of Dew on Meiailic Surfaces. 9 
being at 40°; that on the grass, 37°; and that elevated seven feet 
above the ground, 414°. It is worthy of remark, that two plain 
sheets of polished tin, placed horizontally on the herbage, had not 
the slightest trace of moisture on them. 
On another night, however, when there was every prospect of an 
abundant deposition of dew, the influence of the grass in promoting 
its formation on metals, was clearly shown. At nine p. m. two plates 
of polished tin, one fourteen inches by ten, and the other six by 
two, were laid on very short grass. Another plate of the same di- 
mensions as the former was placed gently on the long grass. Its 
weight necessarily compressed the herbage on which it rested, so 
that the polished surface was surrounded on all sides by grass, 
reaching twelve inches above it. In fig. 15 the long grass is repre- 
sented on two opposite sides of the tin M N, together with the com- 
pressed herbage below it. At eighteen inches above the ground, 
or two inches above the average height of the grass, a similar 
plate, O P, was placed on slender props. The temperature of the 
grass at the moment the plates were exposed was 60°, and of the 
air 65°; being a difference of 5° in the small space of three feet. 
At five the next morning, a great quantity of dew was formed on 
the grass. A register thermometer on the short herbage, indicated 
the maximum cold to have been 52°, and of the air, at the elevation 
before mentioned, 60°. ‘The difference between these maximum 
depressions of temperature was, therefore, by no means considera- 
ble; and the copious deposition of dew observed was to be regarded 
rather as the result of the abundance of moisture in the atmo- 
sphere, than as a consequence of great difference of temperature. 
The metals presented the following particulars for observation. 
The plates resting on the short herbage had a few scattered patches 
of dew on their upper surfaces, but nothing like a regular and uni- 
form deposition, The plate M N, surrounded by the long grass, 
had its superior surface completely covered with minute but distinct 
particles of moisture ; but the plate O P, elevated above the grass, 
was perfectly dry. This difference in the results must be regarded 
as arising from the different conditions, under which the plates 
were situated, Tee latter surface, it will appear, had not its tem- 
