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370 PHILOSOPHICAL TRANSACTIONS. [anNO 1753. 



I have now, says Dr. B. before me the copy of a letter of Mr. Gascoigne to 

 Mr. Oughtred, which I made myself from the original, written in 1 640-1 ; 

 which original was in the possession of the late William Jones, Esq. f. e. s. and 

 is now in the library of the Earl of Macclesfield. It consists of several sheets of 

 paper, all about his invention for measuring small angles to seconds ; where he 

 not only gives the geometrical and optical principles of his contrivance, and the 

 construction of the instrument, but also a series of observations actually taken 

 V/itb it ; some of which I shall transcribe. 



l04O, Aug. 5. Jupiter's diameter o' 51" 



Mars's O 38 



Dec. 24. Mars's O 25 



Venus's O 25 



1640, Aug. 25. Moon's semidiam h. 8 p. m. . 15 17 



Sept. 19 15 U 



Oct. 9 h. 8 p. m. . 16 36 



10 16 36 



27 h. 7 p. m. . 15 38 



29 15 41 



30 15 43 



31 15 49 



These may suffice to prove that Mr. Gascoigne's micrometer was not a thing 

 merely in embryo, but brought to a good degree of perfection above 40 years 

 before that of the French gentlemen was ever so much as mentioned. 



XXX. Ohservations of the Transit of Mercury over the Sun, May 6, 1753. 

 By Mr. John Short, F. R. S. p. 192. 



The instrument prepared for these observations was a reflecting telescope, of 

 2 feet focal length, of the Gregorian form, magnifying about 65 times, and so 

 constructed in its machinery as to move in a plane parallel to the horizon, and 

 also to move in a plane parallel to the equator. This telescope had 2 eye-pieces, 

 each a combination of 2 glasses, viz. one eye-piece for the horizontal motion, 

 with wires at right angles to each other, the wires being between the glasses, 

 and one of the wires placed parallel to the horizon, and consequently the other 

 was vertical ; the other eye-piece was also a combination of 2 glasses, and adapted 

 to a micrometer, the glasses being placed between the wires of the micrometer, 

 and the eye of the observer, and was to be used when the telescope moved in a 

 plane parallel to the equator. 



Mr. Short's house in Surry-street, being so situated as not to see the sun at 

 rising, the Rev. Dr. Birch, was pleased to allow the use of the leads on his 

 house in Norfolk-street, from whence we should be enabled to see the sun soon 



