308 Mr. Ferguson 07i the Cause of the Holes of Ice. 



niption, appeared from the possibility of melting a hole in 

 the most transparent ice by the same lens. Hence it appears 

 that heat, radiated from the sun, will pass through ice with 

 little interruption to any opake foreign body, by which it will 

 be absorbed, and by which it will be communicated, by con- 

 duction, to the contiguous ice. 



The sun is evidently not the only source of the radiated 

 heat absorbable by such foreign body. Thus, we can scarcely 

 suppose that all the sun's heat is intercepted by the clouds, 

 even in a day the most overcast, as such a circumstance never 

 happens with his light; and, when ice is melting in a warm 

 room, not only the fire, but the walls and furniture, are 

 sources of radiant heat. The natural exposure appeared to 

 be most favourable to the formation of the holes; for, in the 

 rays of the sun, a sheet of ice, with a stone above it, was often 

 entirely perforated before the sheet had melted a quarter of 

 its thickness, while, in a warm room, it would first have to 

 melt nearly half its thickness. Perhaps the reason was, in 

 part, the more equal action of heat derived from the atmo- 

 sphere by conduction, and, partly, the greater absorption by 

 the ice of heat radiated from terrestrial objects, than of heat 

 emanating from the sun. 



The explanation of the form of the holes will now be easily 

 understood. As the bodies that produce them, generally 

 either are of themselves round or oval, like most of the small 

 stones found on a river side, or are collected by the washing 

 of the water into circular heaps, as in the case of sand, it is 

 evident that the holes must, from the first, tend to assume a 

 circular form. But, as an additional widening of the hole, 

 during and after its formation, always takes place, owing to 

 the continued melting of the ice, the first irregularities be- 

 come ultimately less observable. The holes are seldom of 

 equal width throughout, being sometimes wider above than 

 below, at other times the reverse. The foreign body that 

 forms the hole has originall}' descended, when the hole is 

 wider above — the first formed part of the hole being widened 

 by its longer subjection to the common melting that the whole 

 sheet undergoes — and, in the reverse case, the body has been 

 below the ice, and has ascended. 



I have never observed any of the holes, not completely per- 

 forating the ice, without having at its bottom some opake 

 foreign body. The holes, during their formation, take a di- 

 rection perpendicular to the horizon ; and the occasional ob- 

 liquity of the holes, in reference to the surface of the ice, is to 

 be accounted for, by their having been formed when the sheets 

 lay in a position oblique to the horizon. 



