47 



sont pouss^es si pen loin qu'elles laissent encore une vaste 

 champ a tons ceux qui voudront perfectionner cette matiere. 

 Mais ne s9ait-on pas que les arts les plus simples ont eu 

 leurs difFerens ages, et que ce seroit comme dtoufFer dans le 

 berceau les ddoouvertes qu'on peut faire dans la suite, que 

 de mdpriser toutes les premieres tentatives, sous pretexte 

 que ce ne sont encore que de foibles commencemens ? 



In the last section he has also considered the intensity of 

 light which has passed through a medium which is not of 

 the same density throughout. By geometrical reasoning he 

 arrives at the conclusion that the curve of intensity (the 

 gradulucique as he terms it) has this property ; its sub- 

 tangent multiplied by the density is equal to a constant. 

 Expressed in the language of the differential calculus this 

 gives rise to a differential equation similar to the one which 

 I obtained by a different method and gave last session in a 

 paper read before the Society, on the intensity of light 

 which has traversed a medium wherein the density is some 

 function of the distance traversed. Except in the considera- 

 tion of the intensity of light which has passed through the 

 atmosphere, Bouguer has made very little use of this highly 

 genera] theorem, for, says he, in most cases we do not know 

 what is the law of density. This may be so, but by assu- 

 ming the density to be some function of the distance, we 

 may deduce some interesting and valuable results. 



"On the Effects of Solar Radiation in Atmospheric 

 Vapour," by the Rev. Thomas Mackereth, F.R.A.S., 

 F.R.Met.Soc. 



Two facts are well known and understood, viz. that solar 

 radiation is the cause of terrestrial evaporation, and that as 

 the vapour so produced in the air condenses and spreads as 

 cloud, the effects of solar radiation upon the surface of the 

 earth are impeded. And it is also well known that invisible 

 vapour from the effects of solar radiation is constantly 



