192 REPORTS ON THE STATE OF SCIENCE.—1914, 
metres for 1 microvolt. The contact-maker used with this apparatus 
is one devised by Professors Callendar and Dalby, which has already 
been described and illustrated in fig. 3. 
Suction Temperature. 
Direct measurements of the suction temperature were made at the 
City and Guilds (Engineering) College during the session 1912-13 on 
a Crossley gas-engine with a cylinder 7 inches in diameter, stroke 
14 inches, and with a compression ratio of 4°8. The object of the 
experiment was to show how the suction temperature varied with the 
speed, with the jacket temperature, and with the mixture. 
The apparatus with which the measurement was made has been 
already described (see pages 180, 181, 182, and 183). The results of the 
experiments are shown by the curves fig. 12. It is proposed to repeat 
these experiments on engines of more modern type and with higher 
compression ratios as soon as the development of the new laboratories 
at the College render it possible to do so. 
The Cyclical Variation of the Temperature of the Charge in a 
Gas-engine Cylinder, 
An example has already been given of the method of determining 
the cyclical variation of the temperature of the charge in a par- 
ticular experiment, deducing it from the temperature measured at a 
point on the compression curve in combination with accurate indicator 
diagrams. The experiment was made at the City and Guilds (Engineer- 
ing) College on the gas-engine already referred to. The engine 
is not of recent construction and therefore the compression ratio, viz. 
4°8, is low compared with the ratios of gas-engines of more modern 
construction. Dr. Coker and Mr. Scoble have measured the cyclical 
variation of temperature on a more modern engine constructed by 
the National Gas-Engine Company in 1907. This engine has a 
cylinder 7 inches in diameter and a stroke of 15 inches. The maxi- 
mum volume occupied by the charge is 5'8 times the minimum volume. 
The method adopted was to measure directly by means of a platinum 
couple the temperature at various points along the compression-curve 
and along part of the expansion-curve, but the highest temperature 
had still to be measured by using the charge itself as a gas-thermometer. 
A value of = is selected from a point on the expansion-stroke, and 
the constant so found is used to calculate the higher temperatures. In 
this method it is unnecessary to make any calculation regarding the 
chemical contraction before and after explosion because the temperature 
is measured after the explosion, but the rate of change of temperature at 
the point where the temperature is measured is very great, and there- 
fore, in comparing the two methods, it is necessary to choose between 
a temperature measured when the rate of change is great with a 
corresponding lag and no correction for chemical contraction, as against 
a method of measuring the temperature when the rate of change is a 
minimum, viz. just after the closing of the suction-valve, and allowing 
