani- 

 cal advantage of this machine is nearly the same as that of the n)ore usual 

 constructions, but it appears to be somewhat more simple than a common 

 engine of equal force. The partition may be extended throughout the 

 diameter of the cylinder, the opposite pairs of cavities being made to com- 

 municate with each other, and thus both sides of the piston may be employed 

 at once. (Plate XXIII. Fig. 313.) 



A piston placed in a similar manner has sometimes been made to revolve 

 continually, and to force the water through a pipe by means of a slider or a 

 spring, which intercepts its passage in any other direction. Machines of this 

 kind have been invented and reinvented, by Ramelli, Cavalleri, Amontons, 

 Prince Rupert, Dr. Hooke, Mr. Braniah, and Mr. Gwynn. Mr. Gwynn's 

 engine, which has been employed in many cases with considerable success, 

 consists of a piston or roller nearly elliptical, well fitted to the cylinder 

 within which it revolves, with a valve pressed lightly against it by a spring, 

 which causes a considerable part of the water contained in the cylinder to 

 be forced in each revolution into the pipe : the whole machine is made of 

 brass; the spring requires very little force, for the pressure of the water on 

 the valve keeps it always close to the roller, and the friction arising from 

 this cause is even an objection to the machine. The stream, although never 



