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WRITINGS OF JOSEPH HENRY. [1855- 



much depend, as we shall presently see, upon the motion 

 or stillness of the air ; and indeed we think that in all cases, 

 in order to obtain comparable results, the bulb should be 

 fanned, so as in every instance to give the same amount of 

 agitation to the surrounding medium. This will be evident 

 from what we have said of the slow diffusion of vapor of fee- 

 ble tension in the atmosphere. A local atmosphere of vapor 

 is soon formed around the bulb, which very much impedes 

 evaporation and consequently the reduction of temperature. 

 Evaporation of water. — Water is constantly evaporated 

 from the surface of the ocean ; the amount however dimin- 

 ishes as we proceed from the equator towards the poles. It 

 is also exhaling from the surface of the earth, but in less 

 quantities. The daily, monthly, and yearly amount of 

 evaporation from a given surface of water and different 

 kinds of earth is one of the most important data in reference 

 to engineering and agriculture which can be furnished, and 

 we would commend the research in reference to it to the 

 special attention of any person who can command the time 

 and desires an opportunity of advancing our knowledge of 

 the operations of nature. A series of experiments on the 

 evaporation from water may be made by carefully noting 

 the quantity which disappears daily from a surface of a 

 square foot freely exposed to air and sunshine. The depth 

 of the box, which may be of tin encased in wood, should be 

 6 inches, and the amount of water measured by a screw, the 

 lower end of which tapers to a point, and on the upper end a 

 divided circle is placed, so marked that the tenth part of the 

 width of the screw or the one-thousandth of an inch may be 

 estimated. Care should be taken to guard this surface from 

 rain, and in high wind to estimate the amount of water 

 which may be blown out ; the latter may be approximately 

 found by surrounding the evaporating vessel with a border 

 of gray paper, on which each drop of escaping water will make 

 a stain; the number and size of these spots being known, the 

 amount of water blown out may be estimated from the re- 

 sult of previous experiments in which the known quantity 

 of the fluid has been sprinkled over the same surface. It is 

 well, in order to make certain corrections, to observe the 



