216 



Evaporation on the open Sea. 



[No. 159. 



Latitude N. 



Thermometer, 

 (Cent, grade.) 



Hygrometer. 



Quantity of water 

 evaporated per hour 

 in millimetres. 



o / 



39 10 



14.5 



o 



82 



0.13 



30 36 



20.0 



857 



0.14 



29 18 



20.0 



83.8 



0.16 



18 53 



21.2 



81.5 



0.20 



16 19 



22.5 



88 



0.13 



12 34 



24.0 



89 



0.13 



10 46 



25.4 



90 



0.12 



11 1 



25.0 



92 



0.09 



" It follows from these researches," says M. Von Humboldt, rt that 

 if the quantity of vapour which the air commonly contains in our 

 middle latitudes, amounts to about three-quarters of the quantity ne- 

 cessary for its 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 supposed it to be from the 

 elevation of the temperature." 



These inferences seem scarcely compatible with the actual indica- 

 tions of my instrument. But it must be observed, that besides being 

 imperfect as a hygroscope, De Luc's instrument takes no cognizance of 

 the important agency of the wind in promoting evaporation. So far 

 from diminishing, the exhalation from the surface of the sea would 

 appear to augment very rapidly as we approach the torrid zone : my 

 observations exhibiting a daily average of 0.398 in. from latitude 

 37° 15' S. to latitude 24° 25', and of 0.809 in. through the tropics. 



