It is remarkable, that the erroneous reasoning of Flam stead appears not to have been noticed by the mathema ticians and astronomers of England. Dr. Wallis publish ed, in the third volume of his works, the letter of Flam stead, in which a very minute account of his observations is given, without any remark as to its inconclusiveness. Even Dr. IIalley appears to have acquiesced in the re sults, which, considering his known opposition to Flam stead, is not easily accounted for.
Roemer seems first to have pointed out the error, in a letter to Flamstead himself; and afterwards James Cassini sheaved at length, in the Memoirs of the French Academy, 1699, the mistake of Flamstead. Cassini himself was soon to have the inconclusiveness of his own attempts to find the parallax of Sirius shown by Dr. Halley.
Roemer, who has so much distinguished himself by adopting and establishing the discovery of the velocity of light, after it had been abandoned by its original author, Dominic Cassini, appears to have exerted himself for many years in attempting to observe a parallax in the fixed stars. Instead of endeavouring to find the changes of declination, as Hooke and Flamstead had done, he ob served the differences of right ascensions of opposite stars. He pursued this method for seventeen or eighteen years, and conceived he had found such results, as fully proved the existence of a sensible parallax in the fixed stars. He found the sum of the parallaxes of Sirius and a Lyra, which two stars are nearly opposite in right ascension, exceeded half, but was less than three-fourths of a minute. But lie died just as he was about to publish his observa tions and conclusions.
Horrcbow, who had assisted in the observations of Roemer, published them in 1727, with the title of " Co pernicus Triumphans." He himself had continued the observations, and his two sons also continued them for some years.
There can be no doubt that the discovery of Bradley put an end to these attempts, in which the errors of ob servation, and actual changes from aberration, Exc. must have been so mixed together, as to have deceived Roemer. It is otherwise difficult to account for his mistake as to the sum of the parallaxes of Sirius and Lyrx, which ap pears to have some reference to aberration. He had cor
rected the errors of Flamstead, and stems to have fallen into a similar one himself.
The two stars, Sirius and a Lyrae, from their difference of declination, were ill chosen. Even now, with the most improved transit instruments, the errors of adjustment would have too great an influence.
In 1714, James Cassini, by means of a fixed telescope, endeavoured to observe the parallax of Sirius. The fu tility of his attempt has been fully explained by Dr. Halley, in the Phil. Trans. vol. xxxi. It would not hare been expected, that at this period the effects of refraction were so little known, that the changes of the meridian altitude of Sirius, (the height of which at Paris is only about twenty-five degrees,) should have been attributed entirely to parallax. But the effects of refraction seem then to have been much better known by the English than by the French astronomers. Cassini concluded, from his obser vations, that the variation in the meridian altitude of Si rius during the whole year, was only about five or six seconds. What then can be thought of Cassini's obser vations, when we now know that there must, independent ly of the changes of refraction, have been a change of at least twenty-three seconds, the star being twenty-three seconds more southerly in the end of March than in the end of September ? This objection could not have been made by Halley, as the effects of aberration were then unknown.
We now arrive at the period when the discoveries of Bradley put an end to all expectations of finding a paral lax greater than a few'seconds. In 1725, Mr. Samuel Molyneaux, (the son of the celebrated William Moly neaux,) and Dr. Bradley, engaged in observations on Draconis, with a view of verifying the observations of Hooke, in 1669, on the same star. Mr. Molyneaux adopted an instrument, (the zenith sector,) constructed nearly on the same principles as that of Dr. Hooke, but far exceeding it in exactness. This superior degree of exactness was owing to the skill of the celebrated Mr. Graham. It cannot be doubted that a vast improvement had been made in the execution of instruments since the time of Hooke.