VOL. LXXXV.] PHILOSOPHICAL TRANSACTIONS. 50? 



to do, my more recent observations, which confirm the former ones, and seem 

 to me very important. Mr. S. here enumerates several instances and expressions 

 in Dr. Herschel's paper, which he endeavours to show are either misrepresenta- 

 tions, or ineffectual in proof of the mistakes in Mr. S.'s paper. 



He then enters on what he calls new observations, confirming the rotation of 

 Venus, her mountainous inequalities, and the twilight of her atmosphere. 

 This collection of observations is very long and numerous, too much so indeed, 

 as well as too unprofitable, to be here given particularly. After recording 

 some of the observations, Mr. S. adds : Whoever is pleased to compare these 2 

 observations impartially, I doubt will not consider them as illusions. To me 

 they rather appear, in more than one respect, convincing and important. In 

 the first evening, the southern horn, as 1 observers agreed, changed its form 

 very quickly, that is in 15 minutes, so much, that the difference between it and 

 the northern was not nearly so striking as before. In the 2d evening, the air 

 being clearer, and the image excellent, this change was still quicker ; for in 1 1 

 minutes, during the observation itself, the end passed very evidently to the form 

 of a separate point of light. Supposing both changes to be the same, and pro- 

 duced by the rotation, the alteration to a separate point of light must have hap- 

 pened on the first evening, at most 1 1 minutes later than 6'' 40*", when I inter- 

 mitted my observation ; that is, about 6'' 51"" ; because on the 2d evening it 

 took place in 1 1 minutes. But on the 2d evening, when I noticed this striking 

 alteration, I no longer knew the time marked the evening before, and I now 

 noted down 6'' 11™. Consequently, this change took place the 2d time very 

 nearly in 24 hours less 40 minutes ; and from these 2 careful observations only 

 we may conclude, very probably, the rotation to be nearly 23*" 20™ ; which agrees 

 extremely well with the approximate period of 1'^ 21™, which I have deduced 

 from observations of 2 years, in my circumstantial memoir already quoted. 



On another observation, Mr. S. remarks : As our own atmosphere was then 

 very clear, that of Venus also seemed to be purer than usual ; for with both re- 

 flectors, and particularly with the 13-feet, Dr. Chladni, as well as myself, enjoyed 

 a magnificent view of the arch of illumination, which seldom presents itself so 

 well to the eye, the image being uncommonly clear and distinct. To both of 

 us the boundary of illumination, toward which the light became very dim, ap- 

 peared (be it ever so much contradicted) not only nebulous, and not sharply 

 terminated, though sensibly sharper than usual, but also very evidently unequal 

 and rugged, with faint shades between, as I have often seen it, but never so 

 plainly. In truth, the appearance, as each declared, was very like the image of 

 the moon at the time of her quadratures, only that the boundary of light was 

 sensibly less sharp, and the faint shadows between were not almost black, but in 



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