DR. FARQUHARSON ON GROUND GRU. 
39 
of temperature. During wind, on the land, the cold air at the surface of the earth 
is continually mixed with, or displaced by, the warmer air above ; and by this pro- 
cess both the earth and air in contact with it are prevented from being reduced to a 
very low temperature by radiation. But in a body of moving water, whose tempe- 
rature is under 39°, the eddies of the current throw down the coldest parts, which 
in still water would remain at the surface, to come into contact with the bottom. 
This last circumstance is the explanation of M. Arago ; and it well accounts for the 
formation of ground gru taking place first in the most rapid parts of the streams ; 
although neither by itself, nor when taken in conjunction with the other two circum- 
stances to which he refers, namely, aptitude to formation of crystals on asperities at 
the bottom, and less impediment to the formation of crystals in a slower motion, will 
it account for the formation of ground gru, as all these circumstances are present 
when the water forms only surface ice. The formation of ground gru requires for 
its explanation an additional element, namely, the radiation, into the clear sky, of 
heat from the bottom of the river ; and the formation never occurs but under a clear 
sky. 
As to the ground gru, observed by Colonel Jackson in the Neva under three feet 
of ice and three feet of snow, that can form no valid objection to the explanation I 
have given, unless it were ascertained that the gru was formed after the surface ice 
and the fall of the snow, and not before them. All rivers issuing from lakes, like the 
Neva, have very clear waters to admit of radiation through them, although as deep 
as it is ; and all rivers are very clear during frost, owing to the freezing up of, the 
little land rills that would convey earthy particles into them. Ground gru formed in 
the Neva would be much more permanent than in our rivers. The mean tempera- 
ture of Alford is 45° Fahr., and that of the earth of course the same ; and on the re- 
mission of its cause, the ground gru is here speedily detached from the bottom, by 
the transmission of heat from below. Not so in the Neva. There, according to 
Kupffer, the mean temperature is only 38 0, 75 ; and under the action of a frost so 
severe as to form three feet depth of ice, although the condition of the clear sky might 
not continue, previously formed gru would, at that mean temperature, be of great 
permanence. This applies also to the Siberian rivers. 
