156 Professor Airy on the Longitude 



In the geodetic inethod it is assumed that the earth is a 

 spheroid, of which the axes are known. When the latitude of 

 a point of departure is determined, the latitude and difference of 

 longitude of any other station visible from the first are calculated 

 without great difficulty, from a knowledge of the beaiing and 

 distance of that station from the first. The bearing is ascertained, 

 by comparing the direction of the line joining the stations with 

 the meridian as determined astronomically ; the distance, by the 

 ordinary geodetic observations. The second station now becomes 

 a point of departure to a third : and by a repetition of this process, 

 the longitude and latitude of places at a great distance from the 

 original point of departure may be ascertained with considerable 

 accuracy. 



The advantage of this method consists in the ease with which, 

 in any country that has been accurately surveyed, the longitude 

 of any point can-be found by comparing it with two conspicuous 

 objects that have been observed in the survey. The disadvantages 

 are, that errors will increase as the number of intermediate points 

 is increased, that they may accumulate to a sensible amount, 

 and that the method cannot be extended to countries beyond 

 the limits of vision. 



The principal astronomical methods are, reciprocal obser- 

 vations of the bearing of two stations ; corresponding observations 

 of occultations of stars by the Moon ; corresponding observations 

 of eclipses of Jupiter's satellites ; corresponding observations of 

 artificial signals; comparison of clocks regulated to sidereal time 

 at the two stations, by transporting chronometers from one to 

 the other; observations of the Moon's distance from the Sun or 

 a star; and corresponding observations of the Moon's right as- 

 cension at the time of her transit. The first has been used in 

 the surveys of England and India, rather for the purpose of 

 forming a scale of longitude, than for the determination of the 



