172 Rev. Mr Eisdale's Observations on Ground-ice. 



for that strange accumulation called ground-ice., which is found 

 nowhere but in streams. 



I would not have brought forward this theory a second time, 

 had T not met with some facts collected by M. Arago, which 

 afford the strongest confirmation of the theory which I had ad- 

 vanced, though he himself scarcely seems to have had a glimpse 

 of their importance. He mentions an observation by Desmarest, 

 that in a cloudy sky the ground-ice accumulates uniformly, but 

 is interrupted when the sun shines. Now, what he calls a cloudy 

 sky I conceive to be an atmosphere loaded with hoar-frost, and 

 rendered hazy by its condensation ; for I do not think it possible 

 that a genuine cloud can exist in the atmosphere during a keen 

 frost. Here, then, this observer furnishes a fact in perfect ac- 

 cordance with the information on which I proceeded, viz. that 

 the ground-ice is formed only during a hazy state of the atmo- 

 sphere, ij3 other words, during a hoar-frost ; whilst he tells us 

 that the process was interrupted when the sky was clear. 



But M. Arago quotes a passage from a paper of Mr Knight, 

 the celebrated botanist, in the 106th vol. of the Phil. Trans., 

 which brings the matter nearer, if not altogether, to a demon- 

 stration, though Mr Knight himself proposes no theory. The 

 passage is as follows : — " In a morning which succeeded an in- 

 tensely cold night, the stones in the rocky bed of the river ap- 

 peared to be covered with frozen matter, which reflected a 

 thread of silvery whiteness, and which, upon examination, I 

 found to consist of numerous frozen spicula crossing each other 

 in every direction^ as in snow, but not having any where, except 

 near the shore, assumed the state of firm compact ice. The 

 river was not at this time frozen over in any part, but the 

 temperature of the water was obviously at the freezing point, 

 for small pieces of ice had every where formed upon it in its 

 more stagnant parts near the shore ; and upon a mill-pond, just 

 above the shallow streams, in the bottom of which I had no- 

 ticed millions of little frozen spicula floating upon the water. 

 At the end of this mill-pond, the water fell over a low weir, and 

 entered a narrow channel, where its course was obstructed by 

 points of rock and large stones. By these, numerous eddies 

 and gyrations were occasioned, which apparently drew the 

 floating spicula under water ; and I found the frozen matter to 



