VENUS AND MERCURY. I4J 



the determination of the period of rotation is impossible. 

 Indeed, it may be remarked that the sun does not rotate 

 in the same manner as a solid globe would necessarily 

 have to do. 



The difficulty about finding the length of the day of 

 Venus simply arose from the fact that the marks on the 

 planet seemed too vague and faint to be relied upon for 

 the purpose. It is true that certain observations of this 

 kind had been made in former years, and it had been con- 

 jectured that the length of the day on Venus was about 

 equal to that of the earth. The question would be, of 

 course, settled if there were some great mountains stand- 

 ing out from Venus which every one could see through 

 their telescopes ; or if there were any black spot or other 

 unmistakable mark, it would answer the purpose. Unfor- 

 tunately, there are no such features discernible none, at 

 least, that are perceptible to ordinary eyes furnished with 

 ordinary telescopes and used in those latitudes on the 

 earth where astronomical observations are most abundant. 

 Much of what we have said with regard to Venus may be 

 applied to the planet Mercury, which lies still nearer to 

 the sun. The period of rotation of both these planeta 

 has been until lately a matter of much uncertainty. 



We are, however, recently indebted to the keen eyes of 

 Professor Schiaparelli, of Milan, for a minute scrutiny 

 of Mercury, on which he has discerned features suffi- 

 ciently recognisable to enable him to solve the great 

 problem of the length of the day on our neighbouring 

 world. He has shown that the earlier notions are appa- 

 rently erroneous. It would have been something to have 

 learned even thus much ; but the positive results at which 



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