Physical Constitution of the Sun. 315 



t a the absolute temperature of the issuing gas in the plane 



of the outlet ; 

 p { the .pressure of the gas in the interior; 

 p a the pressure in the plane of the outlet. 

 According to the mechanical theory of heat, and upon the 

 above-mentioned hypotheses, the following relation holds good 

 between these nine quantities* :-— 



aJ=« c (/,-0, ". (i) 



Further, let 



;=©" < 2 > 



a l signify the mean height of the barometer in metres of 

 mercury ; 



p the density of the gas under consideration at the tempe- 

 rature of melting ice, and under the pressure of the 

 column «j on the earth's surface ; 



<r the density of the gas in the interior space under the 

 pressure p { and at the absolute temperature U ; 



ex. the coefficient of expansion of the gas for 1° C. 

 According to Mariotte and Gay-Lussac's law we have, there- 

 fore, the following relation, 



#=&4 (3) 



u x ol t { v 



The pressure p a in the plane of the outlet may, according to 



Our assumptions, be considered to be equal to that exerted by the 



solar atmosphere at the level of the layer of separation, or at the 



lowest point of the atmosphere. | 



Let p a signify the pressure at the lowest point of the atmosphere ; 



h a given height above this point ; 



p h the pressure at this height ; 



t the absolute temperature of this atmosphere assumed to 



be constant throughout in the absence of knowledge of 



the laws of temperature ; 



g the gravity of the sun at the bottom of its atmosphere ; 



r the radius of the layer of separation ; 



Pj the specific gravity of mercury at the temperature of 



melting ice ; 



g l the intensity of gravity at the earth's surface ; 



«! the mean height of the barometer ; 



p the density of the gas forming the atmosphere at the 



temperature of melting ice and under the action of^ 1 



and a v 



* Zeuner, Grundzuge der mechanischen W'drmetheorie, 2te Aufl. 1866, 

 p. 165. 



Y2 



