232 THE PHYSICAL GEOGRAPHY OF THE SEA. 



height is about 29.9 inches. The hypothesis of an open sea in 

 the Arctic Ocean becomes necessary to supply a source for this 

 vapor ; for the winds, entering the Arctic Ocean as they do after 

 passing over the land and mountain heights of America, Europe, 

 and Asia, must be robbed of much of theii* moisture ere they reach 

 that ocean ; it will require an abundant supply of vapor to create 

 there by precipitation and the liberation of latent heat a degree 

 of rarefaction sufiicient to cause a general movement of the air 

 polarward for the distance of 40° of latitude all around. That 

 there is an immense volume of comparatively warm water going 

 into the Arctic Ocean is abundantly shown by observation, and 

 the rising up there of this water to the surface would afford heat 

 and vapor enough for a vast degree of rarefaction. 



459. The records of arctic explorations, together with the whale- 

 The middle ice. mcu's accouuts of " middle ice" in Baffin's Bay and 

 Davis' Straits, go to confirm this view, which is further elaborated 

 in the next chapter (§ 475). The facts there stated, and this 

 "middle ice," go to show that every winter a drift takes place which 

 brings out of the Frozen Ocean a tongue of ice a thousand miles or 

 more in length : it is the compact and cold " middle ice." In our 

 fresh-water streams it is the middle ice that first breaks up ; that 

 which is out of the way of the current remains longest. Not so 

 in this bay and strait ; there the littoral ice first gives way, leav- 

 ing an open channel on either side in spring and early summer, 

 while the "middle ice" remains firm and impassable. The ex- 

 planation is simple enough. The middle ice was formed in 

 the severe cold of more northern latitudes, from which it has 

 drifted down, while that on the sides was formed in the less se- 

 vere climates of the bay and straits. This winter tongue of ice, 

 which we know by actual observation is in motion from Decem- 

 ber till May, must, during that time, be detached from the main 

 mass of ice in the Arctic Ocean, consequently there must be wa- 

 ter between the ice that is in motion and the ice that is at rest. 

 Not only so : in early summer the whalemen will run up to the 

 north in the open water at the side of the " middle ice" in Davis' 

 Strait and Baffin's Bay, even as far sometimes as Cape Alexander 

 in 78°, to look for a crossing-place. Here, though so far north, 

 they will find the "middle ice" gone, or so broken up that they 

 can cross over to the west side. They trace it up thus far, be- 



