Fig. 4. 



376 Mr. Joule 07i the Changes of Temperature produced by 



which it could be connected by Fig. 3. 



a coupling nut, it had a piece 



D attached, in the centre of 



which there was a bore of ^th 



of an inch diameter, which 



could be closed perfectly by 



means of a proper stop-cock. 

 I must here be permitted to 



make a short digression, in 



order to explain the construc- 

 tion of the stop-cocks, as it may 



save those who may in future 

 attempt similar experiments, the useless trouble of trying to 

 make the ordinary stop-cock perfectly air-tight under high 

 pressures. The one I have used is the invention of Mr. Ash, 

 of this town, a gentleman well known for his great mechanical 

 genius ; and he has in the most obliging manner allowed me 

 to give a full description of it. Fig. 4 is a full-sized sectional 

 view of the stop-cock, a is a 

 brass screw, by means of 

 which a thick collar of lea- 

 ther, /, is very tightly com- 

 pressed. The centre of a is 

 perforated with a female 

 screw, in which a steel 

 screw, 5, works, the threads 

 of which press so tightly 

 against the leather collar as ^^ j 



effectually to prevent any 

 escape of air in that direc- 

 tion. The end of the steel 

 screw is smooth and conical, 

 and the conical hole h is 

 plugged with tin. When 

 the stop-cock is shut, the 

 ismooth end of the steel screw 

 presses against the soft 

 metal, so as to prevent the escape of the least particle of air; 

 but when opened, as represented in the figure, it leaves a pass- 

 age for the air round the conical point. I have tested this 

 stop-cock in the most severe manner, and have found it to an- 

 swer perfectly. 



Having filled the receiver R (fig. 3) with about 2ii atmo- 

 spheres of dry air, and having exhausted the receiver E by 

 means of an air-pump, I screwed them together, and then put 

 them into a tin can containing 16^ lbs. of water. The water 





r 



L-Jwwi 



