CHAP. VIT.] DEEP-SEA TEMPERATURES. 285 
liarities in temperature, owing to the mixture of. 
hot and cold currents of air; but in the main, iso- 
thermal lines, that is to say, lines drawn through | 
places having the same mean temperature, would 
coincide with parallels of latitude. A glance at any 
isothermal chart, whether for the whole year, for 
summer, for winter, or for a single month, will show 
that this is far from being the case. The lines of 
equal temperature deviate everywhere, and often 
most widely, from their normal parallelism with the 
parallels of latitude and with each other. A glance 
at the same chart will also show, that while there 
is an attempt, as it were, on the part of the iso- 
thermal lines to maintain their normal direction 
through the centre of great continents, the most 
marked curves, indicating the widest extensions of 
uniform conditions of temperature, are where there 
is a wide stretch of open sea extending through 
many degrees of latitude, and consequently includ- 
ing very different climatal conditions. 
The lands bordering upon the ocean partake in 
this general diffusion of heat and amelioration of 
climate, and hence we have the difference between 
continental and insular climates—the former giving 
extremes of summer heat and winter cold, and the 
latter a much more uniform temperature, somewhat 
below the normal temperature within the tropics, 
and usually greatly above it beyond their limits. 
The islands of Ireland and Great Britain and the 
west coast of the Scandinavian peninsula are in- 
volved in the most extreme system of abnormal 
curves which we have in any of the ocean basins; 
and to this peculiarity in the distribution of tem- 
