244

THE PLANET MAES.

northern hemisphere there are 372 Martial clays in the summerhalf of the year, while in the winter half there are only 296 ;the lengths of the four quarters being respectively;—of thespring, 191 clays; of the summer, 181 clays ; of the autumn,149 days ; and of the winter, 147 clays. In the southernhemisphere the durations are of course interchanged.

Such a variation in the length of the seasons, althoughconsiderable, would, however, not he very troublesome. Andthe smaller amounts of heat and light which are received byMars might coincide with a slower growth of vegetation and ofthe crops, so that a year and seasons of about double the lengthof our own might be very convenient. There are, nevertheless,other reasons connected with the diminished amount of solarlight and heat, to which we have just referred, which, in ouropinion, involve the question of the habitability of Mars in veryconsiderable difficulties.

Upon an average this light and heat would only be of anintensity equal to -/^ths, or rather more than -fths, of thatwhich the Earth receives, which may be shown as follows:—The Sun would present to an observer upon Mars a diameterabout two-thirds of the size of that which it exhibits to us,because Mars is at a distance from it equal to about timesthat of the Earth ; and the light and heat received would varyas the surface of the apparent disc, i.e. as the square of frds,or nearly as 4th s ; the more accurate value being, as abovestated, f^ths.

In the preceding paragraph we have used the expression“ upon an average,” because it must be remembered, that aconsiderable variation in the above value would also be causedby the ovalness of the orbit of Mars . Its minimum distancefrom the Sun being 129,000,000 miles, its mean distance142,000,000 miles, and its maximum distance 155,000,000miles, there would he a corresponding variation in the Sun sapparent diameter of about T xth part of its mean value. Inother words, the diameter of the Sun’s apparent disc, atdifferent times in each Martial year, would be as the numbers10, 11, and 12. The light or heat received would be asthe squares of these numbers, i.e. as 100, 121, and 144; or