The direct action of the sun's rays upon the earth, particularly those portions which lie north and south of the tropics, is not the only source from which the supply of heat is derived. Although there is a general increase of heat in spring and summer when the sun travels north, and of cold when he travels south in winter, yet there are frequent irregularities attending both. Very sudden and great changes occur in each of them. Frost sometimes, cool weather often, occurs in midsummer, and considerable heat and tornadoes in midwinter. And ordinarily the maxima and minima of each month and, indeed, of each week are widely apart. Even in the polar regions, in midwinter, _where the sun does not shine at all_, the same moderating changes with which we are conversant occur in degree. An extract or two from the register found in Dr. Kane's narrative of the "Grinnell Expedition" will illustrate this. JANUARY 1851, (LATITUDE ABOUT 74 deg., LONGITUDE ABOUT 70 deg.). Date. Wind. Force. Ther. Bar. Sky and Weather. Jan. 3 calm -26.1 29.62 blue sky, m. " 4 W. gent breeze -21.3 29.53 blue sky, detached clouds, m. " 5 W. by N. gent breeze -3.9 29.59 blue sky, m., clouded over. " 6 W. by S. light breeze -0.8 29.67 clouded over, m., snow. " 7 W. gent breeze -14.4 29.96 blue sky, detached clouds, m. " 8 W.S.W. light air -21.2 30.14 blue sky, m. " 29 W.N.W. light air -18.9 30.19 blue sky. " 30 NW. by W. light air -13.5 30.17 clouded over, m. " 31 NW. by W. gent breeze -4.4 29.35 clouded over, snow. Feb. 1 W. light breeze -11.7 29.27 cloudy, blue sky, m. " 2 W. light air -25.1 29.62 blue sky, detached clouds, m.