6?) 



The astronomer royal, Mr. Pond, observing with the nevv 

 mural circle, made by Mr. Troughton, has not hitherto con- 

 firmed my results, although he finds indications of parallax in 

 « Lyrje and a. Aquilas. * I had felt such confidence in my re- 

 sults that 1 did not doubt that one of the first services that 

 would be rendered to astronomy, by the Greenwich mural cir- 

 cle, would be the confirmation of the existence of annual pa- 

 rallaxes in certain stars. But, allowing the greatest accu- 

 mcy in the observer, and excellence in the instrument, I 

 conceive a very probable account has been given, why 

 this has not yet taken place. Many of the stars, even of 

 the second magnitude, such as Polaris, y Draconis, &c. may 

 be affected by a parallax in declination, amounting to a 

 fraction of a second. Were we certain that the standard 

 stars were not affected by parallax, or had we ascertained 

 the quantity, if any, then the method of observing by the 

 mural circle would be far preferable to the methods of ob- 

 serving in which the plumb line is used.-)- 



* Phil. Trans. 1813, part 2. 

 f I can feelingly bear testimony to the great superiority of the mural circle over our 

 instrument, as to the convenience of the observer, and the consequent facility of multi- 

 plying observations. In the mural circle no care is necessary but in making and reading 

 off the observations. In our circle the previous examination of the plumb line is often 

 a very tedious and sometimes unsatisfactory operation. Many observations have been 

 lost thereby, a serious inconvenience in a climate ill adapted to astronomical observa- 

 tions. The calm weather which we so often experience during a high state of the 

 barometer, both in summer and winter, is generally unfavourable to the astronomer, be- 



L 2 



