1878.] arising from Inequalities of temperature. 305 



32 ^ 



second measure, and J—=— 0'315, where p is the pressure, the unit 

 P p 



of pressure being one dyne per square centimetre, or nearly one- 

 millionth pai't of an atmosphere. 



If a sphere of one centimetre in diameter is T degrees centigrade 

 hotter than the air at a distance from it, then, when the flow of heat 

 has become steady, the temperature at a distance of r centimetres will be 



0=T o+ _,and_=_. 



Hence, at a distance of one centimetre from the centre of the sphere, 

 the pressure in the direction of the radius arising from inequality of 

 temperature will be 



T 



- 0*315 dynes per square centimetre. 



4 In Mr. Crookes' experiments the pressure, was often so small 

 that this stress would be capable, if it existed alone, of producing 

 rapid motion in small masses. 



Indeed, if we were to consider only the normal part of the stress 

 exerted on solid bodies immersed in the gas, most of the phenomena 

 observed by Mr. Crookes could be readily explained. 



5. Let us take the case of two small bodies symmetrical with respect 

 to the axis joining their centres of figure. If both bodies are warmer 

 than the air at a distance from them, then in any section perpendicular 

 to the axis joining their centres, the point where it cuts this line will 

 have the highest temperature, and there will be a flow of heat outwards 

 from this axis in all directions. 



d?$ . . . ... 



Hence — t will be positive for the axis, and it will be a line of maxi- 

 mum pressure, so that the bodies will repel each other. 



If both bodies are colder than the air at a distance, everything will 

 be reversed ; the axis will be a line of minimum pressure, and the bodies 

 will attract each other. 



If one body is hotter, and the other colder, than the air at a distance, 

 the effect will be smaller ; and it will depend on the relative sizes of the 

 bodies, and on their exact temperatures, whether the action is attrac- 

 tive or repulsive. 



6. If the bodies are two parallel disks, very near to each other, the 

 central parts will produce very little effect, because between the disks 



(PO 



the temperature varies uniformly and —^—0. Only near the edges will 



there be any stress arising from inequality of temperature in the gas. 



7. If the bodies are encircled by a ring having its axis in the line 

 joining the bodies, then the repulsion between the two bodies, when 



