84 Means of making Ice by the Natives of Bengal. (Fes. 
only prevents the formation of more ice, but dissolves what may have 
been formed. On such occasions a mist is seen hovering over the 
ice-beds, from the moisture upon them, and the quantity of humidity 
contained in the wind. A mist in like manner forms over deep tanks 
during favourable nights for making ice. 
Another important circumstance in the production of ice is the degree 
of wind. When it approaches a breeze, no ice is formed. This is ex- 
plained by such rapid currents of air indicating a considerable difference 
of temperature between the situation from whence it passes, thus re- 
moving the cold air before any accumulation has taken place in the 
ice-beds. It is for these reasons that the thickest ice is expected when 
during the day a breeze has blown from the N. N. W. which thoroughly 
dries the ground, and towards evening and during the night diminishes 
to gentle airs, which steadily proceed from the same quarter, so as to 
allow the full influence of radiation and the impressions from the clear 
sky. 
The ice dishes present a large moist external surface to the dry 
northerly evening air, which cools the water on them, so that, when 
at 61°, it will in a few minutes fall to 56°, or even lower. But the 
moisture which exudes through the dish is quickly frozen, when the 
evaporation from the external surface no longer continues to produce 
much effect. 
To detect the influence of evaporation in producing ice, one of the 
dishes was placed in the evening upon a patch of grass, five feet above 
the level of the ice-beds, so as to be exposed to the full influence of 
the sky and the cold northerly wind. This was the most favourable 
situation for promoting evaporation. The night proved a favourable 
one for the formation of ice, and in the morning the dishes in the beds 
were covered with it, but the dish upon the elevation had lost weight 
‘during the night, and had no ice upon its surface; the water soon after 
sun-rise was at 46°, on another morning the water stood at 50°. This 
experiment was varied by placing a brass vessel of the same size and 
form as the common plates upon a sprinkling of straw on an elevated 
piece of ground near the ice-beds. In the morning it was found about 
the same weight, without any ice, although the plates on the beds were 
covered with thick ice. On the same morning one of the porous 
earthen vessels similarly situated, and covered on its under side with 
tinfoil, presented the same result. 
As a further proof of the cold not being produced by evaporation, 
I next carefully weighed a number of the unglazed dishes in the even- 
ing and again in the morning, when I found that they had gained con- 
