of the Cambridge Observatory. 159 



discrepancies in meridian arcs apply warrant ns to assert), it may 

 be expected that the results of one of these methods will not 

 agree with those of the other. The degrees of longitude on the 

 same parallel found by the geodetic method must be equal, be- 

 cause they are assumed to be equal: those found by astronomical 

 methods may be imequal. The scale of longitude determined on 

 a short extent of the parallel by astronomical methods may not 

 apply to the whole. This in fact is found to be the case with 

 regard to the difference of longitude of Dover and Falmouth, as 

 deduced by proportion of distance from that of Beachy Head and 

 Dunnose. 



Let us now consider the different purposes to which the re- 

 sults of these different methods ought to be applied. In mapping 

 a country, it is desirable to divide the map by cross lines into equal 

 spaces. This object is of paramount importance : and it is of 

 comparatively small consequence that the longitudes thus assigned 

 should be exactly the same, except for the coasts, as those de- 

 termined by astronomical operations. But in fixing the longitude 

 of a place in which astronomical observations are to be made 

 that will be compared with those made at some other place, it 

 is only the difference of the apparent time (as determined by 

 transits, equal altitudes, or some equivalent operation) at which 

 the same phenomenon is observed, that at all interests the as- 

 tronomer. The exact distance in fathoms east or west is of no 

 consequence, provided he knows the number of seconds which 

 elapse between the passage of the same star over the two meridians. 

 For his purposes then the results of the geodetic method are useless, 

 except there is a high probability that they coincide with the re- 

 sults of the methods described under qur second head : and if on 

 trying both methods the inferences cannot be made to agree, the 

 geodetic longitude must be rejected, and the astronomical must 

 be adopted. 



