11 APPENDIX. 



roaring sound will run off till lost in the extreme distance. This 

 sound is almost continuous in very cold nights, and there must 

 be millions of such cracks formed. 



All these, sooner or later, by the hydrostatic pressure, are 

 infiltrated with water, which, in the thin fissures, freezes imme- 

 diately. They can be seen of all sizes from a mere crack to some 

 inches in width where the ice has parted through its whole thick- 

 ness and yielded to the contractile effort. When the temperature 

 rises during the day, this cracking ceases. The ice expands to 

 suit its increased mean temperature, and its edges encroach upon 

 the shores more and more day by day. A permanent increase of 

 size results from the filling of the contractile fissures by frozen wa- 

 ter, and on all the beaches of the lake a ridge parallel to the shore 

 is formed above the level of the water of the lake, composed of 

 sand, gravel, stones, and even large boulders, which are each win- 

 ter pushed further and further up the beach, until they reach the 

 limit of the ice edge. This may be likened to a secular variation 

 or expansion, its period being the existence of the ice field. 



The daily expansion and nightly contraction, arising from the 

 diurnal change of temperature of the ice, gives rise to effects 

 even more striking and important to the residents on the shores 

 than this annual variation. The lake is irregular in form. Its 

 shores, and those of the islands with which it abounds, form wide 

 bays or lobes of water, separated from each other by narrower 

 straits, which are limited by opposite advancing or receding 

 points or reefs of land or of rock. One bay or lobe three or four 

 miles in width and several miles in length will be connected with 

 the next by a narrower portion, perhaps only a mile in width. 

 In the contraction and expansion of these great fields the weaker 

 lines, or lines of least resistance, are across these constrictions, and 

 it is along these lines, which are the same year after year, that 

 the principal visible effects of diurnal expansion and contraction 

 are to be observed. The ice breaks or parts on these lines, some- 

 times leaving an open crack or line of water several feet in width, 

 difficult to cross on foot or in the carriole, as the northern sledge 

 is called. The common winter road of the shore inhabitants is on 

 the ice ; and I have often, at certain well-known points, driving 

 out early in the morning, found an open crack difficult to pass. 

 Returning in the evening, after the heat of the day had produced 

 its effect, the edges of the ice would be found to have met and in- 

 flected either upwards or downwards to such a degree that an axe 

 would be needed to effect a passage. 



This action is to be seen winter after winter at the same places, 

 and the formation of the ridge or gutter is observed by all, for 

 all are put to inconvenience and sometimes in peril by their move- 

 ments. When the edges of the ice fields happen to bend down- 

 wards under the effects of expansion, the passage is most danger- 

 ous, I have seen horses, approaching too near the edge eon- 



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