514 Mr. Dugald OhrJc [Feb. 22, 



The fall, as you see, is gradually decreasing revolution by revolution. 

 This fall, however, is not entirely due to heat loss. It is partly due 

 to work done. There is a certain amount of heat converted into 

 work at each reciprocation. This, however, can be allowed for, and 

 then a cooling curve is obtained which shows the real temperature 

 drop due to cooling up on the expanding and compressing lines. 

 From this curve, by somewhat troublesome calculations into which I 

 need not enter here, the apparent specific heat of the charge can be 

 obtained for each expansion line. 



I show upon the screen a curve giving the apparent specific heat 

 of the particular working fluid in this engine at temperatures of 

 from 70° C. up to 1500° C. Seven points are marked. These are 

 the points of observations, and each point is the mean taken from 

 twenty-one separate diagrams. 



This method of obtaining specific heat appeared to me likely to 

 give the true specific heat of the charge, without fear of continued 

 combustion. In these particular experiments the engine was run at 

 120 revolutions per minute, so that the completion of the first com- 

 pression after ignition occurred half a second after the explosion. 

 In that half second it seemed to me probable that the greater part, 

 if not all, of the combustion would be completed. Calculations 

 made, however, from the expansion curves shown, proved that for 

 some reason the lower end of each expansion line was disturbed in 

 such a way as to make it highly probable that some combustion was 

 continuing within the cylinder, even two seconds or so after the 

 beginning of the explosion. I have, therefore, used the term 

 apparent specific heat to characterise the values given by the curve 

 I have shown. 



Two tables have been calculated, giving the apparent instan- 

 taneous specific heats and the apparent mean specific heats for the 

 different temperatures, as obtained in this manner. Assuming a 

 certain proportion of continued combustion, these numbers give a 

 very fair indication of the heat loss incurred in the cylinder, and 

 curves have been prepared illustrating the heat loss incurred during 

 the whole stroke, and a portion of the stroke, of this engine. These 

 curves give an interesting indication of the probable mean tempera- 

 tures of the cylinder walls under certain conditions. The curves 

 show that for the whole stroke the mean temperature of the whole 

 enclosing walls is about 70° C, when the water-jacket is cold, and 

 about 200° C. when the water-jacket is hot ; but for the inner part 

 of the stroke, the first three-tenths of the stroke, the mean tempera- 

 ture is much higher — 170° C. when cold, and 400° C. when hot. 



This method of investigation gives a more accurate knowledge of 

 the properties of the working fluid, so far as the thermodynamics of 

 the engine are concerned, and it enables us to make an entire heat 

 balance sheet from the diagram only. I have calculated full load 

 diagrams taken from the engine by this method, and accounted lor 



