The instrument was erected at Hesse in 1725, and was 241 feet long. The observations having commenced in December, when the parallax was nearly greatest, no per ceptible change was to be expected for some time. On the contrary, the observations having accidentally been continued, a perceptible change was soon seen, and that not according with parallax. A continuation of the aber rations for a year, exhibited a series of phenomena entire ly inexplicable by any theory then known.
of the death Mr. Molyneaux, Dr. Bradley, to pursue this inquiry with more ease and convenience to himself, erected at Wanstead an instrument 12.4 feet ra dius by the help of the same excellent artist Graham. With this instrument he was able to extend his observa tions about six degrees from the zenith, whereas Mr. Molyneaux's instrument, fitted up for 7 Draconis, only extended a few minutes from the zenith.
About three years from the commencement of the ob servations with Mr. Molyneaux's instrument, 1)1.. Bradley completed his beautiful discovery of the aberration of light.
This discovery rendered it highly probable that the parallax of y Draconis did not amount to a single second. All the changes of place that had been observed in y Draconis had been exactly deduced from the theory of aberration.
Thus, as far as parallax, was concerned, the observa tions were against its existence, as it could not be made perceptible with these most exact instruments. Dr. Brad ley playfully remarks, " There appearing, therefore, after all, no sensible parallax in the fixed stars, the Anti-Coper nicans have still room, on that account, to object against the motion of the earth ; and they may have, if they please, a much greater objection against the hypothesis by which 1 have endeavoured to solve the fore-mentioned pheno mena, by denying the progressive motion of light, as well as that of the earth.
"But I do not apprehend either of these postulates will be denied me by the generality of the astronomers and philosophers of the present age." (Phil. Trans. vol. xxxv.) Dr. Bradley having experienced the exactness of his own observations with a telescope so much shorter than that of Hooke, expresses his surprise that 'Hooke, after all the care he pretends to have taken, should so egregi ously have been deceived. He at the same time acknow ledges the goodness of Flamstead's observations, and their accordance with the laws of aberration.
When it was found that the phenomena of the aberra tion bad been observed so many years before by Flam stead, a claim of a prior discovery was put in for the French astronomer Picard. Indeed, it was almost impos
sible to suppose that observations of tolerable exactness should be made without the aberration appearing an un known cause of irregularity. Roemer observed it; and all those who, for 50 or 60 years preceding Bradley's dis covery, attempted the discovery of the parallax, must have done the same. It adds so much more to Bradley's fame, that, in so short a time, he so exactly observed the phenomena, as to enable him to discover the cause and verify the laws thereof.
Of all the stars observed by Bradley, there was only one of the first magnitude, Capella, within reach of his instru ment, and the position of this star is not favourable for showing the effects of parallax in declination.
The beautiful star Sirius, has been always supposed nearer to us than any other ; but the uncertain effects of refraction in this star, when observed in our northern lati tudes, oppose any attempts to observe its parallax in de clination. La Caillc availed himself of his stay at the Cape of Good Hope to make many observations on this star. Dr. Maskelyne, on examining these observations, found they appeared to show a parallax of several seconds. This induced Dr. Maskelyne, when at St. Helena, for the pur pose of observing the transit of Venus, in 1761, to institute a series of observations on Sirius, but the result turned out quite unsatisfactory as to parallax ; but here, as in almost every other attempt that had been made to discover the parallax of the fixed stars, an important advantage was gained for practical astronomy. Dr. Maskelyne disco vered that the usual method of suspending-the plumb-line by a loop induced very considerable errors. This was af terwards changed for the present method of making it pass over a notch.
It is to be remarked, that, although by its brilliancy, even in our climate, (it must be far greater in southern latitudes,) Sirius appears to surpass all the other fixed stars ; yet, from its situation, its declination is only changed by a little more than half its parallax. In this respect, then, it is not nearly so favourable for the investigation as stars of less brightness, that have considerable latitude, such as a Lyrx, i Draconis, the stars of Ursa Major, and the Pole Star.