TOI.. LIX.] PHILOSOPHICAL TRANSACTIONS. (JqI 



separation from divers necessary means of information, render him, in his own 

 opinion, very unfit for literary inquiries. 



LXFII. Observation of the Transit of Mercury over the Sun, Oct. ^5, 1743, 

 By John Winthrop, F. R. S., New England, p. 505. 



11' This observation, Mr. W. thinks, will determine the longitude of Cambridge, 

 New England, with more exactness than any of the observations that have been 

 used for that purpose. He adjusted his clock by correspondent altitudes of the 

 bright star of Aries, taken the night before the transit with a quadrant of 2 feet 

 radius; and on the day of the transit by correspondent altitudes of the sun; all 

 of which agreed within 5'; and allowed for the difference of the sun's declina- 

 tion, morning and afternoon. The morning was fair and calm, but hazy; yet 

 he had a good view of the planet, and with a 24 feet telescope observed that at 

 gh lym 5s ^ i„ his egress touched 0's limb. At S*" 18"* 58' it went off entirely. 

 Mr. W. could not be so certain of the moment when the planet left the sun, 

 as of its interior contact. For the sun's limb, undulating in the vapours of the 

 horizon, made it somewhat difficult to judge when the indenture, formed by the 

 planet, entirely ceased. However, he believes this latter observation may be 

 relial on to 4 or 5S The comparison of this observation with those made in Europe 

 will, he presumes, determine the difference of meridians within a few seconds of 

 time. 



LXVIII. A Method of fVorking the Object Glasses of Refracting Telescopes 

 Truly Spherical. By the late Mr. James Short, F. R. S.* p. 507. 



Prepare two plates or tools of brass, the one convex, and the other concave 

 being both portions of a sphere of the same radius as the focal length of the 

 object glass wanted, or rather of a radius somewhat longer than the focal length 

 wanted, for a dioptrical reason ; let these plates or tools be between 2 and 3 times 

 the breadth of the object glass desired; or, in long focal lengths, twice the breadth 

 will be sufficient: let these tools be of a sufficient thickness in proportion to their 

 breadth or diameter, and let them be ground with fine emery exactly true to one 

 another, working them alternately, the one above the other, to preserve the same 

 focal length; or, if it is desired longer, you must work the convex above the 

 concave; or, if desired shorter, you must grind the concave above the conxex. 



After this, prepare another brass-plate or tool, of the same breadth and thick- 

 ness as the two former, and of the same radius of concavity; its being truly 

 turned on a lathe will be sufficient for this purpose; which tool is to serve after- 



• This paper, which was delivered, sealed up, by Mr. Short, at the society, on the 30th of April, 

 1752, was, after bb death, opened by the council, and ordered to be printed. — Orig. 



