250 REPORTS ON THE STATE OF SCIENCE. 
(2) Clerk’s Experiments. 
In the first Report it was pointed out that the values of the volumetric 
heat of gas-engine mixture obtained by Dugald Clerk by the compression 
and expansion of gas heated by combustion were (at a temperature of 
1000° C.) about 10 per cent. higher than the corresponding values 
given by Holborn and Henning’s experiments at constant pressure. Cal- 
lendar expressed an opinion that the constant-pressure methods of deter- 
mining specific heat were subject to systematic errors which would tend 
to make the results toolow. This view has received further confirmation 
in the publication of Swann’s work, to which reference was made in the 
last section ; for Regnault’s experiments were of the same character as 
those of Holborn and Henning, and any error of defect in his values might 
be expected to be of even greater magnitude in their experiments. On 
the other hand, Hopkinson gave reasons for supposing that Dugald Clerk’s 
values were too high because of a difficulty in determining the division of 
heat loss between the compression and expansion lines. In support of 
this view Hopkinson stated that he had found that when cold air was 
compressed and expanded in a gas-engine driven by an electric motor the 
values of the volumetric heat deduced by Clerk’s method from the 
compression and expansion lines were too high, and that the air took in 
heat during the first half of the expansion stroke, though its mean 
temperature was above that of the walls. 
This point has been made the subject of detailed experiments by 
Dugald Clerk, who communicated a preliminary account to the Com- 
mittee in Note No. 11. His experiments were made on the ‘ R’ engine 
of the Institution of Civil Engineers’ Tests,! of 9 inches diameter and 17 
inches stroke, which was driven by an electric motor. The exhaust valve of 
the engine was permanently closed during the experiments, and commu- 
nication was made with a large reservoir containing dry air by means of 
the charge inlet valve, which was held partially open until the desired 
engine speed was attained, when the valve was tripped and acted on by 
the usual cam, so as to take an air charge into the cylinder. The trip gear 
was so arranged that after one full opening and closing the valve was 
held closed during the experiments so that the air thus trapped in the 
cylinder was alternately compressed and expanded. In this respect 
Clerk’s arrangements differed from Hopkinson’s, who kept both inlet and 
exhaust valves working continuously in the ordinary way, thus taking in 
a fresh charge of air at every other revolution, compressing and expand- 
ing it once and then discharging it. An optical indicator was used to 
take the diagrams, one of which is reproduced in Fig. 1.* 
An analysis of the last three-tenths of the. first compression line and 
the first three-tenths of the first. expansion line (AB and BC on the 
figure) by Clerk’s method, in which the heat loss is treated as the same in 
compression and expansion, subject to an allowance for the higher tem- 
perature in the former, gave as the value of the volumetric heat of air 
20°9 foot Ib. per cubic foot, or 5°28 calories per gm. molecule. This was 
the mean of six cards, three of which were taken at a speed of 120 r.p.m. 
and three at 180 r.p.m.; the maximum value found was 21°1 and the 
minimum 20°5. The mean temperature on the expansion line was about 
‘ Minutes of Proceedings Inst.C.H., vol. clxiii. p. 288. : 
2 The particular diagram shown in Fig. 1 belongs to a later set of experiments. 
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