VOL. XLVIII.] PHILOSOPHICAL TRANSACTIONS. 420 



be observed that, during both these transits, there was a constant hard gale of 

 wind ; and as lie had, by other observations, formerly found that the images of 

 the planets, in the night-time, did not appear so distinct in windy weather as 

 when it was calm, he therefore imputed the indistinctness of the sun's and Mer- 

 cury's limbs to the air's being agitated by the wind.* Of this we may be made 

 sensible by a familiar instance : 



Suppose a vessel full of water, having any thing lying at the bottom, as a 

 shilling, the water being at rest; you will then perceive the image of the shilling 

 distinctly ; but if you give any commotion to the water, the image of the shilling 

 will then appear indistinct and confused. 



Somewhat analogous to this is this other appearance : if you look through a, 

 telescope at any of the planets, when the stars appear hazy, dim, and languid, 

 you will see them distinctly : but look at them again, when the stars appear most 

 bright and sparkling, you will then find their images less distinct. This may be 

 accounted for by the just-mentioned instance of the vessel of water, by supposing 

 air instead of water. And if we consider the infinite number of heterogeneous 

 particles which continually float in the air, and suppose these to be at rest, or put 

 into motion, we shall find that it is not at all surprizing, that we see the images 

 of objects placed beyond the medium of air, more or less distinct. We are not 

 so sensible of this indistinctness, arising from the agitation of the air, in re 

 fracting telescopes, as in reflectors : because the errors of reflexion, caused by 

 any irregularity in their figure, or confusion in the air, are about 5 or 6 times 

 greater than the same errors in refraction ; even though both telescopes magnify 

 the same number of times; as has long been demonstrated. 



We also took notice of M. de Barro's first phenomenon ; viz. the seeming 

 greater velocity of Mercury when he was near the egress : which we thus ac- 

 counted for. When he was at a considerable distance from the limb, there being 

 nothing near enough to refer his velocity to, he seemed in a manner stationary ; 

 but being advanced near the sun's edge, we could refer his motion to that with 

 ease; which thus becoming sensible, it might be esteemed rapid, in comparison 

 of the former. Mr. S. had often made the same remark on the gradual approach 

 of two luminous bodies, as the appulse of the moon's lucid limb to a star or 

 planet. 



The expedition with which the author observed his 2d phenomenon, is extra- 

 ordinary ; viz. that he should first observe what he names the final contact ; 2dly, 

 that he should take away his green glass ; and thirdly, that he should be able 

 suddenly to alter the conformation of his eye, so as to see distinctly with a much 

 greater influx of light, and then take another observation, and all in the short 



* Since this paper was read, Mr. Short has been informed by M. le Monnier, the French king's 

 aitronomer, that, during the last transit at Paris, they had a hard gale of wind fiom the n. £, — Orig^ 



