﻿the Rays producing Aurora Borealis. 217 



producing these forms, whatever their nature may be, have 

 properties which are indeed most characteristic, and which 

 are also found for a-rays, or, in another way of expression, 

 hy giving to the solar rays "properties which are //ftysically 

 known from ct-raya we have succeeded in giving a simple ex- 

 planation to a large class of auroral, forms, the explanation of 

 which has so much puz/Jed earlier investigators in this field. 



Tt would be premature, from these facts only, to identify 

 the solar rays in question with a-rays ; but in view of the fact 

 that the drapery forms must be produced by some kind of 

 electric radiation which cannot be of the /3-ray type, I think 

 we may reason from similarity in properties to similarity 

 in nature, and suppose that these solar rays consist of atoms 

 or molecules carrying an electric charge. It will be a matter 

 of further investigation to determine more definitely the kind 

 of carrier and charge necessary for the production of the 

 aurora belonging to the class considered showing suddenly 

 stopping straight-lined streamers. 



Tiie similarity between the electric rays from the sun and 

 the a-rays makes it possible that at any rate certain forms of 

 aurora may be produced by real a- rays from radioactive 

 substances, and it will be of' considerable interest to draw 

 the consequences of such an assumption to see how far the' 

 at- ray hypothesis is in agreement with observed facts. 



Absorption of a.- Rays by the Eartlis Atmosphere. 

 Height of Aitr&rw. 



4. We shall suppose the barometric pressure (B) given as 

 a function of the height (h) above the ground. If wo do 

 not take into account the variation of gravity with the height, 

 the mass of unit area of stratum going from the upper limit 

 of the atmosphere to the height (A) is s.B, where B is given 

 in cm. mercury of specific weight s. If this mass (M) 

 consist of a mixture of substances its equivalent mass of air 

 (M') is found from equation (3). Provided the air equivalent 

 is found corresponding to the height (A), equation (2) gives us 

 the initial velocity of an ^-particle which would penetrate 

 down to this height. 



In the numerical calculations we shall use values of B 

 given by Wegener *. These numbers are based on various 

 assumptions, e. g. they '"depend on the assumptions we make 

 with regard to the gases present Thus the distribution of 

 pressure, especially in the strata above 100 km. say, cannot 



* P/n/s. Zeitsehr. xii. Nos. 5 and 6 (li>] I ), 

 Pliil Mag: S. 6. Vol 23. No. 131. Feb. 1§'12, Q 



