170 The Trials of Suction Gas Plants at Derby, 1906 . 
known such an arrangement cannot give a true indicator 
diagram, and at any rate is quite useless as regards obtaining 
the I.H.P. Many of the gears were insufficiently stiff, and 
errors were imported thereby. A certain number of light 
spring cards were also taken and gave useful information with 
regard to the running of the engines. A number of diagrams 
were taken with the Mathot indicator, which is shown in Fig. 8, 
and some extremely interesting results were obtained which 
will subsequently be referred to. 
Devolutions. — The total revolutions for each test were 
obtained by means of revolution counters. Readings were 
taken of these counters at intervals, from which the revolutions 
Fig. 8. — Mathot Indicator and Recorder. 
per minute could be deduced. These observations are, however, 
difficult to take, and in many cases the result was unsatisfactory ; 
there were apparent fluctuations of speed which certainly did 
not exist. These counters, however, gave the correct mean 
speed when taken over a long interval of time. 
Explosions per minute . — The total number of explosions 
during each test was recorded by a counter. Several of these 
counters gave much trouble and were quite unreliable. Readings 
at intervals were also taken to obtain the explosions per minute 
to see if any variation took place during the progress of the 
trial ; these readings, however, are subject to the same errors 
as noted above in connection with revolution counters. 
