of the Cambridge Observatory. 157 



difference of longitude of two important points. The scale of 

 longitude thus formed in England appears to be incorrect: Captain 

 Kater has stated {Phil. Trans. 1828.) that the instrument with 

 which the observations were made is not so perfect as had been 

 supposed: and this is the only remark that I have seen which 

 appears in any degree to account for the errors in longitude*. 

 If the observations are correct, there must be some disturbance 

 in the direction of gravity, of that kind which we attribute to 

 local attraction. Occultations give a more accurate determination 

 than any other known method: but the occultations of bright 

 .stars occur seldom, and corresponding observations of the occul- 

 tations of small stars are rarely found. If there are no corres-^ 

 ponding observations at both stations, the determination rests on 

 the exactness of the Lunar Tables, and the method is not accurate. 

 The observations of eclijises of Jupiter's satellites, of the lunar 

 distances, and of the lunar transits, far inferior in accuracy, are 

 in other respects subject to the same remarks. Corresponding 

 observations of artificial signals have been used within a few 

 years in determining the difference of longitude of Greenwich 

 and Paris, and in measuring extensive arcs of parallel on the 

 continent: but they are found, though giving pretty accurate 

 results, to require a degree of system and an extent of co-operation 

 which can only be obtained when the observations are made under 

 the immediate authority of the government. And even with this 



* One of the most eminent mathematicians of England has attempted to explain them- 

 by considering the bearinj; of a station as dclerniined by the azimuth of the tangent of the 

 geodetic line which joins that station with the place of observation. But it seems ))lain that 

 the bearing, as determined by the view of one station from the other, must depend ou the 

 direction of the ray of light proceeding in a straight line from one station to the other: and 

 that it will therefore be the same as that of the vertical plane belonging to the point of ob- 

 servation, and passing through the station observed. This is the assumption in the cal- 

 culations of the English survey. 



