88 



fectly serene weather, the thermometer being at 

 twenty-four degrees. On the banks of the lake 

 of Geneva*, the mean humidity of the same 

 month is only eighty degrees, the average heat 

 being nineteen degrees. Now, on reducing these 

 hygrometrical observations to a uniform tem- 

 perature, we find, that the real humidity, in the 

 equinoctial basin of the Atlantic Ocean, is to the 

 humidity of the months of summer, at Geneva, 

 in the ratio of twelve to seven. This enormous 

 humidity of the atmosphere explains in a great 

 measure the strength of vegetation, which we 

 admire on the coasts of South America, where 

 no rain falls for several years. 



As the quantity of vapors changes, not with 

 the elasticity of the air, but with the tempera- 

 ture, we may comp&re 3 either the absolute quan- 

 tities of vapour contained in the atmosphere in 

 two places, or the proportion of their quantities 

 to those necessary to the complete saturation of 

 the air in different climates. We know by very 

 accurate experiments the capacities of saturation 

 of the air at different degrees of the thermome- 

 ter ; but the relations which exist between the 

 progressive lengthening of a hygroscopical body, 



tioned. The numbers always mark the apparent humidity, if 

 the contrary be not expressly stated. 



* Under the temperate zone, on the continent, the extremes 

 were commonly in summer sixty-seven and eighty-eight de- 

 grees, the temperature of the air being from twenty-six to 

 eighteen centesimal degrees. 



