150 



HEAT. 



The vanes are enclosed in a glass bulb, which is exhausted by a 

 mercury-pump till the pressure is exceedingly small, and then sealed. 

 On exposing the bulb to a source of radiation, 

 the lampblacked surfaces of the vanes are more 

 heated than the bare surfaces and move away 

 from the source, and so rapid rotation results. 



The complete explanation of the action is 

 due to Osborne Reynolds (" On Certain Dimen- 

 s i na l Properties of Matter in the Gaseous 

 State," Phil Trans., part ii., 1879). The action 

 was also sufficiently explained very shortly after 

 by Maxwell (" On Stresses in Rarefied Gases 

 arising from Inequalities of Temperature," Phil. 

 Trans., part ii., 1879). The theory is altogether 

 beyond our scope, but the following account of 

 what occurs may give some idea of the action. 

 It is to be remembered that it is an account, 

 and not an explanation. * 



Let us imagine that a plane is suddenly 

 introduced into a gas, one side of the plane 

 being hotter than the gas, while the other side 

 is at the same temperature with it. Consider 

 a small area on the plane far from the edges. 

 The molecules which come up on the hot side 

 are raised in temperature by contact with the 

 plane and rebound with a greater velocity than 

 that with which they arrived, while those on the 

 cold side go off with the same velocity. For a 

 moment after the introduction of the plane the number coming up to the, 

 area is the same on the two sides, since it is conditioned by the tempera- 

 ture and pressure of the sur- 

 rounding gas, and these are not 

 yet affected by the presence of 

 the plane. Hence the extra 

 kick off of the molecules re- 

 bounding from the hot side 

 implies a greater pressure on 

 that side. But very quickly 

 this excess of pressure will fall 

 off, the rebounding molecules 

 on the hot side sharing their 

 extra energy with the mole- 

 cules with which they collide, 

 and as soon as a uniform 

 temperature slope outwards is 

 established, the decrease in 

 density, and therefore the de- 

 crease in the number of mole- 

 cules coming up to the hot side, 



compensates for the extra impulse at each collision, and so the pressure 

 falls to the same value as on the cold side. 



* Lord Rayleigh (Nature, July 15, 1909, p. G9) considers the extreme case of a 

 M.F.P. large compared with the dimensions of a vane, and shows that the pressure 

 ia proportional to area of vane x density of gas. 



FIG. 79. 



Fio. 80. 



