PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. GST 



Btroke, when, on the return of the piston, it would have to meet 

 the resistance of the contents on the other side ; so that to gain 

 every advantage it would be necessary to work it expansively, to 

 avoid counteracting back pressure. For instance, if an engine 

 were worked with four atmospheres on the inch by cutting off at 

 one quarter of the stroke, on the piston reaching the end of the 

 stroke expansion would have attenuated it to atmospheric pres- 

 sure as it is blown off. At whatever density the atmospheric air 

 may be worked the conditions are the same ; for, in the greatest 

 practicable velocities, the increased density of this pure element 

 has an increased power of transmission equal to the increased 

 velocity of the piston. Not being clogged with any vapor, nor 

 liable to condense as steam does when not in contact with the 

 water and heat of the boiler; but always being dry, and expan- 

 sive and pure, every advantage of the principle of expansion may 

 be embraced in this system. 



In a reservoir of compressed air, the propulsive force of the 

 whole body follows the current through all the passages to the 

 induction ports of the cylinders, leaving the quantity, when cut 

 ofi', that has moved the piston all its expansive force to carry it 

 to the end of the stroke. Should the density of the current be 

 greater than the power required to overcome the resistance of 

 the piston, the overplus power will give it a speed equal to the 

 density, an attenuation will take place that will equalize the 

 moving force and resistance ; which, being sufficient to keep the 

 machine in motion, the extra density will not be lost power, but 

 only serve to increase the speed ; which extra pressure in the 

 use of steam is blown off at the safety valve. Thus it is the 

 time, and not the speed, that determines the quantity of power 

 requisite for a given load for a given distance ; for if a certain 

 pressure on the piston moves the engine over a mile in two min- 

 utes, in which time the piston has travelled 150 times ; by apply- 

 ing a duplex densit}^ which increases the speed of the piston to 

 150 in one minute, the speed of the engine will be increased, the 

 time halved, but the distance and the power used will be the 

 same, or very nearly so, as it is only applying the requisite power 

 quicker ; because the piston cannot wait to equalize the moving 

 density with the higher density of the current supply. 



With a reservoir of prepared power, the working can be relied 

 on, not being subject to the fluctuations of combustion, artificial 

 temperatures, and the difficulties of steam. 



