10 Mr. Harvey on the Formation 
perature depressed below that of the stratum of air reposing on it, 
during the night; but the former must have been considerably 
colder than the column of air hovering above it. The cooling 
power of the grass surrounding the plate MN, and on which it 
also rested, must have necessarily extended its influence to the me- 
tal; and by lowering its temperature considerably, have occasioned 
the copious deposition observed. The upper plate not being in 
contact with the grass, permitted the air to pass freely on each 
side of it; and being itself a bad radiator, attained no condition 
during the night favourable to the deposition of dew. With re- 
spect to the formation of dew being less abundant on the plates 
resting on the short herbage, than on that surrounded with the long 
grass, it may, in one point of view, be regarded as a consequence 
of the ‘curious fact observed by Mr. Six, that the temperature of 
short grass is always greater than that of long grass. The state 
of the herbage has always a considerable influence on the quantity 
of dew deposited, and the greater the body it presents, the more 
abundant it is likely will be the formation. That the quuntity of 
herbage has a considerable effect, may be inferred from the expe- 
riment, that when one mass of wool was placed on short herbage, 
and another of equal size and weight on the summit of a mass of 
recently cut grass, fifty inches above the ground, the moisture 
gained by the former during the night, was only fifteen grains, 
whereas the increment to the latter was twenty-three. 
In consequence of the plate O P having had its surface exposed 
to the entire canopy of the sky, but the view from the plate M N 
being confined to a comparatively small circular space, in the zenith 
of observation, it might be inferred from a principle adopted by 
Dr. Wells*, that the former would have gained more moisture than 
the latter. But the maxim of this ingenious philosopher is evidently 
limited to the consideration, that the bodies are in other respects 
similarly circumstanced. For instance, in one of the experiments 
* Essay on Dew, page 14, second edition. The principle here alluded to ig 
the following; * Whatever diminishes the view of the sky, as seen from the 
exposed body, accasions the quantity of dew which is formed upon it, to be fega 
than would have occurred if the exposure to the sky had heen complete,” 
