de as the southern extremity of
Hudson Bay; but the isotherm of London, as Humboldt outlines it, passes
through Cincinnati.

Of course such deviations of climatic conditions between places in the
same latitude had long been known. As Humboldt himself observes,
the earliest settlers of America were astonished to find themselves
subjected to rigors of climate for which their European experience had
not at all prepared them. Moreover, sagacious travellers, in particular
Cook's companion on his second voyage, young George Forster, had
noted as a general principle that the western borders of continents
in temperate regions are always warmer than corresponding latitudes of
their eastern borders; and of course the general truth of temperatures
being milder in the vicinity of the sea than in the interior of
continents had long been familiar. But Humboldt's isothermal lines for
the first time gave tangibility to these ideas, and made practicable a
truly scientific study of comparative climatology.

In studying these lines, particularly as elaborated by further
observations, it became clear that they are by no means haphazard in
arrangement, but are dependent upon geographical conditions which in
most cases are not difficult to determine. Humboldt himself pointed out
very clearly the main causes that tend to produce deviations from the
average--or, as Dove later on called it, the normal--temperature of any
given latitude. For example, the mean annual temperature of a region
(referring mainly to the northern hemisphere) is raised by the proximity
of a western coast; by a divided configuration of the continent into
peninsulas; by the existence of open seas to the north or of radiating
continental surfaces to the south; by mountain ranges to shield from
cold winds; by the infrequency of swamps to become congealed; by the
absence of woods in a dry, sandy soil; and by the serenity of sky in the
summer months and the vicinity of an ocean current bringing water which
is of a higher temperature than that of the surrounding sea.

Conditions opposite to these tend, of course, correspondingly to lower
the temperature. In a word, Humboldt says the climatic distribution of
heat depends on the relative distribution of land and sea, and on the
"hypsometrical configuration of the continents"; and he urges that
"great meteorological phenomena cannot be comprehended when considered
independently of geognostic relations"--a truth which, like most