ON GASKOUS BXPLOSIONS. 251 
170° G. and the value of © obtained by extrapolating slightly from 
Swann’s results would be 20°16 at this temperature and at atmospheric 
pressure. Unless, therefore, the difference in density gives rise to a 
greater difference in the volumetric heat than seems at all probable, 
Clerk’s value is about 3 per cent. too high. 
Clerk then went on to calculate from the diagram the actual heat loss 
on the compression and on the expansion stroke, assuming the true 
value of the volumetric heat to be 20. Comparing the heat loss in the 
last three-tenths of the compression (AB) and the first three-tenths of the 
expansion (BC), he found that they were in the ratio of about 3 to 1, 
whereas the mean temperature on compression was only about 11 per 
cent. greater than over the corresponding range in expansion. Clerk 
further found that the heat loss from 0'1 to 0°4 on the expansion 
stroke was practically nil, while on the corresponding part of the com- 
pression stroke it amounted to about 7 per cent. of the work done on 
the gas. 
Commenting on these results, Clerk says: ‘ The experiments show 
that the gas does not on the whole gain heat during the first half of the 
expansion stroke, as was found by Hopkinson.* But they do show that 
for some reason the heat loss is divided very unequally between the 
compression and expansion strokes. The proportion varies from point 
to point of the stroke, and also varies largely with the temperature of the 
walls, but for the inner one-tenth and the first three-tenths of the stroke 
the compression heat loss appears to be about three times the expansion 
heat loss. 
‘ From this it follows that Hopkinson was correct in his expectation 
that the specific heat of air determined by division of heat loss in propor- 
tion to mean temperature would be too high. The experiments show 
that this method of division leads to a value about 3 per cent. higher than 
the true value.’ 
Clerk also finds from these experiments that the greater part of the 
heat loss was incurred at the inner tenth of the stroke during compression 
and expansion at the higher temperature and density; 80 per cent. of 
the loss on the three-tenths was due to the inner tenth. The loss on the 
compression line from 0°4 to 0°1 of the stroke was small, and that on 
the expansion line was less. Calculating C as the mean value on these 
lines, the value is 20°7 foot lb. per cubic foot. The mean temperature in 
this part of the diagram was 120° C. 
In view of these experiments on the compression and expansion of cold 
air, the Committee consider that the division of heat loss in the high 
temperature compressions on which Clerk’s values of the volumetric 
heats are based may require some revision, and that these values may on 
this account be rather too high. The results given by air (as to ratio of 
heat loss between compression and expansion lines) at temperatures of 
the order 200° C. may, however, not be quantitatively applicable to gases 
cooling at the high temperature of 1000° C. It will be necessary to 
experiment further on high temperature compré&sion before the amount 
of the correction necessary on this account can be decided. Clerk has 
* The gain of heat found by Hopkinson may have been due to the fact that he 
did not trap a single charge, but continually compressed and expanded fresh 
charges, in consequence of which the temperature of the cylinder walls, and espe- 
cially of the face of the piston, must have been materially higher than in Clerk's 
experiments. 
