3ro THE DEPTHS OF THE SEA. [CHAP. VIII. 
depth of course, the brine sinking downwards,—must 
greatly outweigh (I give this as what Petermann 
would call a gratuitous speculation) the slight ex- 
pansion caused by the heating of the surface layer. 
The more restricted arctic basin on the other hand, 
as was long ago pointed out by Capt. Maury, parti- 
cipates to a certain extent in the characteristics of 
the Baltic; and I am greatly mistaken if the low 
specific gravity of the polar sea, the result of the 
condensation and precipitation of vapour evaporated 
from the intertropical area, do not fully counter- 
balance the contraction of the superficial film by 
arctic cold. 
The North Atlantic ocean bears a proportion 
in depth to the mass of the earth considerably 
less than that of the paper covering an eighteen- 
inch globe to that of the globe it covers, while 
the film heated by direct solar radiation may be 
represented by its surface coating of varnish, 
and is not actually thicker than the height of 
St. Paul’s. Physicists seem to find a difficulty in 
giving us the amount of palpable effect in pro- 
ducing currents in this shell of water, six thousand 
miles in length by three thousand in width and 
two miles in thickness, which may be due to causes 
such as those relied upon by Dr. Carpenter, acting 
under the peculiar circumstances and to the amount 
in which we find them in nature; and probably we 
are not yet in a position to give them sufficient data 
to enable them to do so. Mr. Croll, a good authority 
in such matters, has attempted to make some calcu- 
lations, and comes to the conclusion that none of them 
are sufficient to overcome the friction of water and to 
