214 Evaporation on the open Sea. [_No. 159. 



surface in a given time, is an important acquisition, not only in meteo- 

 rology, but in agriculture and in the various arts and manufactures. 

 The rate of exhalation from the surface of the ground is scarcely of 

 less consequence than the fall of rain, and a knowledge of it might 

 often direct the farmer advantageously in his operations. On the 

 rapid dispersion of moisture depends the efficacy of drying houses, 

 which are often constructed most unskilfully, or on very mistaken 

 principles." 



The instrument which I have found to answer extremely well, 

 consists of a glass tube the bore of which must be equable, and may 

 vary from one or two-tenths of an inch in diameter to a much larger size, 

 according to the pleasure of the constructor. If the bore be not quite 

 equable, its varying capacity must be ascertained and allowed for on 

 the scale to which it is to be attached. One end of this tube, after 

 being ground quite flat and smooth, is to be closed with a porous sub- 

 stance, which space permits the free transudation of water, but yet not 

 so freely as to accumulate in drops or to fall. I find that common cedar 

 wood possesses the requisite quality, and forms a plug which swells 

 so as to become water-tight ; and by its porous structure permits the 

 fluid to permeate as rapidly as the atmosphere removes it from the 

 exposed surface. The tube thus prepared, and filled with distilled 

 water, is to be attached to a scale divided into fiftieths or hundredths 

 of an inch, upon which as the evaporation proceeds and the column of 

 fluid descends, the daily amount of evaporation may be conveniently 

 observed. No other precaution seems necessary in using this Atmometer 

 than to supply it with very pure rain or distilled water ; for any saline 

 matter it might contain would be deposited upon the evaporating 

 surface, and would interfere very materially with the result. To 

 prevent error from this source, the entire tube should be very frequent- 

 ly (say every time that it is filled,) washed in a quantity of clean 

 water to remove accidental impurities ; and the cedar plug occasion- 

 ally renewed. 



The following observations made with this instrument on board of the 

 ship " Southampton," on her recent voyage from England to Calcutta, 

 showing the rate of evaporation on the open sea in tropical latitudes, 

 may not be altogether uninteresting to such as are curious in oceanic 

 meteorology. The instrument was suspended in a shaded part of the 

 vessel, exposed freely to the action of the wind. 



