70 Mr Harvey on the Formation of Deie. 



ground, were found to be each 51°; and it is remarkable, that no 

 alteration of temperature took place during the night, since a 

 register thermometer left on the tower, and another placed at 

 the above mentioned elevation in the meadow, indicated that 

 the quicksilver had not in either case been below 5] °. At the 

 time the first observations of the temperature were made, that 

 of the grass indicated 49^°, at which time there was laid on it 

 equal plates of glass and tin, and on them equal masses of wool, 

 *(12 grains each,) exposing equal radiating surfaces to the 

 heavens. On the summit of the tower, at the same time, simi- 

 lar parcels of wool were placed, similarly circumstanced. At 

 half- past six the next morning the masses of wool in the meadow 

 had gained equal increments of moisture amounting to 14 grains, 

 and those on the tower equal increments of 7g grains. Hence 

 it appears, that during the whole night the temperature of the 

 air, at the respective elevations of three and one hundred and 

 ten feet, remained stationary, and that the increments of mois- 

 ture obtained in the meadow, by equal masses of the same 

 substance, in contact with bodies possessing different radiat- 

 ing powers, were the same ; as likewise, equal but smaller in- 

 crements, by equal masses similarly placed on the summit of 

 the tower. In the whole course of my experiments on this 

 very interesting subject, I never before met with so many re- 

 markable states of equality as appeared during this night. 



Gersten remarks,* that an horizontal surface is more abun- 

 dantly dewed, than one perpendicular to the ground, a pheno- 

 menon arising from the latter radiating less copiously than the 

 former. To confirm experimentally the remark of Gersten, 

 an evening was selected, distinguished by its beautiful sereni- 

 ty, and for the clear and perfect transparency of the sky. A 

 six inch hollow cube of block-tin was placed in my meadow, 

 two inches above the herbage, with its vertical faces equally 

 exposed to the cloudless horizon. To these faces, and to the 

 upper surface of the cube, equal parcels of wool were attach- 

 ed, with equal radiating surfaces. The air was so perfectly 

 tranquil, that the flame of a candle remained entirely undis- 

 turbed. At five the next morning the wool on the upper sur- 

 face of the cube had gained 15 grains of moisture ; and the 



* Wells on Dew, p. 197, 2d edition. 



