of Cyclically Varying Temperature. 283 



During the first stroke a charge of mixed air and com- 

 bustible gas is drawn into the cylinder by the forward motion 

 of the piston, the air- and gas- valves are now closed automati- 

 cally, and the piston, on its second stroke, compresses the 

 mixture, nearly adiabatically *, into the clearance space. 

 This space forms a prolongation of the working cylinder, and 

 has a capacity of about 45 per cent, of the volume swept 

 through by the piston. The pressure reached at the end of 

 the compression stroke is thus about four atmospheres. At 

 the commencement of the third stroke communication is 

 opened to the ignition-tube ; the mixture then explodes, and 

 the pressure rises to about fifteen atmospheres. The piston 

 moves forward and the pressure falls, as the heated gases 

 expand, until it is about three and a half atmospheres. This 

 takes place just before the end of the stroke, and at this point 

 the exhaust-valve opens. During the fourth stroke the pro- 

 ducts are discharged from the cylinder. 



In order to measure the extremely high temperatures which 

 were known certainly to be reached during such a cycle, some 

 special form of thermometer had to be employed : not only is 

 the temperature high, but the pressures to be dealt with are 

 such as to destroy any thermometer not constructed of 

 materials capable of resisting great mechanical stresses. It 

 appeared to me that the platinum resistance -thermometer, 

 which has been brought to such perfection by Callendar and 

 Griffiths, and whose constants can be determined with such a 

 high degree of accuracy, gave the greatest promise of 

 success. 



The general form and construction of such thermometers 

 are now so well known, that I will only describe the special 

 form which I have employed to fulfil the following conditions, 

 which had to be satisfied in this particular case : — 



(i.) The thermal capacity must be small, as the variations 

 of temperature are extremely rapid. A whole stroke 

 of this engine is completed in less than one fourth of 

 a second ; during this time the temperature varies 

 about 500° C, and as the temperature has to be mea- 

 sured at any fraction of the stroke, a lag of 0*01 second 

 corresponds to one tenth of the stroke of the piston, 

 (ii.) The body of the thermometer must be made of some 

 strong material such as iron or steel, as the impulsive 

 pressures would immediately destroy any weaker 

 material. 



* During compression the pressure and volume are approximately 

 connected by the relation pv l ' 3 = constant. 



U2 



