:g96 . M. De la Rive on the Formation of' Hail. 



column must be very considerable, since it is admitted that the 

 temperature of the atmosphere at its upper limit is at least 

 — 50"^ Cent. This difference ought also to be greater in 

 summer than in winter, and in hot than in cold countries, 

 since tlie temperature of the base of the column of air is deter- 

 mined by that of the soil upon which it reposes, whilst the tem- 

 perature of its summit, being that of the limit of the atmosphere, 

 is every where and at all times the same. This difference of 

 temperature, which extends itself uniformly between all the points 

 of the vertical mass of air, is necessarily the result of a continual 

 propagation of heat from below upwards, and should consequent- 

 ly be accompanied by a development of electricity, the intensity 

 of which should be increased in proportion as it ascends; that is 

 to say, in proportion as the difference of temperature becomes 

 greater. Now this is precisely what we learn from direct obser- 

 vation; we in truth find that the atmosphere, when it is calm 

 and serene, is charged with a positive electricity, whose intensity 

 is continually greater as we ascend. As to the negative electri- 

 city, which should accumulate at the base of the column, it is 

 absorbed by the earth ; for many observations of various kinds, 

 and amongst others those of De Saussure and of Volta, demon- 

 fctrate that the earth is endowed with a negative electricity. 



Taking, then, this view of the matter, which reposes solely 

 upon experiment, and perfectly accords with what we discover in 

 a body heated at one of its extremities, the permanent electrical 

 condition of the atmosphere will essentially depend on the man- 

 ner in which the heat is distributed, and propagates itself through- 

 out the air, and not upon evaporation, vegetation, or any other cause 

 variable in intensity, and unequally distributed, to which the at- 

 mospheric electricity has been erroneously, as we think, attributed. 

 It would be easy to demonstrate that this explanation can account 

 with the greatest accuracy for the variations which atmospheric 

 electricity undergoes ; and in particular that it is not in opposi- 

 tion to the fact observed by De Saussure, and after him by 

 other naturalists, that this electricity is stronger in winter than 

 in summer. In truth, the greater intensity of atmospheric elec- 

 tricity during the winter is owing to this circumstance, that the 

 electroscope, by means of which we perceive it, is at this period 

 of the year put into communication with a larger portion of the 



