-1859] WRITINGS OF JOSEPH HENRY. 239 



of 60° : in still air the evaporation was 21 grains in a minute ; 

 in a gentle wind, 27; and in a strong wind, 3-3. From all 

 the experiments he deduced the important result that the 

 amount of evaporation in all cases is proportional to the 

 difference of the elastic force of the temperature of evapora- 

 tion and that of the dew-point or the vapor actually in the 

 air. 



The empirical rule deduced from his table of results will 

 serve approximately to calculate the amount of evaporation 

 under the different conditions of temperature, dryness, &c., 

 of the air, the temperature of the evaporating surface, and 

 that of the dew-point being known. For still air multiply 

 the difference of the tension of vapor due to the temperature 

 of the evaporating surface, and of the vapor in the atmos- 

 phere, by 4, and this will express in grains the weight 

 of the vapor given off from a circular surface of water of 6 

 inches in diameter in one minute of time. If a gentle wind 

 be blowing multiply the same difference by 5, and if a high 

 wind exists during the experiment multiply the same differ- 

 ence by 6. If for example the temperature of the evapo- 

 rating surface be at the boiling point, and the temperature 

 of the dew-point be 60°, we shall have 30 inches, the tension 

 of the evaporating surface, and 0"5 for that of the tension of 

 the vapor in the atmosphere at the time, the difference will 

 be 29'5, which multiplied by 4 gives 118 grains. Again, if 

 the temperature of the evaporating surface be 90, and that 

 of the dew-point 70, then we shall have 1-4 — 07 = 07. If 

 we suppose a gentle wind blowing at the time this must be 

 multiplied by 5, and we shall have 07 X 5 = 3*5 grains as 

 the amount of evaporation per minute from a circle of 6 

 inches in diameter. 



The formula of Dalton, in the absence of other data, 

 may be considered a valuable approximation ; still results 

 derived from direct observations in different parts of the 

 earth, as we have said before, are desiderata of great value. 



Physical effects of vapor in the atmosphere. — Before consider- 

 ing the more important meteorological changes produced in 

 the general condition of the atmosphere by the vapor which 



