392 Intelligence and Miscellaneous Articles. 



which is of interest, the simplest case only shall be treated here ; 

 the body attracted is assumed to be spherical, to absorb the whole 

 of the energy which falls upon it, and to radiate it then uni- 

 formly in all directions ; the numerical calculations will be made 

 with great allowances. 



Assuming with Langley as the value of the solar constant, 

 that is the quantity of heat which falls perpendicularly in a 

 minute on 1 square centimetre, C=3 gramme- calories, and the 

 mechanical equivalent of heat B = 425 gramme-metres, the quan- 

 tity of energy E which falls in a second on a square centimetre 

 will be c 



E= — B = 21 gramme-metres. 



Taking the velocity of light V=3.10 8 metres, we can from (1) 

 calculate the pressure P which a pencil of rays of 1 sq. cm. section 

 exerts on an absorbing body at the distance of the earth 



P=|=|lO"V*; 



or in absolute units 



P=O6xl0- 4 dynes (2) 



If we take the earth's distance from the sun p = 15 . 10 12 cm., 

 and the velocity of the earth's path <r = 3 . 10 6 cm., the sun's acce- 

 leration 



a=— =0-6 cm. 

 P 

 Accordingly the sun attracts 1 gramme mass, at the earth's dis- 

 tance, with the force A, 



A = 0-6 dyne (3) 



The action which the sun exerts on a body which rotates around 

 it, consists on the one hand in the Newtonian attraction, and on 

 the other hand in repulsion from radiation. Given a spherical 

 body at the earth's distance, which absorbs the whole of the solar 

 energy which falls upon it, and then radiates it uniformly in all 

 directions ; if r is its radius in centimetres, and B its density in 

 reference to w 7 ater, we can calculate both the force G with which it 

 is attracted, and the force H with which it is repelled : 



G = |7rr 3 2A, 



H = 7rr 2 P. 



From this we can calculate the resultant force F, and express it in 

 fractions of the force of gravitation with which the body is 

 attracted by the sun : 



1 G^ =1 -G- 1_ 4A7g (4) 



* The pressure of a pencil of rays of 1 square metre section amounts 

 to | mg. 



