686 PHILOSOPHICAL TRANSACTIONS. [aMNO 1 768. 



well acquainted with astronomy and astronomical calculations, who has made 

 and examined many belonging to the Nautical Almanac, and has been so oblig- 

 ing as to come here and assist in making astronomical observations, during the 

 absence of Mr. M.'s assistant, Mr. Wm. Bayley, who is gone to the North 

 Cape, by appointment of the K. s., to observe the transit of Venus there. The 

 others are, the Rev. Wm. Hirst, who observed the former transit of Venus 

 in 1761, at Madras; John Horsley, Esq. a gentleman whom Mr. M. had the 

 pleasure of first commencing an acquaintance with during his voyage from St. 

 Helena to England, in the Warwick East India ship, and who then, and in se- 

 veral voyages since to the East Indies and home again, observed and calculated 

 the longitude from distances of the moon from the sun and fixed stars with the 

 greatest accuracy; Mr. Samuel Dunn, who has had a good deal of practice in 

 making astronomical observations, and who carefully observed the former transit 

 of Venus, in 1761, at Chelsea; Mr. Peter DoUond, whose great skill in con- 

 structing achromatic and reflecting telescopes ; and Mr. Edward Nairne, whose 

 skill likewise in the same way, and in making all kinds of mathematical and phi- 

 losophical instruments, are sufficiently known to the public. 



Mr. Horsley and Mr. Dunn observed with Mr. M. in the great room ; Mr. 

 Hitchins and Mr. Hirst in the eastern summer-house ; and Mr. Dollond and 

 Mr. Nairne in the western summer-house ; by 3 clocks placed in the respective 

 rooms, which were compared with the clock in the transit room, before the ex- 

 ternal contact, and again after the internal contact was passed ; whence the times 

 of the observations, as noted by the clocks, were reduced to the time of the 

 transit clock, and thence to apparent time. The observations are given in the 

 following table, as reduced to apparent time. 



External Regular cir- Threadof light com- '; MagnU 



contact cumferences pleted, or the in- Telescope made use of. fying 



in concact. ternal contact. power. 



N. Maskelyne 7' 10"" 58' 7' SB" 31' 7" ■29" 23' 2 feet reflector 140 



Mr. Hitchins 7 10 5+ 7 28 47 7 28 57 6" f. reflector «J0 



W. Hirst 7 11 11 — 7 29 18 2 f. reflector 55 



J. Horsley 7 10 44 7 28 15 7 29 28 10 f. achromatic 50 



S. Dunn 7 10 37 7 29 28 7 29 48 3j f. achromatic 140 



P. Dollond 7 11 19 — 7 29 20 3i f. achromatic 150 



E. Nairne 7 11 30 — 7 29 20 2 f. reflector 120 



Mr. Dollond and Mr. Nairne used telescopes of their own construction ; but 

 they did not wait till the thread of light was formed at the internal contact, but 

 noted the time when they judged it was just ready to be formed. The 34- feet 

 achromatic telescopes were those made with 3 object-glasses. 



The differences between the different observations seem pretty considerable, 

 and greater than Mr. M. expected, considering that all the telescopes may be 

 reckoned pretty nearly equal, excepting the 6-feet reflector, which is much su- 



