VOL. XV.] PHILOSOPHICAL TRANSACTIONS. ihf 



had with him a very good instrument of his own, furnished with telescopic 

 sights, the better to compare together the two ways of observation. 



The same day that Mr. Halley arrived, he did in Mr. Halley's presence, as a 

 specimen of his manner of observing, take, with his large brass sextant, the 

 distance of Regulus and Spica ; which he found to be 54° l' 55", the same 

 exactly, even to seconds, which he had six times before observed in the years 

 ]658, 1661, 1671 : at which Mr. Halley being much surprized, it was again 

 for his greater satisfaction, that he might not think it only a piece of good luck, 

 observed the same exactly, June 1, and again June 7. 



Many more observations were made from day to day, during all the time of 

 his abode there, such particularly as Mr. Halley directed. To which he at- 

 tended constantly, and strictly exanjined them, that he might not by any mis- 

 take be imposed on. Which observations are all here particularly set down- 

 And among the rest, a list of several observations made by the author, with 

 a small instrument and common sights, and the same made by Mr. Halley, with 

 a much larger, and telescopic sights : by which the author thinks it will appear 

 to an indifferent judge, that those performed by the plain sights, though in a 

 smaller instrument, are the more accurate. And he professes that he could not, 

 with Mr. Halley's instrument, make observations with equal accuracy and quick- 

 ness, as with his own. 



Upon the whole matter, Mr, Halley finding every thing to his satisfaction, 

 and beyond his expectation, thought fit to leave in writing his attestation there- 

 of, in form of a letter to the author, dated July 8-18, 1679, declaring himself 

 "abundantly satisfied of the use and certainty of these his instruments and 

 observations. And whereas he had before been always doubtful, that his ob- 

 servations by naked sights might, as to some minutes, be uncertain ; and had 

 therefore wondered why he declined the use of telescopic sights; he had, 

 partly to gratulate the author's publishing of his observations, and partly to 

 satisfy his own scruples, undertaken that journey ; which he now considers as 

 no small happiness; and declares himself abundantly pleased with it. And offers 

 himself a voluntary witness, of the almost incredible certainty of those instru- 

 ments.' As having seen, with his own eyes, not one or two, but a multitude 

 of observations of the fixed stars, performed with his great sextant, even by 

 divers observers, and by himself sometimes, though less expert therein, being 

 often repeated, most accurately, and almost incredibly to agree ; and never to 

 differ more than by an inconsiderable part of a minute." With further expres- 

 sions of joy and admiration ; as wondering at nothing more, than to find them 

 so accurate. 



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