Mr Harvey on the Formation of' Dew. 71 



parcels attached to its vertical surfaces, equal increments of 

 dew, amounting to five grains. Hence, if the quantities of 

 moisture deposited be regarded as measures of the radiating 

 powers of the respective surfaces, the former exceeds the lat- 

 ter, in the ratio of three to one ; — a difference arising entirely 

 from position.* All the exposed parts of the cube were co- 

 vered with particles of dew, those on the top being the great- 

 est; and it was most interesting to remark the gradual diminu- 

 tion of magnitude in the particles, from the upper part of the 

 vertical faces to the lower. 



Another evening was selected for repeating the same expe- 

 riment, when a gentle breeze was blowing from the east, one 

 surface of the cube being placed directly exposed to its influ- 

 ence. During the night the parcel of wool on the top gained 

 an increment of moisture, amounting to 10 grains, notwithstand- 

 ing the breeze swept freely over its surface. The wool, on the 

 eastern side, being exposed to the full influence of the wind, 

 gained only a grain and a half of dew ; but that on the west- 

 ern side, from its being comparatively sheltered, obtained an 

 addition of five grains and a half. -J- The masses of wool on 

 the northern and southern surfaces of the cube being equally 

 exposed to the lateral action of the wind, gained equal incre- 

 ments of dew, amounting to two grains ; an effect due to the 

 equally diminished radiation of the wool, by the equal action 

 of the wind on the two surfaces. 



This experiment shows clearly the influence of wind in 

 checking the formation of dew. In the former experiment, 

 when the air was perfectly tranquil, all the parcels of wool at- 



* Dr Wells very properly remarks, p. 59, 2d edition of his Essay, that 

 " the same degree of cold in the precipitating body, may he attended with 

 much, with little, or with no dew, according to the existing state of the air 

 in regard to moisture ; and from which it appears, that the relation be- 

 tween the temperature of the body on which the dew is deposited, and the 

 quantity deposited at different periods, must be variable. But the quanti- 

 ties deposited at the same time, on surfaces of the same kind, differently 

 circumstanced with respect to the aspect of the sky, may, without impro- 

 priety, be regarded as comparative measures of the radiating powers of 

 those surfaces. 



' + This result is in conformity with a remark of Dr Wells ; " A body 

 placed at the leeward end of the raised boards generally acquired more dew 

 than a similar body at the windward extremity." 



