12 INTENSITY OF SUN'S HEAT AND LIGHT. 



Another feature worthy of mention, is the resemblance of the earth to the planet 

 Mars; upon which Sir W. Herschel has remarked: "The analogy between Mars 

 and the Earth is, perhaps, by far the greatest in the whole solar system. The 

 diurnal motion is nearly the same, the obliquity of their respective ecliptics not 

 very different ; of all the superior planets, the distance of Mars from the Sun is by 

 far the nearest alike to that of the Earth; nor will the length of the Martial year 

 appear very different from what we enjoy, when compared to the surprising duration 

 of the years of Jupiter, Saturn, and Uranus. If we then find that the globe we 

 inhabit has its polar region frozen and covered with mountains of ice and snow, 

 that only partly melt when alternately exposed to the sun, I may well be permitted 

 to surmise that the same causes may have^ the same effect on the globe of Mars ; 

 that the bright polar spots are owing to the vivid reflection of light from frozen 

 regions; and that the reduction of those spots is to be ascribed to their being 

 exposed to the sun." 



Recurring now to equation (9) and the proposition following, it will readily be 

 inferred that during each of the four astronomic seasons of Spring, Summer, Autumn, 

 and Winter, the intensities received from the sun are precisely 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 globe regarded as one 

 aggregate, and is consistent with local alternations, by which it is summer in the 

 northern hemisphere when it is winter in the southern. Deferring the consideration 

 of these local inequalities, however, we may here illustrate the connection 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, con- 

 tinues ninety-three days; autumn, commencing at the equinox, September 23, con- 

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

 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. 1 



At the present time the earth is in perihelion, or nearest the sun about the 1st 

 of January, and farthest 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 

 exceeds the temperature in December, although the earth in the latter month is 

 nearer to the sun. This result is attributed by that meteorologist 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 portion 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. 



1 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. 



