8TR080 IN RAIUFIEO GASES 



- 



variation of the temperature for a point moving along this line will be denoted 

 U ?, and the ipeia whtinn of this quantity along the same line by 



There will in general, be a particular direction of the line h for which 



u a maximum, another for which it is a minimum, and a third for which it is 

 a maximum-minimunx These three directions are at right angles to each other, 

 and are the axes of principal stress at the given point; and the part of the 

 trees arising from inequalities of temperature is, in each of these principal axes, 



tl* (fv 



pO <//<' ' 



where it. is the coefficient of viscosity, p the density, and 6 the absolute 

 temperature. 



3. Now for dry air at 15'C., /t=r9xlO~ 4 in centimetre-gramme-second 



measure, and *'*,= 0'315, where p is the pressure, the unit of pressure being 

 p6 p 



one dyne per square centimetre, or nearly one-millionth part of an atmosphere. 



If a sphere of 2a centimetres in diameter is T degrees centigrade hotter 

 than the air at large distances from it, then, when there is a steady flow 

 of heat, the temperature at a distance of r centimetres from the centre will be 



Ta , d*0 2Ta 

 e=0.+ - , and-T3= j-. 

 r dir r 



Ht-nce, at a distance of r centimetres from the centre of the sphere, the 

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

 will be 



Ta 



^ 0'63 dynes per square centimetre. 



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

 thi stress would be capable, if it existed alone, of producing rapid motion 

 in a radiometer. 



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 wanner than 



