VOL. LI.] PHILOSOPHICAL TRANSACTIONS. 501 



LXXVIll. A Proposal for discovering the Annual Parallax of Sirius. By the. 

 Rev. Nevil Maskelyne* A. M., F. R. S. p. 889. 



The Royal Society had resolved to send persons of ability to proper places, to 

 observe the approaching passage of Venus over the sun, the 6th of June in the 

 year following; (as first proposed by Dr. Halley 44 years ago, as a proper means 

 of determining the sun's parallax to a great degree of exactness, Dr. M. recom- 

 mends a very important object in astronomy, which he apprehends may be 

 cleared up at the same time, by the astronomers sent to one of those places 

 which will probably be judged convenient for the observation of Venus's transit, 

 viz. the island of St. Helena.-f- 



This object is the determination of the annual parallax of the Orbis Magnus; 

 the finding out of which, from observation, would be the fullest and most direct 

 proof of the Copernican system, as the want of this proof hitherto has been the 

 strongest argument made use of by those who have withheld their assent to an 

 hypothesis, which so fully satisfies all the other phenomena. 



No one indeed will now venture to assert that, even if no annual parallax 

 could be found, after the greatest exertion of human art and industry, the Coper- 

 nican system was not therefore true; since the quantity of this parallax may be 

 so small as to escape tlie reach of our sight, though assisted to the utmost. But 

 though the defect of it would be no just argument against the Copernican system, 

 yet the actual demonstration of it, from observation, would be a direct and con- 

 vincing proof of the truth of that system. It remains then to be considered, 

 what hope there is now left, after astronomy has been brought to such a great 

 degree of perfection, of being able to find out an annual parallax in any of the 

 fixed stars. 



Mr. M. is sensible he may here seem to be presumptuous, in venturing to 

 treat on this subject, after the many accurate observations made by Dr. Bradley, 

 with an instrument constructed for this very purpose. He would just beg leave 

 to take notice, that the stars which this astronomer observed, were such only as 

 lay within a few degrees of his zenith ; and though his observations do not seem 

 to show a sensible parallax in any of them, yet we cannot thence absolutely con- 

 clude, that among the great number of visible stars, there are none in which it 

 may be perceptible, till they have all of them, especially those of the greatest 

 lustre, been observed in proper places near the zenith, with the like care and ac- 

 curacy which he has used: for, as Dr. Bradley has himself remarked, where any 

 stars are remote from the zenith, the uncertainties of refraction, and the irre- 



* The present astronomer royal, 1807. 



t Mr. M. was himself the person sent to St. Helena, to make the observations, as will appear 

 hereafter in due place. 



