76 PROFESSOR FORBES’ INQUIRIES 
centres of extreme cold in the northern hemisphere, connected with the predomi- 
nant masses of land in Siberia and Arctic America; and he ingeniously suggested 
that the isothermal lines might form a connected set of double curves, analogous 
to lemniscates, round these centres as poles. With this assumption, he was led to 
propose a formula for the mean temperature of any place depending on the pro- 
duct of the sines of its angular distances from the two cold poles. M. Dove con- 
tests the existence of two centres of minimum temperature in the annual means ; 
yet his researches make it exceedingly probable that they do exist in winter, as 
at most seasons there are also two centres of greatest heat in the tropical regions, 
depending partly on the local accumulation of land, partly on the peculiarity of 
the marine currents. 
9. But the most instructive mode of indicating the climatic irregularities of the 
globe, subsequently to the isothermal lines of Dz Humsoupt, is Dove’s represen- 
tation of what he calls the “Thermic Anomaly,” by means of “Isabnormal 
Curves.”* The principle of these curves is the following :—The average tempera- 
ture of every tenth parallel of latitude is taken by tabulating its temperature at 
36 equidistant points—that is, on every tenth meridian. These numbers are best 
obtained from the isothermal curves on the plan of De Humpoupr. The tempera- 
ture of any place on the given parallel is then compared with the mean of the 
parallel, and lines are drawn through all those places whose temperature is 1’, 
2°, 3°, or more degrees above the mean of the parallel; and other curves are drawn 
through points which are 1°, 2°, &c., be/ow the mean of their respective parallels. 
These deviations + or — from the mean of the parallel are called the ‘‘ Thermic 
Anomaly,” representing, in fact, all the heating or cooling causes in action at a 
given place, independently of its distance from the equator. The curves so drawn 
are called the ‘‘ Isabnormal Curves of Temperature.” The whole globe is thus di- 
vided into two vast, but not necessarily continuous regions; the one having a 
temperature higher than the mean due to the latitude, and the other having a 
temperature below the mean. Centres of re/ative maximum cold, coinciding 
nearly with the Cold Poles of Sir Davin BrewsvtsEr, are found in the regions 
of Hudson’s Bay and of Eastern Siberia. 
10. The isabnormal curves may also be formed for different seasons, when they 
become very instructive. In winter, the relatively cold centres of the northern 
hemisphere are within the continents, and the relatively warm centres are in the 
North Pacific and North Atlantic respectively ; in summer these positions are re- 
versed, beautifully pointing out the effect of the ocean in equalizing the tempera- 
ture of the globe. } 
11. To ascertain the “ Thermic Anomaly,” M. Dove, of course, required to fix 
as exactly as possible the mean temperature of every tenth parallel of latitude, 
* The Distribution of Heat over the surface of the Globe, &c. Plates IV.and V. 4to. Lon- 
don, 1853 
