Diurnal Variations of the Magnetic Needle. U9 



creasing with the distance from this pole; this explains why the 

 positive electricity increases with the height of the atmosphere. 



The causes which determine the accumulation of negative electri- 

 city at the surface of the earth, and of positive electricity in the 

 upper regions of the atmosphere, act in a continuous manner : 

 there should thence result an unlimited tension of the two opposite 

 electric states, if, having attained a certain degree of energy, they 

 did not neutralize each other by the aid of different circumstances. 

 In other words, having reached a certain limit of tension which 

 varies with the state of the atmosphere and the surface of the earth, 

 the two electricities cannot go beyond it, and unite or neutralize 

 each other as regards the excess over that limit. This neutraliza- 

 tion is effected in two ways, in a normal or constant manner, and in 

 an irregular and accidental manner. 



This second mode is exhibited under a variety of forms; some- 

 times it is simply the humidity of the air, and better still, the rain 

 or snow, which re-establish the electrical equilibrium between the 

 earth and the atmosphere ; in some cases waterspouts manifest in an 

 energetic form the mutual action of the two electricities, which tend 

 to unite. Sometimes the winds, by mixing the air in contact with the 

 surface of the earth, and like it negative, with the positive air of 

 the more elevated regions, give rise to sheet-lightning, or to storms, 

 when there is at the same time a formation of clouds and condensa- 

 tion of aqueous vapours, owing to the humidity and different tempera- 

 ture of the strata of air which become mixed. The attraction of 

 clouds by mountains, the luminous phenomena exhibited at the ex- 

 tremity of elevated points, are likewise due to the same cause. But 

 I will not stop to discuss further all these natural and intelligible 

 consequences of the theory which I expound. I shall confine my- 

 self to one single remark, which is, that we must bear in mind, that 

 in observations of atmospheric electricity, the intensity of the electric 

 signs perceived is not always a proof of the intensity of the electri- 

 city itself; for the humidity of the atmosphere, by favouring the 

 propagation of the electricity of the upper strata, may give rise, as 

 is frequently seen in winter, to very powerful electrical manifestations 

 even when the cause producing them is not very powerful. The 

 contrary is frequently seen in summer. 



I now pass to the regular and normal mode of neutralization of 

 the two electricities. I had already suspected the existence of this 

 mode in my notice of 1836; but I did not announce it positively, 

 because there was then wanting a fact, which science now possesses, 

 viz., the perfect conductibility of the terrestrial globe, with which 

 the employment of the electric telegraph has made us acquainted. 



To make it understood how I conceive this mode of neutralization, 

 I divide the atmosphere into annular strata parallel with the equa- 

 tor ; the positive electricity accumulated at the external portion of 

 this layer cannot exceed a certain degree of tension without travel'- 



