52 PHILOSOPHICAL TRANSACTIONS. [aNNO 1763. 



lax ought also to be 0".17 less. If then this correction be admitted, which is 

 warranted by the best observations, the sun's horizontal parallax will be Q".Q2. 



There is still another method by which we are enabled to determine the sun's 

 parallax, by comparing the observations made in different places, where the 

 effect of parallax on the planet is considerable at the times of the two contacts. 

 It was more convenient to make use of the 2d internal contact for this purpose, 

 and the observers were very advantageously stationed at St. Helena and the Cape 

 of Good Hope : for by comparing the observations made there, with those at 

 Tornea°, Tobolski, and in some of the eastern parts of Asia, the difference of 

 the times of the contacts, when reduced to the same meridian, will be found to 

 be very considerable, amounting to more than Qi minutes at the first two places 

 above mentioned, and being greater, as the places are farther situated to the 

 north-east. But if this method be used, it is absolutely necessary that the lon- 

 gitudes of the places should be determined with the utmost accuracy, since an 

 error of a few seconds would have a considerable influence on the result, and 

 would increase or diminish the quantity of the sun's parallax, in proportion. 

 The unfavourable state of the heavens at the time of the internal contact pre- 

 vented the Rev. Mr. Maskelyne from making an observation at the Isle of St. 

 Helena; which is the more to be lamented, as his observation would have con- 

 firmed or corrected the observation at the Cape if necessary; since the effect of 

 parallax at both places would have been very nearly the same. The observers at 

 the Cape were more fortunate, and differed only 4' in their observation of the 

 internal contact. But before we proceed to deduce the quantity of the sun's 

 parallax, by comparing as well the observation made at Greenwich as those at 

 other places, with the observation at the Cape, it will be necessary to lay before 

 the reader the authorities on which the longitude of each place has been deter- 

 mined. 



The longitude of the Cape of Good Hope was not even nearly known till the 

 Abbe de la Caille went thither in the year 1731. By a comparison of Q eclipses 

 of Jupiter's satellites, as well immersions as emersions, observed at the Cape, 

 with the corresponding observations made at Paris, the Cape was found, by the 

 Abbe de la Caille himself, to be l*" 4" 14' to the east of Paris, or l*" IS" 31' to 

 the east of Greenwich. Messrs, Mason and Dixon observed many eclipses of 

 Jupiter's satellites at the Cape, but the weather was not so favourable here in 

 England. However, by comparing 4 observations made in Surry-street, and 

 one at Greenwich, with those made at the Cape, the difference of longitude at 

 a mean is found to be l'' 13"" 28', which Mr. H. has used in the following com- 

 putations. 



' The internal contact, as reduced from sidereal to apparent time by Mr. Ma- 

 son, happened at 21*' 39"" 52^ But on examination it will be found to have 



