244 PHILOSOPHICAL TRANSACTIONS. [aNNO I716. 



superior planet, and accompanied with so remarkable a satellite. Therefore, at 

 a mean, supposing the earth's semi-diameter, seen from the sun, or which is 

 the same thing, the sun*s horizontal parallax, to be 12 seconds and a half, the 

 moon will be less than Mercury, and the earth larger than Venus, and the sun's 

 distance from the earth come out nearly 16500 semi-diameters of the earth. 

 I shall admit of this distance at present, till its precise quantity be made to 

 appear more certain by the trial I propose; not regarding the authority of 

 such as set the sun at an immensely greater distance, relying on the observa- 

 tions of a vibrating pendulum, which do not seem accurate enough to deter- 

 mine such minute angles; at least, such as use this method will find the 

 parallax sometimes none at all, and sometimes even negative; that is, the 

 distance will become either infinite, or more than infinite, which is absurd. 

 And it is scarcely possible for any one certainly to determine, by means of in- 

 struments, however nice, single seconds, or even 10 seconds; and therefore, 

 it is not at all surprising, that the exceeding minuteness of such angles has 

 hitherto baffled the many and ingenious attempts of artists. 



While I was making my observations in the island of St. Helena, about 40 

 years since, on the stars round the south pole, I happened to observe, with the 

 utmost care. Mercury passing over the sun's disk : and contrary to expectation, 

 I very accurately obtained, with a good '24-foot telescope, the very moment in 

 which Mercury, entering the sun's limb, seemed to touch it internally, as also 

 that of his going off; forming an angle of internal contact. Hence I discovered 

 the precise quantity of time the whole body of Mercury had then appeared 

 within the sun's disk, and that without an error of one single second of time; 

 for, the thread of solar light, intercepted between the obscure limb of the 

 planet, and the bright limb of the sun, though exceedingly slender, affected my 

 sight, and in the twinkling of an eye, both the indenture made on the sun's 

 limb by Mercury entering into it, vanished, and that made by his going off, 

 appeared. On observing this I immediately concluded, that the sun's parallax 

 might be duly determined by such observations, if Mercury, being nearer the 

 earth, had a greater parallax, when seen from the sun; for, this difference of 

 parallaxes is so very inconsiderable, as to be always less than the sun's parallax, 

 which is sought; consequently, though Mercury is to be frequently seen within 

 the sun's disk; he will scarcely be fit for the present purpose. 



There remains therefore Venus's transit over the sun's disk, whose parallax, 

 being almost 4 times greater than that of the sun, will cause very sensible dif- 

 ferences between the times in which Venus shall seem to pass over the sun's 

 disk in different parts of our earth. From these differences, duly observed, 

 the sun's parallax may be determined, even to a small part of a second of time; 



