VOL. LII.] PHILOSOPHICAL TRANSACTIONS. 553 



having taken off the micrometer, with the 2 feet reflector, Mr. Bird, mathematical 

 instrument-maker in the Strand, with a reflector 1 8 inches focal length, ofliis own mak- 

 ing, and Mr. B. with the refractor, the telescopes used by Mr. Bird and him magnify- 

 ing about 55 times, that by Mr. Green 1 '20 times, June 5th 1761, at 20*^ 19™ 00' 

 apparent time, all three agreeing to the same second. The final egress by Mr. 

 Green and Mr. Bliss, was only one second later than by Mr. Bird, at 20*^ 37"™ 9* 

 apparent time. At 20^ 26"^ 56', by the mean of 5 observations, the centre of 

 Venus was north of the sun's south limb in declination, by his micrometer, 3' 20''. 

 The diameter of Venus was once measured by Mr. Green, with Dollond's micro- 

 meter, 57'' ; by Mr. Canton in Spital-Square, being the mean of 3 good observa- 

 tions, with the same kind of micrometer, 58". The sun's horizontal diameter 

 was observed by Mr. Bird, with the reflector, 3l' 36", which Mr. B. suspects is 

 3 or 4 seconds too large, as the telescope was not accurately adjusted for parallel 

 rays to his eye, 



The internal contact was observed by Mr. Hornsby, on the north side of the 

 observatory at Shirburn-Castle, with an excellent 12 feet telescope and micro- 

 meter, made by Mr. Bird, of the old form ; and by Mr. Phelps, on the south side, 

 with a 1 4-feet telescope ; the telescope used by Mr. Hornsby magnifying 68 

 times, and that by Mr. Phelps about 55 times ; by Mr. Hornsby at 20^ 15"" 10* 

 apparent time, by Mr. Phelps 4 seconds later, Mr. Bartlet counting the clock, 

 which each observer could hear. Mr. Phelps lost the final contact by mistaking 

 the teller of the clock. Mr. Hornsby makes it at 20^ 33™ 17' ; but supposes it 

 to have happened a few seconds later; for at 20^ 33"" 12^, it was not quite gone 

 off the sun, when he was obliged to move his eye-stand, and at 20* after, it was 

 certainly totally emerged. They make the diameter of Venus 56'^, and Mr. 

 Hornsby, by a mean of 12 observations, made a little before and after the noon of 

 the 5th, makes the diameter of the sun at right angles to the equator, with his 

 micrometer, 3l' 32*. At 20^ 12"^ apparent time, Mr. Hornsby, by one obser- 

 vation, makes the centre of Venus north of the sun's south limb in declination, 

 3' 26". The latitude of the observatory at Shirburn-Castle is 51^ 39' 22'^, being 

 to the north of the Royal Observatory lO' 43''. The difference of longitude 

 between them has been determined, by some former observations, to be 4' 1^, 

 Shirburn being to the west. 



XXXI I I. The Transit of Venus over tl\e Sun, June 6, 176], at Savile- House, 

 about 8' of Time West of St. PauVs^ London. By Mr. Ja. Short, p. 178. 



The instruments, made use of on this occasion, were a reflecting telescope of 

 18 inches focus, with a helioscope adapted to it, and having a field of more than 

 the sun's diameter, proper for showing Venus on the sun's disk, with great ease 



VOL. XI. 4 B 



