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222 Wm. A. Norton on the Variations 



the mean daily temperature becomes lower; but the effect of 

 this cause is much less than appears at first sight, for, if less dew- 

 falls in any short space of time, at the lower temperature, the 

 heat given out will be less, and therefore the cooling will be 



greater. 



We have next to enquire into the absolute amount of dew de- 

 posited at any one season. The experiments of Professor Brock- 

 lesby, detailed in an article "upon the Influence of Color on 

 Dew," published in the No. of this Journal for September, 1848, 

 furnish data upon which we may base a calculation. The ex- 

 periments I shall use are a suite of eleven, made at intervals from 

 July 9th to August 4th. Strips of flannel, of the size of from 

 15 to 17 5 square inches, and of various colors, were placed upon 

 closely shorn turf, or upon a smooth board elevated six or seven 

 inches above the turf, and the amount of dew gained by them 

 ascertained by weighing them. The average amount deposited 

 upon the pieces of green flannel, in the eleven experiments, I find 

 to have been 28'1 grains for a surface of twelve square inches.^ 

 This is equivalent to a film of water of the thickness of T ^o °f 

 an inch. The average is very nearly the same for the other col- 

 ors. It is to be observed with respect to these experiments that 

 they were made at the season of the year when there is the least 

 dew, and that the nights do not appear to have been selected with 

 reference to the degree of humidity of the air, for the amount 

 deposited on the different nights varies from about 20 grs. to 60 

 grs., and the first ten days are included within a space of twenty 

 days. They were probably a fair average of the nights that oc- 

 cur at that season of the year. The radiating power of mold 

 is 92, and of vegetation over 100, that of lampblack being 100. 

 There is no reason to suppose that the radiating power of flannel 

 is materially greater. I shall therefore consider myself entitled 

 to assume that on clear nights in July the average amount of dew 

 is not less than t | ¥ of an inch. 



Connected with the question of the amount of dew is that of 

 the height in the atmosphere to which the abstraction of vapor 

 extends. We may obtain an estimate of the distance through 

 which the vapor falls, by observing how much the dew point falls j 



near the surface of the earth. At Philadelphia, where the hygvo- I 



metric observations were made at the height of about four teet 

 above the ground, this does not exceed 2°, on the average, in the 

 months of July, August, and September. Obtaining the requisite 

 data, and making the calculation, I find, that to furnish in *0l of 

 dew at the temperature of 70°, this reduction of the dew point 

 must extend no less than 736 feet ; and that this amount of dew is 

 equivalent to all the vapor in the air, when the air is saturated, 

 within forty-three feet of the earth's surface. In the average hy- 

 grometric state of the air (for both day and night) it is only half 



