THE SPECIFIC GRAVITY OF THE SEA, ETC. 219 



of sea water into -which the navigator vill take the trouble to dip 

 these two instruments. 



449. These specific gravity and thermal curves, as they are pre- 

 Light cast by Plate scutcd ou this Plate (X.), throw light also on the 

 in The Atretic occMi. qucstion of an open sea in the Arctic Ocean. That 

 open sea is hke a boiling spring (§ 427) in the midst of winter, 

 which the severest cold can never seal up ; only it is on a larger 

 scale than any spring, or pool, or lake, and it is fed by the under 

 currents v/ith waim water from the south, which, by virtue of its 

 saltness (see Fig. 2), is heavier than the cool and upper current 

 which rmis out of the polar basin, and which is knovTi as an ice- 

 bearing cmTent. It is the same which is felt by mariners as £ar 

 down as the Grand Banks of Newfomidland, and recognized by 

 philosophers off the coast of Florida. This upper cmi'ent, though 

 colder than its fellow below, is lighter, because it is not so salt. 

 Figure 2 reveals to us a portion of sea between the parallels of 

 34"^ and 40^ north, exactly in such a physical category as that in 

 which this theory presents the Arctic Ocean. Here, along our 

 o^n shores, the thermal curve loses 12^ of heat ; and what does 

 the specific gravity cm've gain in the same interval ? Instead of 

 increasing up to 1.027, accordmg to the thermal law, it decreases 

 to 1.023 for the want of salt to sustain it. Now recollect that the 

 great American chain of fi'esh-water lakes never freezes over. 

 XVliy ? Because of their depth and their vertical circulation. 

 The depths below are continually sending water above 32° to the 

 surface, which, before it can be cooled dovn to the freezing-point, 

 sinks again. Now compare the shallow somidmgs in these lakes 

 "^ith the great depths of the Arctic Ocean ; compute the vast ex- 

 tent of the hydrographic basin v;hich holds this polar sea ; gauge 

 the rivers that discharge themselves into it ; measure the rain, 

 and hail, and snow that the clouds pom' down upon it ; and then 

 contrast its area, and the fresh water drainage into it, mth the 

 like of Long Island Sound, Delaware Bay, and the Chesapeake ; 

 consider also the volume of diluted sea water between om' shore- 

 line and the Gulf Stream ; strike the balance, and then see if the 

 arctic supply of fresh water be not enough to reduce its salts as 

 much as our o^vn fresh water streams are diluting the brine of 

 the sea under our own eyes. The very Gulf Stream water, vrhich 

 the observing vessel left as she crossed 34° and entered into those 

 hght httorai waters, was bound northward. Suppose it to have 

 flowed on as a smiace current until it, vdth its salts, was reduced 



