376 THE DEPTHS OF THE SEA. [CHAP. vin. 



depth of course, the hrine 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 hasin on the other hand, 

 as was long ago pointed out by Capt. Maury, partici- 

 pates 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- 

 tions, and comes to the conclusion that none of them 

 are sufficient to overcome the friction of water and to 



