158 Professor Airy on the Longitude 



advantage the method has sometimes totally failed. It is, how- 

 ever, that which is on the whole best adapted to the determination 

 of the difference of longitude of two observatories on the same 

 continent or large island. The method of transporting chro- 

 nometers has been used with great success for finding the difference 

 of longitude of Dover and Falmouth, and that of Falmouth and 

 Madeira: and an expedition is now employed by order of our 

 government in finding the difference of longitude of several 

 stations on the shores of Africa and South America by this 

 method. In practice it is undoubtedly the best of all, when the 

 stations compared are situated so as to admit of an easy sea- 

 voyage from one to the other. 



Now there is this important difference between the geodetic 

 method and the astronomical methods to which Ave have just 

 alluded. It is necessary in the former to assume that the earth 

 is exactly a s])heroid of known dimensions, at least that it is 

 exactly a solid of revolution, and the meridian plane of any 

 station is then defined to be the plane passing through the station 

 and through a certain fixed line called the axis of the earth. In 

 the latter, there is no such assumption : the figure of the earth 

 might be of the most irregular kind, yet the determinations would 

 be made with the same ease, and the same independence of the 

 figure, as if it were a [)erfect spheroid. The method of occultations 

 ought to be excepted from this remark, as the parallax of the 

 Moon which enters into the computation requires an approximate 

 knowledge of the earth's form and magnitude. But all the others 

 are absolutely independent of any knowledge of the earth's form: 

 the definition of meridian which they assume is, the plane passing 

 through the vertical of the station and the celestial pole : and thus 

 the meridian is determined from elements which in no degree refer 

 to any other jiart of the earth. 



If now the earth's figure be not perfectly regular (which the 



