^"h 843 THE GENESEE FARMER. 



and at other times a mass of fine, cold particles gain greatly in size and coalesce into 

 large drops in falling through a stratum of saturated air. 



To measure the quantity of dew deposited each night, an instrument is used called a 

 drosometer. The most simple process consists in exposing to the open air bodies whose 

 exact weight is known, and then weighing them again after they are covered with dew. 

 According to Weels, locks of wool, weighing five decigrammes, are to be preferred, 

 which are to be divided into spherical masses of the diameter of about five centimeters. 



Alchemists used carefully to collect dew, which they regarded as an exudation from 

 the stars, and in which theyho]iei! to find gold. It is only recently that the true theory 

 of the formation of dew has been established ; although Aristotle's observations pointed 

 out the fact that most dew falls in "calm, clear nights. In the cool of evening, different 

 bodies part with the heat imbibed during the day, unequally ; some cooling faster than 

 others. The leaves of trees, grass and other plants radiate heat rapidly, and soon 

 become cold enough, to do what 1 Condense, like ice-water in a pitcher, aqueous vapor 

 in the atmosphere on the cold surface. White sand and clay are poor conductors of heat, 

 cool slowly, and while green herbage is covered with a heavy dew, naked earth has little 

 or none. At the South, and even in the climate of Washington, a heavy dew runs into 

 a raornino- shower ; a phenomenon which we never saw at the North. By greatly 

 cooHng the atmosphere during night, and lessening its capacity to hold water as vapor, 

 not only are vessicles formed which constitute the body of fog and mist, but a shower of 

 fine rain is not uncommon. 



The dew-point, which once attracted a g<^od deal of attention, is a very simple affair. 

 It is the point of temperature at which the precipitation of water takes place ; and this 

 point varies with the humidity of th(.' atmosphere. A comparatively damp atmosphere, 

 other things being equal, precipitates a p.)rtion of its vapor in dew, sooner than a dry 

 atmosphere. If the temperature "is below freezing, the dew is congealed into a white 

 frost, or a black one, according to the oiicumstances. 



We have been led into tliis train of th(>ugh by reading in a foreign journal an account 

 of some interesting researches made by an eminent French chemist, M. Barrel, showing 

 the amount of foreign bodies, and the good manures, brought to the earth in rain water 

 during a year. Had the water fallen in a remote rural district instead of the city of 

 Paris, the result would have been more satisfactory. M. Barrel states that although 

 the quantity of tlie following substances varied in different months, yet the monthly 

 average from July to December, inclusive, was as follows : 



StrB6TA2;CBP IN A CUHIC MBTER 01" KADf 'WATBB. 



Nitrogen, (^.BC (rrainmes 129 grains. 



Nitric acid, 19.09 do 394 do 



Ammonia, 8.61 do 85T do 



Chlorine, . 2.27 do 85 do 



Lime, 6.4.S do 100 do 



Magnesia, 2.12 do 32.T do 



Allowing water to fall only twenty-four inches in twelve months, and the above 

 figures indicate a gain per acre as follows : 



Nitrogen 4C^ pounds. 



Nitricacid, 103 do 



Ammonia, 19/^ <io 



Chloride, l'.^>^ do 



, Lime, 35 do 



Magnesia, 11 do 



One hundred and three pounds of nifi-ic acid, and nineteen and a half of ammonia, 

 give a very good dressing of the most valuable constituent of crops. Deep plowed and 

 finely-tilled earth absorbs most of the manures which fall in rain, snow, and dew, and 

 this i? one of the great secrets of successful agriculture. Prepare the land well before 

 the seed is planted, and Nature will reward you liberally for so doing. 



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