42 Heat, Moisture, and Evaporation. 



2. In an atmosphere where the dew point is at 42*3 and 

 the temperature 70-5°, required the depth of water evaporated 

 per hour ? This will be the difference between 1 3*8 and 5'8, or 

 8 grains per minute. Now gfc is "0134 inches per hour, or 

 a little more than 3 tenths of an inch in 24 hours. 



3. A forcing house contains 4000 cubic feet of air, and it is 

 desired to know the quantity of water that would saturate it, 

 the temperature being 70°. 



Opposite the temperature 70° in the first column, the 

 moisture w T hich combines with a cubic foot of air at 70° is 

 7*8 grains, and 7*8 X4000 is 31200 grains. 



4. If the ventilation of a house be 300 cubic feet per minute, 

 and the dew-point qf the air admitted be 32° required the 

 surface of water that would maintain the dew-point of the air 

 in the house at 50 when its temperature is 70° ? 



When the dew-point is at 32° we find each foot of air con- 

 tains 2-3 grains, and at 50° it contains 4 grains, the difference 

 has therefore to be added by evaporation, but 4 — 2*3 is 1*7 

 grains, and 300 x 1*7 is 510 grains. The evaporation from 

 each foot at 70° is nearly 13*8 grains, and at 50° about the 

 mean between 6-8 and 7*8, or 7'3 grains, therefore 13*8 — 7'3 

 is 6'5 grains from each foot of surface ; and as we have found 

 that 510 grains will be required in that time, we have the 

 quantity by dividing 510 by 6'5, which is very nearly 80 feet 

 of surface. 



By this time the reader will have felt that an important in- 

 quiry is but slightly entered into, it requires a more complete 

 table, and experiments on the evaporation from moist earth, 

 leaves, &c. to render it more useful. It is not to artificial 

 atmospheres alone that the investigation applies, it may be 

 extended to the face of the globe, and may enable us to trace 

 the effects of cultivation, of stagnant waters in confined dis- 

 tricts, and the proper distribution of wood, cultivated land, 

 and water which will preserve a healthy element. Men, as 

 well as plants, feel the exhausting influence of dry air, or 

 perish under the effect of a cold and saturated atmosphere, and 

 perhaps a warm and saturated one is equally noxious. 



Those who are acquainted with the researches of Mr. 

 Dalton, on evaporation, will find that his experimental ana- 

 logy is abandoned, and the subject referred to those first 

 principles which must be involved in the question ; and I trust 

 I have done sufficient to show the basis of an accurate theory. 



Mr. Dalton's analogy gives nearly true results in low 

 temperatures, but in high ones it is very erroneous ; besides 

 not accounting for the well known depression of temperature 

 which must take place where the heat is not supplied from an 

 artificial source. 



