of Dew on Metallic Surfaces. I! 
instituted by Dr. Wells, to illustrate the principle in question, by 
bending a sheet of pasteboard into the form of the roof of a house, 
and placing it with its ridge uppermost, and ends open, over a mass 
of ten grains of wool laid on the grass ; and at the same time placing 
another equal mass on the herbage, fully exposed to the sky, the 
former gained, during the night, an increment of only two grains, 
whereas the latter gained sixteen. In this experiment, the two 
masses were placed under the same circumstances, so far as con- 
tact with the grass was coucerned; but in the case relative to the 
plates of tin, one was not only in contact with the herbage, but also 
surrounded by it; whereas the other was completely detached. 
The gradual manner in which dew is deposited on the metallic 
side of gilded glass was pleasingly exemplified on another occasion. 
The parallelogram of glass was six inches by four, as represented 
in fig. 14. It was first exposed to the atmosphere with its metallic 
side uppermost, at half-past six, rp. m., being about three quarters 
of an hour after sunset. The atmosphere was clear, and highly 
charged with moisture ; and dew had formed on glass in a shady 
place, three quarters of an hour before the departure of the solar 
orb. A mild and gentle breeze prevailed also at the same time. No 
perceptible change took place in the metallic surface until eight, 
when minute particles of dew were visible at A, the leeward end. 
From the last-mentioned hour to ten, the moisture gradually in- 
creased from A to the middle part of the surface ; and distinct drops 
were likewise deposited at D, B, E,C. As the particles increased 
in size round the three edges, other minute drops were successively 
deposited, more distant from them ; and it was observed, that they 
accumulated with most rapidity at the leeward sides A and C. At 
eleyen, Pp. M., when the sketch represented in the figure was made, 
an oval portion of the metallic surface was found entirely free from 
moisture. The same figure was also perfectly visible at midnight, 
when the drops at A had increased to at least an eighth of an inch 
in diametey; those at C being rather less, The particles at the 
corners D and E also preserved their superiority in size above those 
at B, 
