8 Mr. Harvey on the Formation 
to infer that the opposite results, observed were produced by the 
substances on which they were respectively placed. A slight trace 
of moisture was perceptible on the zinc, but not the least degree 
on the copper and tin. » 
An example of the slowness with which polished tin permits 
moisture to be deposited on it, occurred when a concave mirror, 
formed of polished block tin, was employed as an /thrioscope, on 
the plan first suggested by Dr. Wollaston. The focus of the in- 
strument, at the time the experiment was performed, was 20 inches 
above the ground. The night was tranquil, and dew was copiously 
deposited on glass, a few minutes after it was presented to the 
chilling influence of the transparent sky. Atnine, vr. M. the ther- 
mometer in the focus of the Athrioscope indicated a temperature 
of 46°; the herbage being at the same time 44° ; and the air, seven 
feet above the ground, 494°. Observations, connected with some 
other phenomena, were made every half hour; but no trace of 
moisture was perceptible on the metallic surface, till two a. M., 
when it appeared slightly dimmed, although other substances had 
gained considerable increments of dew in the same time ; masses 
of wool, for example, having increased in weight from twelve grains 
to thirty. At the same moment, the focal thermometer indicated a 
temperature of 424°; that on the grass, 39°; and that elevated in 
the air, 45°. In five hours, therefore, the cold of the upper sky 
only underwent a change of 33°; whereas the grass lost by radia- 
tion in the same time 5°; and the eleyated stratum of air diminished 
its temperature 45°. From two o’clock to three, the thermometer 
remained stationary, but the moisture had sensibly increased on 
the surface of the Athrioscope, and increments amounting to se- 
veral grains, were likewise found on other substances ; a proof, 
that if the general temperature remains stationary, after the tempe- 
rature of a body is sufficiently lowered to permit the formation of 
dew on its surface, the farther deposition of moisture is not pre- 
vented. At four a.m. the whole metallic surface was covered with 
visible drops, the temperature, at the same moment (just before 
sunrise), indicating the maximum of cold, the focal thermometer 
