232 



Royal Society : — 



would seem, then, that the velocity has relation rather to the mean 

 area of the tube than to that of the entrance and exit orifices, as the 

 latter were the same in the tube of 3 inches uniform diameter, and 

 in the conical tube on its base, while the revolutions of the disc 

 were 3'0 per minute in the former, and 8*8 per minute in the latter. 



Fig. 13. 



Fig. 14. 



Fig. 15 



Fig. 16. 



a 



L 



^ 



When the exit orifice of the tube of 6'75 inches diameter was re- 

 duced to 3'5 inches, the rapidity of the revolutions was reduced only 

 about 10 per cent. 



The influence of temperature in accelerating or retarding the cur- 

 rents through the tubes next engaged the author's attention ; but 

 before entering into direct experiments, he found by very numerous 

 observations, that on some occasions no appreciable difference could 

 be observed in the temperature of the atmosphere of the room near 

 the floor and the ceiling, while on others there was a mean excess 

 of 0°' 1 7 Fahr. near the ceiling without causing any perceptible dif- 

 ference in the velocity of the revolutions of the discs. In forty com- 

 parative observations of the temperature of the external surface of 

 the tube (fig. 1 3) and of the surrounding air, that of the tube was 0"09 

 higher ; in twenty-three it was equal, but in only five was it lower 

 than the surrounding air. 



Of thirty-six comparative observations of the temperature of the 

 air within, and external to the tube, by a delicate mercurial thermo- 

 meter, it was found to be slightly higher within the tube in twenty- 

 seven, and in the remaining nine it was equal, but never lower 

 than that of the external air. The greatest excess was 0°-4 Fahr., 

 and the mean excess 0°'14 Fahr. 



The accuracy of these results was tested by an extremely delicate 

 difl"erential thermometer, which also indicated a minute excess of 



