1912-1913-] Some Astronomical Problems. 43 



extent that even the blaze of an equatorial sun is not able to 

 make good the loss. 



We see, then, that efficiently as the earth is protected, as a 

 whole, from the great cold of space, we nevertheless have 

 great extremes of climate, and we are able to learn also that 

 under these extreme conditions life can be well maintained. 

 The most energetic forms of life are found in the temperate 

 parts of the earth, life in the torrid zone being distinctly 

 more languid ; but the severe conditions found in the arctic 

 regions do not forbid the existence of many varied forms. 

 This fact should be taken into consideration when dealing 

 with the problem of life in other planets. At the same time, 

 the immense importance of an atmosphere, the density of 

 which must not be less than half that of ours, and a large 

 expanse of ocean, to make possible any form of life such as 

 we know, even of the lowliest kind, must, not be lost sight of. 



Now there are three planets besides our earth which may 

 be called terrestrial, as having a certain likeness to our world. 

 They all lie within the great gap which separates Mars and 

 Jupiter, — Mercury, Venus, and Mars. 



To some people the great question of astronomy appears to 

 be, Is Mars inhabited ? Venus is really a world much more 

 like our own than is Mars. She is nearly the same size, and 

 without doubt has a sufficiently dense atmosphere, but her 

 greater proximity to the sun gives her a supply of light and 

 heat about double what we receive. Supposing that Venus 

 revolves on her axis in twenty-four hours or so, she might 

 very well be the seat of life as far as favourable conditions 

 are concerned. There is a doubtful point, however, about this 

 planet. The time of rotation on the axis has not yet been 

 determined; observers have given times varying from 22 

 hours to 227 days. No definite markings on the disc 

 can be observed by which the rotation can be fixed, but the 

 balance of opinion seems to be that Venus turns always one 

 face to the sun, and revolves once only on her axis during 

 her whole period of revolution. If this should be an estab- 

 lished fact, it would of course prove fatal to the thought that 

 life of any kind could be found there. 



Mercury is in a hopeless condition, being so much nearer 

 the sun than Venus, and, moreover, in all probability turning 



