184 



as erroneous ; and this leads the author to inquire, what confidence 

 is due to the Greenwich observations of '.the present day. His prin- 

 cipal reliance on their accuracy he founds on the coincidence of the 

 results obtained by the two independent methods of direct vision and 

 reflection, and he shows in what manner this coincidence of the two 

 results is a proof of the accuracy of either. The general tendency 

 of all instruments, he observes, is to undergo equal and opposite 

 flexures, at equal elevations on either side of the zenith ; whence it 

 cannot be inferred, with regard to instruments that turn in azimuth, 

 that flexure does not take place, from stars of different altitudes giv- 

 ing the same error of collimation. But on the contrary, if, by the 

 method of reflection, stars of different altitudes give the same hori- 

 zontal point, the strongest proof will be afforded of the non-flexure 

 of the instrument. These equal and opposite flexures on opposite 

 sides of the zenith, will, in our latitudes, affect stars near the equator 

 by errors in polar distance about double of the errors committed in 

 stars near the zenith. These views are illustrated by a comparison 

 of the Greenwich Catalogue with those of Dr. Brinkley and Mr. Bes- 

 sel ; and since the two latter, in conformity with the law of flexure, 

 differ from one another by 5" near the equator, and by only half that 

 quantity near the zenith, it is inferred that the instruments used by 

 those two astronomers, one or both of them, are liable to flexure. 



Since those stars must be most correctly determined by the method 

 of reflection, whose altitude is the least, on account of the shortness 

 of the arc to be measured, the author considers himself intitled to 

 decide what is the share of error which, in stars near the equator, be- 

 long to each of these catalogues. In stars so situated, Dr. Brinkley's 

 polar distances exceed the author's by about 2", and Mr. Bessel's 

 fall short of them by about 3" ; and since the Greenwich Catalogue 

 everywhere divides the difference of the two catalogues in nearly the 

 same ratio, it is considered probable that the errors of each through- 

 out are nearly in the same proportion. 



On the Parallax of a Lyree. By John Pond, Esq. Astronomer Royal, 

 F.R.S. Read November 14, 1822. [Phil. Trans. 1823, p. 53.] 



The author, not satisfied with his former attempts to discover the 

 parallax of a Lyra with the mural circle, has resumed his examina- 

 tions of this star with the same instrument, being prevented from 

 employing the method of a fixed telescope, for want of an opposite 

 star sufficiently bright to be observed throughout the year. The 

 Dublin observations having indicated that the parallax of y Draconis 

 is insensible, and that of a Lyra about 2", he investigates first the 

 difference of parallax between these stars ; secondly, the absolute 

 parallax of a Lyra. 



The change in the angular distance of the two stars at opposite 

 seasons, he finds not to exceed one tenth of a second ; which perfect 

 coincidence he attributes in great measure to the mildness of the 

 winter of 1821-1822, enabling him to equalize the temperatures of 



