152 Royal Society, 



will acquire heat for some time after they have begun to recede, and 

 lose it after they have begun to approach. There will, in fact, be 

 a certain lagging in the effect of the heat on the pith, like that 

 which is apparent in the action of the sun on a comet, which causes 

 the comet to be grandest after it has passed its perihelion. Prom 

 this cause it is easy to see that the mean temperature of the ends 

 will be greater during the time they are retiring than while ap- 

 proachiug, and hence the driving force on that end which is leaving 

 will, on the whole, more than balance the retarding force on that 

 which is approaching ; and the result will be an acceleration, so that 

 the bar will swing further each time until it passes the candle, after 

 which the hot side of the bar will be opposite to the light, and will 

 for a time tend to counteract its effect, so that the bar will for a 

 time be quieter. This fact is independent evidence as to the nature 

 of the force ; and although it does not show it to be evaporation, 

 it shows that it is a force depending on the temperature of the pith, 

 and that it is not a direct result of radiation from the candle. 



Since writing the above paper, it has occurred to me that, accord- 

 ing to the kinetic theory, a somewhat similar effect to that of eva- 

 poration must result whenever heat is communicated from a hot 

 surface to gas. 



The particles which impinge on the surface will rebound with a 

 greater velocity than that with which [they approached ; and con- 

 sequently the effect of the blow must be greater than it would have 

 been had the surface been of the same temperature as the gas. 



And, in the same way, whenever heat is communicated from a 

 gas to a surface, the force on the surface will be less than it other- 

 Avise would be, for the particles will rebound with a less velocity 

 than that at which the} r approach. 



Mathematically the result may be expressed as follows- — the 

 symbols having the same meaning as before, e representing the 

 energy communicated in the form of heat, and §v the alteration 

 which the velocity of the molecule undergoes on impact. As before, 



and 



dv 2 A /%> ; 



dv ( v 4- %vf — v 2 du 2 cu i 



: — ~ =—7. — nearly, 



6 ' 2g %g 



dv * 





Therefore, in the case of steam at a temperature of 60°, 



/•- e • 

 J 2000 ' 

 and in the case of air 



J 1400 



