Mr Forbes on Polar Temperature. 21 



ening the defence of ice ; the mean temperature of the air may 

 thus be indefinitely low, whilst that of the junction of ice and 

 water cannot be under 28°. But mark the contrast of the pro- 

 cess of thawing. Here the material for operation, or the ice, 

 is always exposed to the atmospheric influence ; here there is 

 no difficultly conducting medium to shelter it from the oppo- 

 site extreme ; but not only this, for we have the authority of 

 Professor Leslie himself for saying that the dissolution com- 

 mences next the water. It is actively going on upon both sides 

 of the ice at once; the currents of warmer water ascending from 

 lower latitudes are able to effect the partial destruction of the 

 fields before the solar rays have acquired sufficient power to 

 commence their active energy above. But not merely do these 

 causes combine to this great end, but the fall of rain, and the 

 insinuation of moisture through the pores of the ice, tend to 

 its rapid dismemberment ; yet Mr Leslie has confined his cal- 

 culations to the mere thickness of a cake of ice, which the so- 

 lar rays, with a given degree of obliquity, can dissolve. 



II. We must attend to the important, but little known, ope- 

 rations which nature is constantly performing through the 

 agency of marine currents, before we can pronounce upon the 

 quantity of ice melted under a given latitude or mean tempe- 

 rature ; and we have just quoted the best authority for the 

 fact, that the commencement of the thaw is from below. The 

 laws of hydrostatics prepare us for the fact, that currents of 

 various kinds, and modified by an infinity of local circum- 

 stances, must effectually prevent stagnation in the ocean, and 

 preserve a certain interchange between the Equatorial and Po- 

 lar regions. Their precise nature and extent must often be 

 disguised or wholly unperceived, especially from our ignorance 

 of causes in action at great depths, which must be materially 

 modified by the remarkable law of the irregular variation of 

 the volume of water by heat, from which it follows that water, 

 perhaps near 50° of temperature, may have a superior specific 

 gravity to surface water at the freezing point of the ocean, or 

 28°. It is easy, therefore, to speculate upon the sudden effects 

 which may ensue from the action of currents at the critical 

 moment of the disruption of the ice, and, though easier to spe- 

 culate upon than to analyze, there are none so bold as to deny 



