
78 PROFESSOR FORBES’ INQUIRIES 
or cold travel to or from the interior of continents, or parallel to their shores, in a 
manner which physical theories may indicate in a general way, but cannot possi- 
bly evaluate with accuracy. Further, the periodical conversion of vast surfaces 
of salt and fresh water into the state of ice must alter, for a portion of the year, the 
laws of temperature. 
14. The secondary anomalies just referred to are in their nature in a great 
degree compensatory. If acurrent of hot water moderates the cold of a Lapland 
winter, the counter current, which brings the cold of Greenland to the shores of the 
United States, in a great measure restores the balance of temperature, so far as it 
is disturbed by this particular influence. The prevalent winds, in like manner, 
including the trade-winds, though they render some portions of continents on the 
average hotter or colder than others, produce just the contrary effect elsewhere. 
Each continent, if it has a cold eastern shore, has likewise a warm western one, 
and even local winds have for the most part established laws of compensation. In 
a given parallel of latitude all these secondary causes of local climate may be 
imagined to be mutually compensatory, and the outstanding gradation of mean or 
normal temperature will mainly depend—l1sé, Upon the effect of latitude simply ; 
2d, On the distribution of land and water considered in their primary or statd- 
cal effect, which in a first approximation we may conceive to be proportional to 
their relative areas. 
15. Assuming, then, the data with which the tables and charts of M. Dove fur- 
nish us, I propose first, to inquire what portion of the average temperature of the 
globe in each parallel is due to the land, and what to the water, which respectively 
belong toit. Next, I propose to obtain a formula, empirical, indeed, yet involving 
the elements of physical geography as data, for expressing the mean temperature of 
a given parallel, and thence arrive at an approximate answer to the inquiry as to 
what would have been the equatoreal or polar temperature of the globe, or that of 
any latitude, had its surface been entirely composed of land or of water,—a result 
not without interest in cosmical speculations, even if subject to some margin of 
uncertainty. 
16. Every one acquainted with physico-mathematical investigations, especially 
those of more recent times, is aware that the flexibility of an algebraic formula in- 
cluding several constants is such, that its coincidence with a series of graduated 
observations must be very cautiously admitted as any proof whatever of the relation 
of the numbers being otherwise than empirical. My object here being to obtain 
a formula necessarily involving several constants, but with a certain physical 
significance, the importance of testing the parts of the formula by independent 
analogies cannot be overrated. In order, therefore, to show that the result of my 
labours has been something more than the mere empirical coincidence to which 
I have referred, it will be necessary to trace the course of the approximation and 
verification which I adopted. 
