92 



i_ 



It follows from these researches, that, if the 

 quantity of vapor, which the air commonly con- 

 tains in our middle latitudes, amounts to about 

 three quarters of the quantity necessary for it's 

 saturation, in the torrid zone this quantity is 

 raised to nine-tenths. The exact ratio is from 

 0*78 to 0*88. It is this great humidity of the 

 air, under the tropics, which is the cause that 

 the evaporation is less than we should have sup- 

 posed it to be from the elevation of the tem- 

 perature. 



I was often surprised, during this passage, and 

 at a later period in the vast basin of the Pacific 

 Ocean, at not seeing the hygrometer make nearer 

 approaches to the point of extreme humidity. 

 This instrument has been sometimes, far from 

 the coasts, at eighty-three degrees ; and gene- 

 rally in the equinoctial zone, it kept between 

 ninety and ninety-two degrees. According to 

 the meteorological tables, published by Messrs. 

 Langsdorff and Horner, we see, that in Kru- 

 sen stern's voyage, as well as in that of La Pe- 

 rouse, the apparent humidity * was found to be 



* Me*m. de 1'Acad. de Petersbourg, t. i, p. 454. I have 

 corrected the indications of Deluc's hygrometer, which was 

 used by the Russian navigators. In their instrument the 

 70th degree corresponded to the point of extreme humidity. 

 Lamanon's hygrometers were well verified, since they indi- 

 cated 100 or 101 degrees in a thick fog. Voyage de La 

 Perouse, t. iv. p, 281. 



