170 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1920. 



suppose that we see there the engineering works of intelligent beings 

 is merely fanciful. As for the polar caps which form and melt with 

 the Martian seasons, the best opinion is in doubt whether these may 

 be thin deposits of hoar frost from the traces of water vapor in the 

 atmosphere, or frozen carbonic acid gas, which, in view of the low 

 temperature of Mars, is perhaps as probable. 



Mercury being surely uninhabitable, and Mars most certainly in- 

 hospitable, there remains only Venus among the planets as a prob- 

 able abode for intelligent life. Here we must be struck by the favor- 

 able prospect. A twin planet to the earth in size and mass, its high 

 reflecting power seems to show that Venus is largely covered by 

 clouds indicative of abundant moisture ; probably at almost identical 

 temperatures to ours, our sister planet appears lacking in no essential 

 to habitability. 



Some writers have said that owing to the supposed period of rota- 

 tion of Venus on its own axis being equal to its period of revolution 

 around the sun, just as occurs in the case of the moon with respect to 

 the earth, Venus must always present the same side toward the sun, 

 and that therefore the one-half would be in extremest cold and the 

 other in a most blistering heat. Dr. A. Graham Bell has pointed out 

 to me in conversation, however, that this view of things is most 

 improbable. The reflecting power of Venus has been carefully 

 determined and, as given above, lies in the neighborhood of 60 

 per cent. So high a reflecting power demands apparently the exist- 

 ence of clouds, and these clouds can hardly be of any other substance 

 than water. If it were a fact that the rotation period of Venus is 

 equal to its period of revolution around the sun, all of the water 

 would be distilled from the hot side to the cold side and these clouds 

 would disappear. These evidences alone seem to me to be sufficient to 

 overcome the observational evidence which indicates the equality of 

 periods of rotation and revolution. That is dependent on spectro- 

 scopic observations to some extent, and these are not competent to in- 

 dicate more than that the period of revolution is large as compared 

 with our day. They are not accurate enough to show that the period 

 is 225 days, equal to the year of Venus, but it may be anything above 

 10 of our days, as far as the present spectroscopic observations would 

 be accurate enough to indicate. As for the reported observation of 

 markings upon the planet which were said to rotate in 225 days, this 

 observation can only be regarded with the greatest doubtfulness. 



It is only because the clouds have always prevented a telescopic 

 view of its surface that Venus excites no popular interest like that 

 aroused by Mars. If it should be reserved for the early future to 

 exchange intelligence with our nearest planetary neighbor after the 



