22 Mr Forbes on Polar Temperature. 



the powers which nature thus holds in store, and which may 

 be operating the most important ends, although out of the 

 reach of our senses. 



III. The last point upon which I have to insist, (for I only 

 touch upon what I consider the most important facts,) is the 

 influence of aerial currents, or the strong north winds which 

 prevail in the Arctic regions at the time of the breaking up of 

 the ice. If we refer to the details of navigators, we shall find 

 that the phenomena of this period are very different from what 

 philosophers in their closets generally set about theorizing upon. 

 The latter would imagine to themselves a lake or pond having 

 acquired a few inches of ice at a temperature little below 32°; 

 here indeed the process of the dissolution is slow enough; — but 

 how different is ice in its own proper region, with a temperature 

 of about 30 below zero, in the month of March ; it suffers a 

 rapid and irregular expansion of volume in the succeeding 

 month, when winter suddenly rushes into the comparatively 

 intense heat of the short-lived Arctic summer. The icy fields 

 are ruptured with tremendous violence, and separate with a 

 fearful crash from the eternal glaciers of the ice-bound shores 

 to which they were cemented. All causes become at once ripe 

 for action ; the expanded volume of the crust, — the liquefying 

 influence of now powerful solar rays, and the penetrating effect 

 of showers, prepare every thing for the action of those prevail- 

 ing northerly winds, which, by the accounts of navigators, 

 in a few hours change totally the whole aspect of the horizon, 

 as far as the eye can reach ; and hurry whole thousands of 

 acres down to more southern climates, where the greater part 

 is speedily melted by the joint influence of solar warmth and 

 a higher mean temperature of the ocean ; but some masses 

 even reach extremely far into the Temperate Zone. Can we 

 then fail to see a vast difference between the process of freezing 

 and of dissolution ; and have we not shown, even in this rapid 

 sketch, sufficient causes to account for the apparent anomaly 

 which we proposed to explain ? I perfectly agree with Mr 

 Leslie, that, " to discuss with accuracy the question of the pe- 

 riodical formation and destruction of the Polar ice, it becomes 

 necessary to explain the true principles which regulate the dis- 

 tribution of heat over the globe ;" and I have only to regret 



