VOL. I.] PHILOSOPHICAL TRANSACTIONS. 69 



mine that the period of this conversion is made in the said space of 24 h. 40m ; 

 and not oftener than once within that time ; alleging for proof; 



1 . That whereas Feb. 6, N. S. he saw the spots of the first face of Mars 

 moving from eleven o'clock in the night until break of day, they appeared not 

 afterwards in the evening after the rising of that planet ; whence he infers, that 

 after 12 hours and 20 minutes, the same spots did not come about; since that 

 the same which in the morning were seen in the middle, upon the rising of 

 Mars, after 13 or 14 hours might have appeared near the occidental limb. But 

 because he might be imposed upon by vapours, whilst Mars was yet so near 

 the horizon, he gives this other determination, viz. 



2. Whereas he saw the first face of Mars the 6th of February at 1 1 o'clock 

 of the night following ; he did not see the same after 1 8 days at the same hour, 

 as he ought to have done, if the period is performed in the space of 12h. 20m. 



3. Again, whereas he saw, Feb. 24 in the evening, the other face of Mars, 

 he could not see the same the 13th and 15th day of March, to wit, after 17 

 and 19 days, as he should have done, if the revolution were made in the newly 

 mentioned time. 



4. Again, whereas the 27th of March in the evening he saw the second face 

 of Mars, he could not see it the 14th and l6th of April. 



From all which observations he judges it to be evident, that the period of 

 this planet's revolution is not performed in the space of 12 hours 20 min. but in 

 about 24 hours 40 min. more exactly to be determined by comparing distant 

 observ^ations : and that those who affirm the former must have been deceived 

 by not well distinguishing the two faces, and by having seen the second, mistak- 

 ing it for the first. 



All which he concludes with this hint, that, when he defines the time of the 

 revolution of Mars, he does not speak of its mean revolution, but only of that 

 which he observed whilst Mars was opposite to the sun, which is the shortest 

 of all.* 



The Phases of the Planet Jupiter. By Mr. Hook. N" 14, p. 245. 



Anno 1666, June 26th, between three and four o'clock in the morning, I 

 observed the body of Jupiter through a 60 foot-glass, and found the apparent 

 diameter of it through the tube to be somewhat more than two degrees, that 

 is, about four times as large as the diameter of the moon appears to the naked 

 eye. 



* The more nice observations of later observers have proved that Cassini was right in his suspi- 

 cions above expressed j the period of the rotation of Mars having been settled by modern astronomers 

 at 24h. 39m. 22 8. 



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