42 THE POPULAR SCIENCE MONTHLY. 



more than half of the mean diameter of the Earth. The density 

 of the planet is rather less than three quarters of the density of 

 the Earth, or about four times the density of water. The force 

 of gravity upon its surface is less than two fifths of that upon 

 the Earth ; more accurately, 0'38. That is to say, if a man from 

 the Earth could visit Mars, he would find that his weight had 

 diminished almost two thirds. Members of terrestrial fat men's 

 clubs could become agile dancers by simply going to Mars. This 

 feebleness of the force of gravity must, it is clear, have an impor- 

 tant effect upon the organization of any forms of life that may 

 exist upon Mars, whether animal or vegetable. The mean dis- 

 tance of Mars from the sun is 141,500,000 miles, that of the Earth 

 being 92,900,000. The length of Mars's year is six hundred and 

 eighty-seven days. Its day is only forty-one minutes longer than 

 our day upon the Earth. The inclination of its equator to the 

 plane of its orbit differs but slightly from that of the Earth. But 

 when we come to consider the eccentricity of its orbit, we find a 

 decided difference between the Earth and Mars. The Earth's 

 orbit is so nearly a circle that its greatest and least distances from 

 the sun differ by only 3,000,000 miles, while the orbit of Mars is 

 so eccentric that that planet is 26,000,000 miles nearer to the sun 

 at one extremity of its orbit than at the other. It follows that, 

 while Mars receives, upon the whole, less than half as much light 

 and heat from the sun as the Earth gets, yet that quantity is 

 variable to the extent of about one third of its greatest value — in 

 other words, the sun gives Mars half as much again heat at its 

 perihelion as it does at its aphelion. It is hardly necessary to 

 point out the important climatic effect of such a variation. An- 

 other remarkable resemblance between the Earth and Mars comes 

 in here. Just as on the Earth, the summer of the northern hemi- 

 sphere of Mars occurs when the planet is farthest from the sun 

 and its winter when nearest. The effect, as Mr. Proctor has 

 pointed out, tends to equalize the temperature of the seasons in 

 Mars's northern hemisphere, but to exaggerate their difference in 

 the southern hemisphere. 



We may dwell for a moment upon this last-stated peculiarity, 

 for it is exceedingly interesting in its suggestiveness. Having 

 summer occurring in the southern hemisphere of Mars at the 

 planet's perihelion, and winter at its aphelion, we should find 

 there a most remarkable disparity both in temperature and in 

 the brilliancy of daylight between the two seasons. The differ- 

 ence would be the sum of the effects produced by the greater or 

 less distance of the sun and the variation in the inclination of its 

 rays to the surface of the planet. Since the first cause alone would 

 produce an inequality amounting, in the extreme, nearly to the 

 ratio of 3 to 2, it is evident that the addition of the second would 



