VI. On the Longitude of the Camhridge Ohsematory . 

 By GEORGE BIDDELL AIRY, MA. 



MEMBER OF THE ASTRONOMICAI, SOCIETY, FELLOW OF TRINITY COLLEGE, AND OF THE 



CAMBRIDGE PHILOSOPHICAL SOCIETY, AND PLUMIAN PROFESSOR OF ASTRONOMY 



AND EXPERIMENTAL PHILOSOPHY, IN THE UNIVERSITY 



OF CAMBRIDGE. 



[Read 'Nov. 24, 1828.] 



The methods of determining the difference of longitude of two 

 places may be classed under two heads: those which depend 

 principally upon geodetic measures, and tho.se which are entirely 

 founded on astronomical observations. The circumstances which 

 are favourable to the application of one of these methods are not 

 in general well adapted to the use of the other: and in few instances 

 have both been u.sed for the determination of longitude. But the 

 difference in the facilities of application is not the only, or the 

 mo.st remarkable difference : they are different in principle, are 

 founded on different definitions of terms, and their results, if they 

 should differ, are equally valuable, but are valuable for different 

 purposes. 



Before giving a detailed account of some observations made 

 in order to determine by one of these methods the longitude of 

 a station which had previously been determined by the other, 

 it may not be amiss to describe more particularly the nature and 

 capabilities of both methods. 



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