262, REPORTS ON THE STATE OF SCIENCE. 
gas temperatures have been measured by a thermo-couple or a platinum 
resistance thermometer. The first method of measurement by pressure 
changes necessitates the use of accurate indicators. In gaseous explo- 
sions, pressures and temperatures before ignition are comparatively 
easily determined, but pressures after ignition are often difficult of deter- 
mination for purely mechanical reasons. The Committee are of opinion 
that for accurate work in gaseous explosions optical indicators offer 
marked advantages, and they recommend that in all investigations de- 
pending upon the use of such indicators the oscillation period of the 
indicator should be given. In the piston indicators used by Clerk and 
Hopkinson this period is very easily determined. Fig. 5 is a photo- 
graphic reproduction of an optical diagram taken by the Clerk indicator 
to determine the period of the instrument used for the experiments de- 
scribed in Note No. 11. Such diagrams are taken by causing the piston 
to strike against a stop before maximum pressure is reached. On the 
return of the piston alter maximum compression or explosion it will be 
found that the expansion line shows an oscillation. rom this oscilla- 
tion the period of the indicator can be readily obtained. In this indi- 
cator the oscillation period is 1-216th of a second. With a light spring 
diagram shown in Fig. 6 the period is 1-108th of a second—double the 
period of the other. To make certain that the indicator registered the 
maximum pressure correctly, the piston was held up by a movable stop, 
so that the spring was compressed to within a few pounds of the maxi- 
mum pressure. ‘The first compression line is, therefore, omitted from 
a diagram produced in this way until nearing the maximum compres- 
sion. The piston is then lifted from its stop whenever the pressure 
exceeds that put upon the spring by the stop, and the maximum pressure 
is indicated, avoiding the momentum effect which causes the piston to 
tend to overrun its true pressure. ‘This is clearly shown in Fig. 7. A 
number of diagrams taken proved that the maximum pressure obtained 
with the stop in and the stop out was the same. 
Oscillation period experiments and experiments with a stop are 
recommended to determine the accuracy and sensitiveness of piston- 
operated indicators. Careful comparisons made by Professor Burstall and 
Professor Hopkinson of the Hopkinson optical indicator and a specially 
selected Crosby mechanical indicator proved that the mechanical indicator 
gave maximum pressures differing considerably from the optical indicator, 
except when the ignition was very slow, though the mean pressure was 
nearly the same. 
Fig. 8 shows a comparison of the two indicators which will be 
readily followed. Experiments made by Clerk with another mechanical 
indicator compared with a Clerk optical instrument showed deviations 
exceeding 5 per cent. in maximum pressures. 
As showing the importance of temperature measurement in gaseous 
explosions, the Committee would refer to diagrams obtained by Callendar 
and Dalby, described in Dalby’s Note No. 12. These diagrams showed 
a maximum temperature of 2500° C. of the gas and air mixture in a 
gas-engine cylinder—a temperature about 300° C. higher than any 
temperature obtained by ignition from atmospheric pressure. Pro- 
fessor Dalby is continuing his investigations, and many interesting points 
can be determined by his method of experimenting. The question -of 
» See Proc. Inst.M.H., July 1909. 
