280 Mr. Dugald Clerk [Jan. 29, 



The centre temperature is then found to vary from 1100° C. to 

 1200^0., while the mean temperature from B to D is 1160° C, and 

 the mean temperature inferred from the pressure is 1100° C. Here 

 the temperature differences are very much smaller than at the 

 moment of maximum pressure. The mass of gas during coohng may 

 l)e described as a hot core in which the temperature is approximately 

 uniform, varied only accidentally by currents and surrounded by a thin 

 layer wherein the temperature falls to the temperature of the walls. 

 Prof. Hopkinson considers that if such a layer were ^ cm. thick, and 

 that if the fall of temperature were uniform, the mean temperature 

 inferred from the pressure would fall short of that of the hot core by 

 the observed amount, that is, GO^ C. 



These experiments are most interesting and valuable in aiding 

 the understanding of what is happening on the rising line. Obviously 

 this experiment proves very clearly that even in an absolutely non- 

 conducting vessel temperature differences will be found at the 

 moment of maximum pressure, apart altogether from any question of 

 heat loss. In such a vessel the point of maximum temperature 

 would always be tbe point where the flame originated, because at 

 that point a flame at the full combustion temperature will be produced, 

 and this combustion temperature will be raised by compression from 

 the initial pressure to the maximum pressure of explosion. Now that 

 it has been proved, it is quite obvious that this temperature rise 

 by compression should occur ; but no one thought of it or predicted it, 

 and it remained for Hopkinson to discover it by these interesting 

 experiments. 



Explosion within Engine Cijlinders having Moving Pistons. — It 

 has been long known that the times of explosion in closed vessel 

 experiments were much longer than the times found within engine 

 cylinders having moving pistons. Indeed, it is evident that an 

 engine, say, of about 21 inches diameter cylinder, approximating to 

 the capacity of Hopkinson's explosion vessel, could not possibly 

 be operated in any commercial way if the time of explosion had 

 been as long as ' 26 second, the time found by Hopkinson for an econo- 

 mical mixture. Allowing 40 degrees of the circle of revolution of a 

 crank for the ignition period in an engine, one revolution would 

 require to take 'l'?A seconds, so that the engine could not revolve at 

 even as low a rate as 30 revolutions per minute and still ignite 

 the gases in time to produce effective results. Engineers long 

 recognized that the rate of the travel of flame usually found in 

 closed vessel experiments was much lower than that in engine 

 cyUnder tests. In a paper published in the Proceedings of Civil 

 Engineers in 1882, Clerk macte the following statement : — 



" The author has found it possible to ignite a whole mass in any 

 given time between the limits of one-tenth and one-hundredth part 

 of a second, by so arranging the plan of ignition that a small volume 

 of gaseous mixture is first ignited, expanding and projecting a flame 



