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XI. On the Latitude of Cambridge Observatory. By George Biddell 

 Airy, M.A. late Fellow of Trinity College, Plumian Professor 

 of Astronomy and Experimental Philosophy, and one of the 

 Flce-Presidetits of the Society. 



[Read April 14, 1834.] 



The accurate determination of the latitude, with an instrument 

 like the Mural Circle now in use at the Observatory, seems at first 

 sight to be an easy business. In practice, however, it is not without 

 difficulties. I do not here allude to the correction for refraction ; 

 since, though there may be a trifling uncertainty in regard to its magni- 

 tude, it is easy to leave , a result subject to that uncertainty, and 

 admitting of correction without any trouble whenever a correction of 

 the refraction shall be established. Nor do I allude to the uncertainty 

 in the corrections by which, from a star's apparent place on any day 

 of observation, its mean place at a fixed epoch can be inferred; since 

 the uncertainty about any of these is far less than the smallest quantity 

 for which we could pretend to answer in fixing the latitude of any 

 place ; and its effects being periodical, would in a comparatively short 

 series of observations, produce no sensible effect. The difficulties to 

 which I allude are instrumental: they are not periodic in time, like 

 the latter; nor do they admit of correction from posterior researches, 

 like the former of the causes of uncertainty which I have mentioned ; 

 they are moreover such as would scarcely be suspected to exist, until 

 their effects are discovered from the discordance of the results of 

 observations. 



The Mural Circle is an instrument which gives simply the reading 

 of that point of the graduated limb which is opposite to an imaginary 

 fixed index when the telescope is pointed to the object of observation. 



M M2 



