471 



degree of Fahrenheit's thermometer, we shall have T ^- of ji^ for 

 the increase of the whole bulk of that in the tube. 



The disc continued to rotate while the thermometer indicated equal 

 temperature in the tube and external to it, in eight of the cases, 

 and was arrested by an accident in the ninth. 



In another experiment, when the lower orifice of the tube was 

 alternately closed and opened by a valve, the temperature appeared 

 under both circumstances to be the same ; hence, if we assume that 

 a minute excess of temperature of the air within the tube, over that 

 of the air external to it, exists, yet the experiment shows that it is 

 not attributable to any heat being disengaged by the movement of 

 the air itself. 



Increase and decrease of the temperature of the room exercised a 

 considerable influence on the velocity of the rotations of the discs, 

 which increased as the day advanced, and declined as the tempera- 

 ture fell towards evening, although the direct rays of the sun never 

 fell upon the window of the room. 



Partial exclusion of light, by a blind covering the whole window, 

 produced a considerable reduction in the velocity of the rotations of 

 the discs, but a screen of a foot in breadth, interposed between the 

 window and an individual tube, merely reduced the velocity of the 

 rotations from 12'5 to 1TO per minute. 



The influence of reduction of temperature of the long branch of 

 the tube, by placing around it two coils of wet tape*, reduced the 

 revolutions of the disc from 4'0 to 1*75 per minute ; a third reduced 

 the revolutions to TO ; a fourth to 0'5 ; and a fifth caused complete 

 cessation. 



To ascertain the influence of the abstraction of aqueous vapour on 

 the rotation of the discs, a shallow vessel, containing strong sulphuric 

 acid, was placed, at the suggestion of the Rev. Dr. Booth, imme- 

 diately below the disc in the short branch of the tube. After the 

 lapse of thirty minutes, the rotation had ceased altogether ; at the 

 commencement the disc was rotating at the usual rate. The same 

 vessel, placed in the tube without the sulphuric acid, had no effect 

 on the rotation. 



In another experiment a bell-glass was suspended over the short 

 branch of the tube, so that the short branch projected into it, and a 



* Half an inch broad, and not so wet that any of the water ran away from it. 



