The direct heat of the sun being unequally distributed over different parts of the globe is the primary ciuse of the variation of climate ; its diurnal action on land is sensible only for a few inches in depth ; the ani.ual action however extends throughout the superior stratum of variable temperature above mentioned. The mean temperature of a place is generally estimated by taking the average of the diurnal temperatures during the four seasons of the year, and again taking the average of these four averages.
As there is a great variety of temperature in the same parallel of latitude, wo cannot have a formula dependent only on this element to express the heat of places on continents. The first approximate formula to that effect which deserves the name, is that given by Slayer, the celebrated astronomer ; though empirical, it is found to possess considerable exactness. If t be the mean temperature in degrees of Fahrenheit's thermometer, he makes t = 84 — 52 sin where L is the latitude of the place. The supposed facts which evidently suggested this formula were the equatorial mean temperature of which is now generally supposed to be too high ; the Polar mean temperature of 32`, or the freezing point, which, from the recent observations of Parry, Scoresby, &c., is now known to be far too great ; and thirdly, that the diminution of beat from the equator to the poles The mean temperature in the latitude of 34° in different continents, and at places near the sea, is found to vary but little, thus :— The sea varies in temperature much less than the air ; the regions of warmest water extends about 54° on each side of the equator, but rather farther to the south than to the north. There are two points of maximum cold near the North Pole ; one of 34- Fahr. in the neigh bourhood of Melville Island, and another of 2° Fahr. somewhere about the 79th degree of north lat., long. 120° east. The isothermal curves, " accordingly, about the North Pole bear no inapt resemblance,' as remarked by Sir David Brewster, " to the isochromatic lines, or coloured spluero-lemniscates exhibited by polarised light in a biaxal crystal, whose optic axes are inclined to each other about 30°, having the pole itself almost centrally situated between them, and their line of junction nearly coincident with that diameter of the polar basin which intersects it, and passes through its two great outlets into the Pacific and Atlantic oceans—a most remarkable feature, strongly indi cative of the absence of land, and of the prevalence of a materially milder temperature (possibly not averaging below 15° Fahr.) at the
actual pole. Of the point or points of maximum cold in the southern hemisphere we know nothing." An unexpected and most interesting consequence has followed from Sir D. Brewster's recognition of the resemblance of the north polar isothermal curves to the isochromatic lines of polarised light in the case described. Sir John F. W. Herschel has found that there is a coincidence between the two sets of curves, " in respect of the arithmetical progression of temperature in the one series corresponding to that of chromat.e sequence in the other," which he points out " as something different from and additional to a lucre general resemblance of form ; " though " it will of course be under stood," as he observes, " that we have not the slightest intention of tracing any physical analogy" between them. What the relation can be, of which this coincidence is a result, it is impossible to say ; but it la important in this place to indicate the bearing of the coincidence upon Mayer's formula. expressing the diminution of heat from the equator to the pole. Sir J. I lerschel shows that if a series of curve. be traced by calculation from an equation derived from and equivalent to the general equation of the optical curves in question, for tempers tures varying from a certain mean temperature thermometrically indi cated, and from each other, by equal thermometric intervals, these curves, " in respect of their magnitudes, distances from their foci and the pole, as well as in their general gradations of flexure, will coincide, or nearly so, with a series of isochrumatie curves equidistant is their orders qf tie." This latter series, if laid down on a map, will divide the meridians in the polar regions in a succession of points distributed over them according to a certain law of progrusion. By calculation from these data, he obtains equations, of which he says : • Now it is remarkable that this is precisely the form in which Mayer endeavoured • to express empirically the decrement of temperature in proceeding from the equator towards the pole, from such as could be obtained about the middle of the last century." The _details of this curious subject are given by Sir J. 1lerschel in his article Meteorology, Eoeyc. Brit.' para. 192 to 194.