Heat, Moisture, and Evaporation. 



39 



ration in our climate will then be equal to about one-fifth of an 

 inch in the tube in twelve hours, this quantity might be made 

 the unit on the scale; and the whole would be evaporated in 

 fifty hours in the mean state of the air. 



Place the tube in a perpendicular position, and fill it to zero ; 

 then place it with the tube horizontally as in the figure. When 

 it is desired to know the quantity, which has been evaporated, 

 invert the remaining portion from the cup to the tube, and 

 the place of its surface in the tube will indicate the quantity 

 evaporated. The lower part of the cup ought to contain as 

 much water as fills the tube to zero. 



This instrument will faithfully indicate the power with which 

 the air is abstracting moisture from plants ; it is a little trouble- 

 some in use, but not more so than the hygrometers, which 

 indicate with equal accuracy. 



The ordinary state of the atmosphere,with respect to moisture 

 in this country, is extremely variable ; but the mean result of 

 many observations, of both the thermometer and the dew point, 

 shows that the temperature of deposition and the actual temper- 

 ature, follow each other in a regular manner. The difference 

 between the actual temperature and that of the dew point is 

 least in January, and gradually increases till June, when it 

 again declines to its winter state. Mr. Daniell has observed 

 these phenomena with much attention, and has given the results 

 for three years. The mean of these observations is shown in the 

 small tablet [Jig. 7-), where the upper shaded line shows the 









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