the Magnetic Needle and on Aurora Boreales. 287 



unequal distribution of temperature in the strata of the atmo- 

 sphere. It is well known that, in a body of any nature what- 

 soever heated at one of its extremities and cooled at the other, 

 the positive electricity proceeds from the hot part to the cold, 

 and the negative electricity in the contrary direction ; it thence 

 results that the lower extremity of an atmospheric column is 

 constantly negative and the upper one constantly positive. 

 This difference of opposite electric conditions must be so much 

 the greater the more considerable is the difference of tempe- 

 rature ; consequently more marked in our latitudes in summer 

 than in winter, more striking in general in the equatorial than 

 in the polar regions. It must be observed that the negative 

 state of the lower portions of the atmospheric columns must 

 be communicated to the surface of the earth on which they 

 repose, whilst the positive state of the upper portions is dif- 

 fused more or less, from above downwards, through nearly 

 the whole of each of the columns, according to the facilities 

 offered by the greater or less degree of humidity of the air to 

 the propagation of the electricity. An atmospheric column 

 therefore resembles a high-pressure battery on account of the 

 imperfect conductibility of the elements of which it is com- 

 posed, — a battery the negative pole of which is in constant and 

 direct communication with the terrestrial globe, discharges 

 itself upon the globe, whilst it becomes itself charged with the 

 electricity of its positive pole, which is distributed over it 

 with an intensity decreasing 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 

 electricity 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 

 diflferent circumstances. In other words, having reached a 

 certain limit of tension which varies with the state of the atmo- 

 sphere 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 neutralization 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 ; 

 sometimes it is simply the humidity of the air, and better still 

 the rain or snow, which re-establish the electrical equili- 

 brium between the earth and the atmosphere; in some cases 

 waterspouts manifest in an energetic form the mutual action 



