190 PHYSICS. 
of Asia, have summers like those of Berlin ; to these, however, follow severe 
winters, in which the mean temperature of the coldest month amounts 
to from — 3°F. to—4°F. Such a continental climate may justly be called 
excessive. The unequal distribution of land and water produces indirectly 
an unequal heating in different places, by influencing in a great measure 
the direction of wind and ocean currents, which diffuse both higher and 
lower temperatures. Europe owes her proportionably mild climate to 
three causes: 1. To her peculiar shape, her coast being broken to an 
extraordinary degree, thus producing a very long outline. 2. To the 
character of the land south of her, the greater part of Africa being barren 
and sandy, and at the same time very hot, thus giving rise to incessant 
currents of warm air. 3. To the gulf stream. This gulf stream comes from 
the gulf of Mexico, where the temperature of the water amounts to nearly 
88°F"., coasts the peninsula of Florida, follows the coast of North America to 
39° or 40° N. L., turning off then in a north-easterly direction towards 
Europe, which owes its freedom from polar ice to the fact that the gulf 
stream tempers the waters on its coasts. Northern Asia has a very cold 
climate compared with Europe. In America, the climate in the interior and 
on the eastern coast is much more severe than on her Pacific shore, cold 
currents from the north traversing the former, carrying the polar cold into 
more southern regions. Thus, at Nain in Labrador, on the eastern coast, 
the mean annual temperature is about 24°F.; at New Archangel, on the 
western coast, nearly in the same latitude, this same mean is about 441°F. 
The mean summer temperature in the former place is scarcely 43°L.. that 
of the latter about 57°F. As a general rule. the eastern coast of a continent 
is colder than the western, a fact which will be explained in the section on 
winds. The southern hemisphere is considerably colder than the northern ; 
in the former the polar ice reaches 20°—30°, in the latter only 9° from the 
pole: ice not rarely occurs in the former at a latitude of 31°. In the new 
world the mean temperature increases more rapidly with increasing latitude 
than in the old. 
The temperature of the ground is often remarkably different from that of 
the air, sometimes higher and sometimes lower, according to circumstances. 
The mean temperature of the earth’s surface agrees very nearly with that 
of the air at a mean latitude, and is generally indicated by the temperature 
of springs; at higher latitudes the mean temperature of the ground is higher, 
and at lower latitudes it is lower than that of the air. The temperature of 
the ground from the equator to the pole diminishes the faster as we approach 
the parallel of 45°. Ata slight depth the variations of temperature are much 
less than at the surface, and at still greater depths there is no variation 
whatever, a constant temperature existing, but little different from the mean 
annual temperature of the place. The depth at which all annual variations 
of temperature vanish entirely, varies in different places ; in the torrid zone 
it amounts to only one foot; in the temperate, as in France, Germany, &c., 
this depth is from sixty to seventy feet ; even here, however, the diurnal 
variations vanish at a depth of from one and a half to three feet. The 
temperature of the earth increases regularly with increasing depth, this 
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