AND THE OTHER PLANETS OF THE SOLAR SYSTEM. 663 



time, though it must produce a great effect on their internal temperatures at points sufficiently 

 remote from their surfaces. It is easy to imagine that these remoter planets may be older 

 than the Earth, as they must be, for instance, according to Laplace's theory or any analogous 

 one ; but no hypothesis can be more perfectly arbitrary, or less sanctioned by all theory, than 

 that of the more recent formation of the remoter than of the nearer planets. For these reasons 

 I consider that the cause now spoken of cannot be admitted among those which can be 

 regarded as exercising any probable influence on the present superficial temperatures of the 

 planets. 



32. The planet Mars is usually recognised as having more in common with the Earth 

 than any of the other planets. It receives a sufficient quantity of heat from the Sun to mark 

 the different seasons of its year, and the inclination of the axis of rotation to a perpendicular 

 to the plane of its orbit, which is about 28°. 42', does not differ much from the corresponding 

 inclination in the case of the Earth. Its mean distance from the Sun is to that of the Earth 

 in the ratio of 1,52 : 1, so that the intensity of solar radiation at the distance of Mars to that 

 at the distance of the Earth from the Sun, is nearly as the ratio ,44 : 1. Hence taking the 

 value of h at the equator and pole of the Earth as 36" (C) and 12°,4 respectively, we may take 

 the corresponding values for Mars as 16° and 5°,5. The value Q at the equator of Mars will 

 be rather less than ,9591, its value at the Earth's equator, on account of the greater obliquity 

 of Mars. Without farther calculation we may assume it to be ,9. At the pole it is equal to 

 the sine of the obliquity = ,48. Hence we have 



hQ = 14°,4 (C) at the equator of Mars, 

 and = 2°,6 pole. 



Now in the entire absence of the Sun's influence, the uniform superficial temperature of 

 the planet, with an atmosphere equal to that of the Earth, would = - 39°,5 (C) (Art. 25). 

 Hence if the Earth with her present atmosphere had been placed in the orbit of Mars, with an 

 obliquity equal to that of Mars, the mean temperature of the equator would have been 



= - 39°,5 + ZhQ, 



= - 10°,7 (C), 



= - 12«,7 (F) ; 



and the mean temperature of the pole would have been 



= - 39°,5 + 5°,2, 



= - 34P,3 (C), 



= - 29°,7 (F). 



If, however, we allow to Mars an atmosphere similar to that of the Earth but of greater 

 thickness, to the amount of some 15000 or 20000 feet, the mean temperature may be 60° (F), or 

 15°,5 (C) at the equator, and about 14° or 15° (F) at the pole, supposing the additional 

 atmosphere to have rather less effect at the pole than at the equator. Any greater increase of 

 temperature would, of course, result from a still greater increase of atmosphere. 



Vol. IX. Paet IV. 85 



