60 Professors Ayrton and Perry on the 



previous explosion. The temperature of the clearance-space 

 fluid may be anything between 410° C. and the temperature 

 of the atmosphere, depending on how recently an explosion 

 has taken place. As the piston moves forward it draws into 

 the space a mixture of gas and air. At the end of the for- 

 ward stroke the pressure of the mixed fluid is nearly that of 

 the atmosphere ; in the back stroke the fluid is compressed. 

 At the beginning of the next forward stroke the fluid is 

 ignited, and rapid development of heat results, causing great 

 increase in pressure, the pressure gradually diminishing until, 

 just before the end of the forward stroke, the fluid is allowed 

 to escape. In the next back stroke the piston drives the fluid 

 out of the cylinder with the exception of what remains in the 

 clearance, and thus completes a cycle of operations. In fig. 1 

 (PL III.) indicator-diagrams show the nature of the alterations 

 in pressure and volume going on during the compression 

 and working parts of the cycle; distances measured from L 

 representing pressure in pounds per square inch from vacuum, 

 and distances measured from P representing volume of 

 the fluid, the unit of volume being the volume described by 

 the piston moving through one foot of the length of the 

 cylinder. 



Four different diagrams are given whose compression parts 

 practically coincide, the differences in their ignition parts being 

 due to differences in the amounts of gas supplied. We have 

 not thought it necessary to give a complete diagram in which 

 the dismission and suction parts of the cycle should be repre- 

 sented. 



The shape of the diagram is materially modified by the 

 recentness of the last explosion, as this affects the temperature 

 of the fluid before compression, and so modifies the actual 

 amount of the mixture of gas and air entering the cylinder. 

 To a less degree the shape of the diagram is affected in the 

 discharge part of the cycle by the recentness of an explosion, 

 as a recent explosion will have given the exhaust passages a 

 higher temperature. 



'6. The Nature of the Working Fluid. — For the purpose of 

 showing the nature of the working fluid we have constructed 

 Table I. It will be seen from this that a mixture of 6'760 



enabling students to illustrate for themselves a course of lectures delivered 

 by one of us on the Gas-engine. Hence we have taken the temperature 

 of 410° C. as the exhaust temperature, instead of 300° C. given by our 

 own measurements with the Siamens pyrometer. A correction of this 

 temperature would perhaps lessen the number 1-57 W and increase 0*37 W, 

 given in § 9. 



