VT)L. I.] PHILOSOPHICAL TRANSACTIONS. 81 



But observing March 21, I was surprised to find the air, though not so clear 

 as to the appearance of small stars, so exceeding transparent, and the face of 

 Mars so very well defined, and round and distinct, that I could manifestly see 

 it of the shape in E, about half an hour after nine at night. The triangular 

 spot on the right side (as it was inverted by the telescope, according to the ap- 

 pearances, through which all the preceding figures are drawn) appeared very 

 black and distinct, the other towards the left more dim ; but both of them 

 sufficiently plain and defined. About a quarter before 12 o'clock the same 

 night, I observed it again with the same glass, and found the appearance exactly 

 as in F ; which I imagined to show a motion of the former triangular spot. 



Also March 22, about half an hour after 8 at night, finding the same spots 

 in the same posture, as at G, I concluded that the preceding observation was 

 only the appearance of the same spots at another height and thickness of the 

 air ; and thought myself confirmed in this opinion, by finding them in much 

 the same posture March 23, about half an hour after Q, as at H, though the 

 air was not so good as before. 



And though I desired to make observations about 3 o'clock those mornings, 

 yet something or other inter\-ened, that hindered me, till March 28, about 

 3 o'clock, the air being light in weight, though moist and a little hazy, 

 when I plainly saw it to have the form represented in I ; which is not recon- 

 cileable with the other appearances, unless we allow a turbinated motion of 

 - Mars upon its centre : which, if such there be, from the observations made 

 March 21, 22, and 23, we may guess it to be once or twice in about 24 hours, 

 unless it may have some kind of librating motion ; which seems not so 

 likely. 



Observations made in Italy, confirming the former, and fixing the 

 Period of the Revolution of Mars. By J. D. Cassini. N° 14, 

 p. 2^2. 



That with a telescope of 24 palmes, or of about 1 6 feet, wrought after S. 

 Campani's way, he began to observe February 6, l666, N. S. in the morning, 

 and saw two dark spots in the first face of Mars, as represented at K, pi. 2, 

 %. 2. 



That with the same glass he observed Feb. -14, in the evening, in the other 

 face of this planet, two other spots, like those of the first, but larger ; as L. 



That afterwards continuing the observations, he found the spots of these two 

 faces to turn by little and little from east to west, and to return at last to the 

 same situation wherein he had seen them first. 



VOL. I. L 



