Sciences of Meteorology and Terrestrial Magnetism. 167 



the physical character of the surface. The heating power is thus greatly 

 diminished at low altitudes of the sun. The changes of temperature 

 have thus a certain general dependence on altitude, and on latitude ; 

 but no law of decrease has yet been established in either direction — the 

 complex elements, indeed, seem to throw great obstacles in the way of 

 our ever determining such a law, — we shall therefore state a few of the 

 best ascertained facts, in order to give a general notion of the subject. 



And, first, as regards the distribution of heat in latitude, the recent 

 corrected isothermals of Dove show that the decrease is extremely 

 different under different meridians ; the decrease, as we advance from 

 the equator, is least near this line, and becomes progressively greater 

 to about lat. 45° ; the temperature of the equator is 79°-8 F., but the 

 warmest parallel does not coincide with the equator ; it is that of about 

 10° N., and hero May is the warmest month. At the equator the 

 maxima temperatures fall in April and November, the minima in July 

 and December. The mean temperature of the pole is 2°'2 F. ; in summer 

 (July), 3°"6; in winter (January), — 26°-6. In July the equator is 

 48° warmer than the pole ; in January, 106° warmer. From latitude 

 40° up to the pole, July is the warmest month ; in latitude 30° August 

 is the warmest ; in latitude 20° the two are equal. From latitude 60° 

 to the pole the temperature may be found with great exactness by the 

 following empirical formula : — 



(! = + 3°-65 + 105-75 cos^x 



X being the latitude, and t the mean temperature of the year in that 

 latitude. . . . From the equator to latitude 40° S. the temperatm-e 

 of the southern hemisphere is lower than that of the northern. But 

 Dov^ thniks that this may not be so in the higher latitudes. East of 

 the meridian of Ferro the decrease of the temperature of January in 

 going north is given, between lat. 0° and 30°, by the formula — 



< = 32° + 47°-5 cos 2 x, 



and in the western hemisphere, between lat. 0° and 40°, by the formula — 



< = 32° + 48°-15 cos {2x - 7°) ; 



for both hemispheres for north latitudes, both high and low, 



«= — 23°-l + 102-4 cos' x 



and still nearer the truth for low latitudes — 



< = — 22° + 101-25 cos' X. 



For the eastern half of the southern hemisphere, the formula 



t = 20°-8 + 59° cos' (x - 5°), 



