VOL. LXXII.] PHILOSOPHICAL TRANSACTIONS. 197 



time the experiment is repeated, we may pronounce it capable of measuring 

 seconds ; if otherwise, it will remain to be examined, whether the fault lies in 

 the mechanical or optical part. 



Let us now suppose that the parallax of the fixed stars does not amount to a 

 single second, yet still the case is by no means desperate ; and though the diffi- 

 culty of measuring seconds will soon suggest to us what extraordinary powers 

 and distinctness of the telescope, and accuracy of the micrometer, are required 

 to measure thirds ; this ought by no means to discourage us in the attempt. 

 Could we measure angles, much smaller than seconds, might we not hope to 

 find the parallax of some of the fixed stars at least to amount to several thirds ? 

 On the other hand, if it should appear indeed that, even with such improved 

 methods of measurement, we could not reach the remote situation of such 

 almost infinitely distant suns, we might still derive a valuable approximation to- 

 wards truth from such repeated observations, even though they should not be 

 attended with all the success we expected from them. On this assurance, Mr. 

 H. endeavoured to take such a method for attempting the investigation of the 

 parallax of the stars as to avail himself of the improvements he had already 

 made, and was still in hopes of making, in his telescopes. 



The next thing necessary to be considered in this undertaking was, the 

 manner of putting it into execution. The method pointed out by Galileo, and 

 attempted by Hook, Flamsteed, Molineux, and Bradley, of taking distances of 

 stars from the zenith that pass very near it, though it failed with regard to 

 parallax, has been productive of the most noble discoveries of another nature. 

 At the same time it has given us a much juster idea o( the immense distance of 

 the stars, and furnished us with an approximation to the knowledge of their 

 parallax that is much nearer the truth than we ever had before. Dr. Bradley, 

 in a letter to Dr. Halley on the subject of a new discovered motion of the fixed 

 stars, says, " I believe I may venture to say, that in either of the two stars last 

 mentioned (y Draconis and n Ursae majoris) it (the annual parallax) does not 

 amount to 2". I am of opinion, that if it were l" I should have perceived it in 

 the great number of observations that I made, especially on y Draconis ; which 

 agreeing with the hypothesis (without allowing any thing for parallax) nearly as 

 well when the sun was in conjunction with, as in opposition to, this star, it 

 seems very probable, that the parallax of it is not so great as one single second." 

 Phil. Trans, n. 406. As it is not known that any thing more decisive has been 

 done on the subject, it will not he amiss to see how far this method of finding the 

 parallax has really been successful. The instrument that was used on this occa- 

 sion, was the same as the present zenith sectors, which can hardly be allowed 

 sufficient to show an angle of 1 or even 2 seconds with accuracy ; yet, on ac- 

 count of the great number of observations, and above all the great sagacity of 



