101 



at 20° of height, 8* 5° of the cyanometer 



30 12 



45 15-5 



50 18*3 



60 21 



between 70&90 



224 



This decrement has a great analogy with that 

 which was observed at Geneva on the 11th of 

 April, 1790, and to which Mr. Prevost * endea- 

 voured to apply calculation. We know, that 

 both follow nearly an arithmetical progression, 

 but that at sea there are great irregularities 

 before twenty degrees of height. This zone 

 near the horizon exhibits tints extremely pale 



on account of the vapor that lies on the surface 

 of the water, and through which the blue rays 

 are transmitted to us. It is for the same reason, 

 that near the coasts, at an equal distance from 

 the zenith, the vault of the sky appears of a 

 deeper color on the land side than on that to- 

 ward the sea. 



The quantity of vapor, which modifies the 

 shades of the atmosphere by reflecting white 

 light, is changing from morning to evening ; and 

 the cyanometer, observed at the zenith, or near 

 this point, indicates with sufficient precision the 

 variations that correspond to the different hours 

 of the day. 



Journal de Physique, t. lvii, p. 372. 



