326 METEOROLOGY. 



From tliis investigation it appears that during each of the four 

 astronomic seasons of spring, summer, autumn, and winter, the in- 

 tensities received from the sun are jirecisely equal. For in each 

 season the earth passes over three signs of the zodiac, or a quadrant 

 of longitude. The equality of intensities, however, applies to the 

 entire glohe regarded as one aggregate, and is consistent with local 

 alternations, h}' which it is summerin the northern hemisphere when 

 it is winter in the southern. Deferring the consideration of these 

 local inequalities, however, we may here illustrate the connexion of 

 the seasons with the elliptic motion from an ephemeris. In the year 

 1855, for example, spring in the northern hemisphere_, commencing 

 at the vernal equinox, March 20th, lasts eighty-nine days ; summer, 

 beginning at the summer solstice June 21, continues ninety-three 

 days ; autumn, commencing at the equinox', September 23, continues 

 ninety-three days ; and winter, beginning at the winter solstice, De- 

 cember 22, lasts ninety days ; yet, notwithstanding their unequal 

 lengths, the amounts of heat and light which the whole earth receives 

 are equal in the several periods. Since the earth is not strictly a 

 sphere, but an oblate spheroid, it evidently presents its least section 

 perpendicular to the rays of the sun at the equinoxes. As the sun's 

 declination increases, the section also increases and attains its limit 

 at the solstice. The variation, however, appears to be not material, 

 and compensates itself in each season. 



At the 2)resent time the earth is in perihelion, or nearest the sun 

 about the 1st of January, and furthest from the sun on the 4th day of 

 July. A special cause must, therefore, be assigned for the striking 

 fact which Professor Dove has shown by comparison of temperatures 

 observed in opposite regions of the globe, namely : that the mean 

 temperature of the habitable earth's surface in June considerably ex- 

 ceeds the temperature in December, although the earth in the latter 

 month is nearer to the sun. This result is attributed by that meteo- 

 rologist to the greater quantity of land in the northern hemisphere 

 exposed to the rays of the sun at the summer solstice in June ; while 

 the ocean area has less power for this object, as it absorbs a large por- 

 tion of the heat into its depths. Had land and water been equally 

 distributed, in other words, were the earth a homogeneous sphere, 

 the alleged inequality of temperature, it is obvious, would never have 

 existed. 



SECTION III. 



LAW OF THE sun's INTENSITY AT ANY INSTANT DURING THE DAY. 



The rays which emanate from the sun's disk into space proceed in 

 diverging lines in the same manner as if they issued directly from the 

 centre. And on arriving at the earth their intensity, as before stated, 

 will be inversely proportional to the square of the distance. 



But the more obvious phenomena of solar heat and light are mani- 

 fested to us under a secondary law. The sun's intensity first becomes 

 sensible in the eastern rays of morning ; it gradually increases to a 

 maximum during the day ; it declines on the approach of the shades 



