1901.] on the Existence of Bodies Smaller than Atoms. 585 



earth's atmosphere and will then or even before then, come under the 

 influence of the earth's magnetic field. The corpuscles when in such 

 a field, will describe spirals round the lines of magnetic force ; as the 

 radii of these spirals will be small compared with the height of the 

 atmosphere, we may for our present purpose suppose that they travel 

 along the lines of the earth's magnetic force. Thus the corpuscles 

 which strike the earth's atmosphere near the equatorial regions where 

 the lines of magnetic force are horizontal will travel horizontally, 

 and will remain at the top of the atmosphere, where the density is 

 so small that but little luminosity is caused by the passage of the 

 corpuscles through the gas ; as the corpuscles travel into higher lati- 

 tudes where the lines of magnetic force dip, they follow these lines 

 and descend into lower and denser parts of the atmosphere, where 

 they produce luminosity, which on this view is the Aurora. 



As Arrhenius has pointed out, the intensity of the Aurora ought 

 to be a maximum at some latitude intermediate between the pole and 

 the equator, for, though in the equatorial regions the rain of corpuscles 

 from the sun is greatest, the earth's magnetic force keeps these in 

 such highly rarefied gas that they produce but little luminosity, while 

 at the pole, where the magnetic force would pull them straight down 

 into the denser air, there are not nearly so many corpuscles ; the 

 maximum luminosity will therefore be somewhere between these places. 

 Arrhenius has worked out this theory of the Aurora very completely, 

 and has shown that it affords a very satisfactory explanation of the 

 periodic variations to which it is subject. 



As a gas becomes a conductor of electricity when corpuscles pass 

 through it, the upper regions of the air will conduct, and when air 

 currents occur in these regions, conducting matter will be driven 

 across the lines of force due to the earth's magnetic field, electric 

 currents will be induced in the air, and the magnetic force due to 

 these currents will produce variations in the earth's magnetic field. 

 Balfour Stewart suggested long ago that the variation in the earth's 

 magnetic field was caused by currents in the upper regions of the 

 atmosphere ; and Schuster has shown, by the application of Gauss' 

 method, that the seat of these variations is above the surface of the 

 earth. 



The negative charge in the earth's atmosphere will not increase 

 indefinitely in consequence of the stream of negatively electrified cor- 

 puscles coming into it from the sun, for as soon as it gets negatively 

 electrified it begins to repel negatively electrified corpuscles from the 

 ionised gas in the upper regions of the air, and a state of equilibrium 

 will be reached when the earth has such a negative charge that the 

 corpuscles driven by it from the upper regions of the atmosphere are 

 equal in number to those reaching the earth from the sun. Thus, on 

 this view, interplanetary space is thronged with corpuscular traffic, 

 rapidly moving corpuscles coming out from the sun while more slowly 

 moving ones stream into it. 



In the case of a planet which, like the moon, has no atmosphere, 



