Heat, Moisture, and Evaporation. 



41 



perature. The third column shows the quantity, in grains, that 

 would be evaporated in one minute, if the air had no moisture 

 in it ; and the fourth column shows the temperature of the air 

 corresponding to this degree of evaporation, when the evapor- 

 ating fluid is not supplied by extraneous heat. 







Quantity evaporated 

 in still air in grains 

 per minute, from 

 a surface of one 

 square foot. 



Temperature of the at-! 



Temperature of 

 evaporating 



Weight of vapour 

 in a cubic foot 



mosphere in which' 

 the evaporation takes 



surface. 



of saturated air. 



place. 



32° 



2'3 grains. 



4*6 grains. 



33-8° 



35 



2-5 



5-0 



37-0 



40 



2-9 



5-8 



42-3 



45 



3-4 



6-8 



47-7 



40 



4-0 



8-0 



53-2 



55 



4-6 



9-2 



58-7 



60 



5-7 



11-4 



64-5 



65 



6'9 



13:8 



70-5 



70 



7.8 



15-6 



76-2 



15 



9-2 



18-4 



82-3 



80 



10-7 



21-4 



88-5 



85 



123 



24-6 



94-8 



90 



14-1 



28-2 



101-2 



95 



16-9 



33-8 



108-4 



100 



19'0 



38-0 



115 



105 



21-2 



42-4 



122 



110 



24-6 



49-2 



130 



115 



28-0 



56-0 



137 



120 



32-5 



65-0 



146 



125 



37-4 



74-8 



154 



130 



42-3 



84-6 



163 



135 



48-7 



97*4 



174 



140 



54-9 . 



109*8 



184 



A few examples will show the application of this table. 



1. If the temperature of the air be 64*5°, then opposite 

 64 - 5 in the fourth column we find 11-4 grains for the evapo- 

 ration in a minute when the air is dry ; but if it has been 

 ascertained that the dew point is at 37°, look opposite 37° in 

 the fourth column, and the evaporation corresponding to that 

 temperature is 5 grains. Take the difference between these, 

 that is, 11'4 — 5—6-4 grains per minute will be the evapo- 

 ration from a superficial foot when the temperature is 64^ 

 and the dew point 37°. 



If the grains evaporated in a minute be divided by 600, the 

 result will very nearly express the inches in depth evaporated 

 in an hour, in a still atmosphere. 



