EXTRACTS 



FKOM 



THE METEOROLOGICAL CORRESPONDENCE OP THE INSTl 



TUTION, WITH REMARKS BY THE SECRETARY, 



PROP. JOSEPH HENRY. 



The object of the publication of the following correspondence and remarks, is 

 to record isolated facts of interest, and to answer inquiries which are frequently 

 made by meteorological observers. 



From W. C. Dennis, Key West, Florida. 



November 10, 1856. 



I have been mindful of your remarks as to my opportunity to gather facts rela- 

 iiive to surface evaporation. 1 have given a great deal of attention to the sub- 

 ject, but my principal object has been to learn the conditions wliich attend an 

 increase of evaporation, for the purpose of applying them to practical utility in 

 the manufacture of salt. There is a difficulty in the way of getting the yearly 

 amount of evaporation from an ordinary surface, as I am convinced that evapo- 

 ration goes on much slower from sea water as it approaches the state of satura- 

 tion ; consequently in ordtr to get useful data to determine the yearly surface 

 evaporation, the water employed should either be fresh, or kept at about the 

 strength of sea water. So much difference is there in amount of evaporation from 

 ordinary sea water and brine, nearly at. saturation, that while the former evapo- 

 rates at say 0.05 inches per day, the latter remains nearly stationary. 



This fact I noticed frequently last summer, when the air was damp and heavy 

 with dew at night. In the morning after a heavy dew, I often found brine that 

 was very strong, weaker than it was the night before. Tliis I suppose may be 

 accounted for by the strong affinity water has for salt. 



The hygrometrical condition of the air makes a great diffijrence in the amount 

 of surface evaporation. The past year has been very damp, but the rains have 

 not been remarkably heavy, yet everything goes to show that there has not been 

 much over half*the evaporation that there was in 185 L and 1854, years in which 

 there were heavy rains. Through this summer I did not observe ihe evapora- 

 tion of quite 0, 10 inch in any twenty-four hours. In 1854, in the mont h of l^lwj, 

 I noted the evaporation in one day (twenty-four hours) of full 0.30 of an inch. 

 From this it can be seen that in order to ascertain tlie average yearly evapoj'ation 

 from a given surface of the ocean, it will be necessary to make accurate daili/ 

 observations for a number of years, with ordniary sea water kept at the same 

 degree of density. 



I have found that evaporation goes on faster, when the weather is clear and 

 cahn, from the surface of very shallow water than from deeper water ; but when 

 the wind blows sufficiently strong to form a ripple or wave, evaporation goes on 

 faster from water that is six inches deep than from that which is not more than 

 one or two inches. Some other of my observations lead me to think that very 

 little heat is absorbed by perfectly dear water in the passage through it of th(; 

 direct rays of the sun, and that the maximum heat to which it is possible for the 

 sun's rays to bring water depends on the color of the bottom that reflects back those 

 .rays, as well as on the shallowness of the water. 



