VOL. LXXXV.] PHILOSOPHICAL TRANSACTIONS. 500 



of small differences in the position of planets, may be exemplified from the late 

 eclipse of the sun on the 5th Sept. 1793> when the projections of the moun- 

 tains Leibnitz and Doerfel, bounding the southern edge, were so different from 

 those of the older observations, under a similar variety of circumstances. The 

 above-mentioned conclusion with respect to Venus becomes still more evident 

 and remarkable, from its agreeing more exactly than could be expected, accord- 

 ing to the circumstances, with the period of 23 hours 21 minutes, which, in my 

 memoir on the rotation of Venus, I had determined as near the truth : for on 

 the 27th of February that appearance took place about 40 minutes earlier than 

 the evening before ; and the middle of the time when the southernmost part of 

 the southern horn appeared as a separated point of light (a phenomenon similar 

 to the present), was by that observation at 6^ 29™. From the 27th February, 

 1793, 6^ 29™ p. m. to the 13th March 11^ a. m. there are IS"* 16" SI"", which, 

 with the period of 23*^ and 21"^, are resolved into 14.04 revolutions, exact to 

 the very inconsiderable fraction of -f J-^ ; which is so much the more surprizing, 

 as no attention could be paid to the inequalities. 



April 2d, 6^ 50™ p. m. with power 160 of the ^--feet Schr. it struck me' with 

 uncommon certainty and precision, after so many similar appearances of both 

 horns, that the southern horn was remarkably slenderer in comparison with the 

 northern ; and that in general the whole southern illuminated part appeared 

 considerably smaller than the northern. I tried this phase with 288 and 370, 

 and found it to be assuredly so ; and with the same certainty I observed it also 

 repeatedly confirmed with the noble 13-feet reflector, till 8 o'clock. My attend- 

 ant, who knew nothing of it, made the same remark, and particularly noticed 

 the irregular form of the arch bounding the illumination, which formed a slen- 

 derer horn, as often happens with the moon ; and also in the same manner in 

 its single parts, the crescent of Venus appeared uneven, like that of the moon, 

 though not sharply so, but faintly and undefined. I did not now see the moun- 

 tains of Venus, by their projection and shadow, as in the moon ; but the ap- 

 pearances above described must indisputably have been occasioned by mountain- 

 ous inequalities. Very often have I perceived similar phases on the moon with 

 my naked eye. It would be inexplicable, if different eyes, with different excel- 

 lent telescopes, and various magnifying powers, should have seen for an hour 

 together such an appearance, with equal confidence, and yet the whole be no- 

 thing but a fallacy, misleading a careless observer. Did not Cassini, Bianchini, 

 and other observers, surely not deficient in caution, perceive similar phenomena, 

 and draw the same conclusion ? 



At 8*^ 35"*, Venus presented not a clear image. She had already passed the 

 pleiades about half a degree, and my hope of seeing perhaps an occultation was 

 frustrated. At 10*' 15™, a very instructive observation, by comparison with the 



