486 Mr. J. P. Joule and Prof. Thomson on the Thermal Effects 



and the thermometer for the temperature of the air flowing from 

 it, were found to be necessary by Mr, Joule, who continued the 

 research alone, and made the experiments described in what 

 follows. 



A piece of brass piping, 

 a (see the accompanying 

 sketch drawni half the 

 actual size), was soldered 

 to the termination of the 

 leaden spiral, and a bit of 

 calf-skin leather, h, ha- 

 ving been tightly bound 

 over its end, it was found 

 that the natural ])ores of 

 the leather were suffi- 

 cient to allow of a mii- 

 form and conveniently 

 rapid flow of air from 

 the receiver. By pro- 

 tecting the end over 

 which the leather dia- 

 phragm was bound \vitli 

 a piece of vulcanized 

 india-rubber tube c, the 

 former could be im- 

 mersed to the depth of 

 about two inches in the 

 bath of water. A small 

 thermometer*, having a spherical bulb -^th of an inch in diameter, 

 was placed within the india-rubber tube, the bulb being allowed 

 to rest on the central part of the leather diaphragm f. 



In making the experiments, the pump was worked at a uni- 

 form rate until the pressure of the air in the spiral and the tem- 

 perature of the thermometer had become sensibly constant. The 

 water of the bath was at the same time constantly stirred, and 

 by various devices kept as uniform as possible during each series 

 of experiments. The temperature of the stream of air having 

 been observed, the same thermometer was immediately plunged 

 into the bath to ascertain its temperature, the difi'erence between 

 the two readings giving of course the cooling eff'cct of the rush- 

 ing air. 



* We had two of these thermometers, one of which had Fahrenheit's, 

 the other an arbitrary scale. 



t The bulb was kept in this position for convenience sake, but it was 

 ascertained that the effects were not perceptibly diminished when it was 

 raised \ of an inch above the diaphragm. 



