334 PHILOSOPHICAL TRANSACTIONS. [aNNO 1793. 



fidence, from the measures I took with the 20-feet reflector, in the morning of 

 the 24th of November, 1791 ; which, when redi'ced to the mean distance of 

 the earth, give 18".79 ^o'" tlie apparent diameter of this planet. This result is 

 rather remarkable, as it seems to prove that Venus is a little larger than the 

 earth, instead of being a little less as has been supposed ; yet, on the nicest scru- 

 tiny, I cannot find fault with the measures. The planet was put between the 2 

 wires of the micrometer, which were outward tangents ; and they were, after 

 each measure, shut so as to meet with the same edge, and in the same place 

 where the planet was measured. In this situation the proper deduction, for not 

 being central measures, was pointed out by the index-plate. The transits of the 

 25th were corrected for a small concavity of the wires, which being pretty thick 

 and stubborn, were not strained sufficiently to make them quite straight, the 

 amount of which was also ascertained by an examination of the division where 

 the wires closed at the ends, and where they closed in the centre. The zero 

 was, with equal precaution, referred to a point at an equal distance from the 

 contact of the wires on each side ; for they are at liberty to pass over each other, 

 without occasioning any derangement. The shake, or play, of the screw is less 

 than 3-tenths of a division. The two planets however are so nearly of an equal 

 size, that it would be necessary to repeat our measures of the diameter of Venus, 

 in the most favourable circumstances, and with micrometers adjusted to the 

 utmost degree of precision, to decide with perfect confidence that she is, as 

 appears most likely, larger than the earth. 



The remarkable phenomenon of the bright margin of Venus, I find, has not 

 been noticed by the author we have referred to : on the contrary, it is said, 

 *' this light appears strongest at the outward limb, from whence it decreases gra- 

 dually, and in a regular progression, towards the interior edge or terminator." 

 But the luminous border, as I have described it, in the observations of the 9th, 

 16th, 20th, and 22d of April, does not in the least agree with the above repre- 

 sentation. With regard to the cause of this appearance, I believe that I may 

 venture to ascribe it to the atmosphere of Venus, which, like our own, is pro- 

 bably replete with matter that reflects and refracts light copiously in all direc- 

 tions. Therefore on the border, where we have an oblique view of it, there 

 will of consequence be an increase of this luminous appearance. I suppose the 

 bright belts, and polar regions of Jupiter, tor instance, which have a greater 

 light than the faint streaks, or yellow belts, o^; that planet, to be the parts 

 where its atmosphere is most filled with clouds, while the latter are probably 

 those regions which are free from them, and admit the sun to shine on the 

 planet ; by which means we have the reflection of the real surface, which I take 

 to be generally less luminous. If this conjecture be well founded, we see the 

 reason why spots on Venus are so seldom to be perceived. For, this planet 



