NOTES 93 



But S = i '7 5 x io 6 ergs per sq. cm. per sec. 

 V = 3 x io 10 cm. per sec. 



GM , 9 



-p- = 0-59 cm./sec. 2 . 



Then m = - . - very nearly 



n io 4 



If then 



Or fully absorbed sunlight can only balance the pull 

 on ^ gramme of matter close behind each square 



centimetre. If the sunlight is partly reflected then 

 something less than twice this amount of matter can 

 have push balancing pull. 



If the absorbing matter is scattered at different distances 

 behind the square centimetre, the amount of matter which 

 can be balanced will be increased. For suppose it at 

 double the distance. The cross section of the cone, with 

 vertex at the sun and the square centimetre as base, will 

 at the double distance have four times the area, and four 

 times the matter can thus be balanced. But we confine 

 the investigation to the case in which the matter is not 

 far behind the square centimetre in comparison with the 

 distance r, the case to which the tails of comets, at any 

 rate, roughly correspond ; and the correspondence is the 

 closer in that the density must decrease rapidly as we 

 recede from its head. With constant acceleration out- 

 wards, half the matter is in the first quarter of the tail. 

 We see at once that no gas can be repelled. For there 

 is no gas known in which the absorption of a layer of 

 mass io~ 4 gm. per sq. cm. at all approaches completeness. 



Let us suppose that the matter consists of opaque 

 absorbing particles, and, for the sake of illustration, 

 suppose n = io, a value probably existing in some 

 comets' tails. 



