86 PROF. C. FREWEN JENKIN AND MR. D. R. PYE ON THE 



weight rests on a plunger in an oil cylinder. There was some doubt as to the 

 effective area of the piston, since it was not exactly uniform in diameter, and there 

 was a small clearance between it and the cylinder. In order to clear up this point 

 the testing machine was checked against a mercury column about 1 m. high. The 

 value of the effective area of the plunger was found in this way ; it only differed by 

 B io from the maximum measured area. The testing machine only gave pressures up 

 to 400 Ibs. per sq. inch. Up to this pressure the gauges showed a practically 

 constant error, and it was assumed that the error remained the same at the higher 



pressures. 



The gauge used for the pressure-temperature curve, and for experiments where 

 accurate high-pressure readings were needed, was subsequently calibrated for its 

 whole range by the N.P.L. ; the results agreed closely with those obtained by the 

 authors. In a few of the experiments, where accurate low pressures were needed, a 

 low-pressure gauge was used, which was calibrated over its whole range. The 

 pressures are believed to be correct to about 1 Ib. per sq. in. 



Adjustments. Before beginning any test the apparatus was run for a considerable 

 time while the conditions were adjusted to what was required ; the test was not 

 begun until a steady regime had been attained and all the parts had reached steady 

 temperatures ; moreover, unless a test was completed without anything more than 

 trifling changes of any of the conditions, the results were discarded. The conditions 

 were adjusted by regulating the speed of the pump, opening or closing the throttle 

 valve, and switching on more or less electrical power to the calorimeters. While the 

 adjustments were being made the flask B was emptied and the flask A filled, so that 

 before the actual test began the C0 2 was circulating through A, which was full. The 

 potentiometer and the temperature of the cold junction were also adjusted. 



As soon as everything was ready, the valve on the top of flask A was closed and 

 that on B opened, and the regular readings of all the instruments was commenced. 

 In most tests these readings were made every minute. Each time the weighing bell 

 rang the time was entered to the nearest second the first ring marking the time of 

 start of the test, which usually continued till the flask was almost empty. 



In Series I. the apparatus was connected as shown in fig. 3, p. 71, the object being 

 to measure the heat represented by the area NBCQ (fig. 7, p. 74) for a series of tempe- 

 ratures 0,, ranging from the highest to the lowest attainable. To keep the rate of flow 

 of CO, within convenient limits the pump was run as slowly as possible for the higher 

 values of 0, and as fast as possible for the lower values of 2 . The temperature 6 a 

 was not directly measured, but was deduced by the pressure-temperature curve from 

 tin- pressure shown by the gauge connected to the pipe leaving the calorimeter ; the 

 tlm.ttle valve was adjusted so as to keep this pressure steady at the figure selected 

 The electrical power was adjusted so as to keep the temperature of 



I the percentage of air present was not always approximately constant, this method of estimating 

 9 t it liable to error. 



