Of M. D E L U C. 57 



in fome refpects, fufficiently jufl, may bear perhaps another in- 

 terpretation. That the precipitation of water limply evapo- 

 rated cannot be produced except by cooling, is an expreffion 

 which, though not contrary to my idea, does not contain pre- 

 cifely my opinion. Water is not precipitated from the atmo- 

 fphere in time of rain by the cooling of the air, in the ordinary 

 fenfe of that expreffion, that is, by the abftraclion of a certain 

 quantity of its heat, which is then communicated to fome other 

 body ; but it is becaufe the air is not able to contain fo great a 

 quantity of water, in proportion to its heat, when it is in a 

 lower temperature. The compound mafs of air, which in the 

 formation of rain precipitates water, is not cooled, fo far as I 

 know, below the mean temperature of the different maffes of 

 unequally heated air which have concurred to form it ; but this 

 mean temperature does not fuffice to evaporate all the water 

 which had been contained in thefe mafles feparately. This, 

 however, is only by the by ; and I now proceed to the material 

 part of his conclufion, where he thus continues : 



" Quant a l'hypothefe que je viens d'examiner, elle etoit 

 " tres naturelle dans l'etat des faits connus ; puifqu'il n'etoit 

 " pas poffible de concevoir d'aucune autre maniere, que des 

 " melanges d'airs a differentes temperatures, puflent produire 

 " des pluies abondantes : et la vraifemblance de cette hypothefe 

 " ne pouvoit etre detruite, que par un genre d'experiences et 

 " d'obfervations, qui ne fait que de naitre en phyfique avec 

 " l'hygrometre." 



In anfwer to this, I have but to obferve, that, had M. de 

 Luc contented himfelf with faying, as he here has done, that 

 the probability of this hypothefis could not be overturned, but 

 by a fort of phyfical experiments and obfervations which have 

 jufl taken their birth with the hygrometer, I mould have waited 

 patiently until thofe experiments and obfervations had arrived 

 at that maturity which might enable them to confute my 

 theory. But M. de Luc has undertaken to confute it upon 



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