258 Mr. Galbraith on the Determination of the Latitude 



new Obsei'vatory on theCalton Hill here; and being unprovided 

 with any very gqod instruments which could be relied on with a 

 tolerable degree of confidence, I thought if it were possible by 

 any means to connect it with the Trigonometrical Survey, my 

 purpose would be accomplished. It is obvious, by observing 

 intersections from a few points or stations in the Survey, well 

 ascertained, my object would be more accurately performed. 

 But this would have required time, expense, considerable 

 trouble, and good instruments. It occurred to me, therefore, 

 that an extension of the problem above noticed, were the sta- 

 tions judiciously chosen, so as to avoid the indeterminate case, 

 would answer the objects I had in view. Thus, conceiving 

 the arcs between the stations visible from each other on the 

 surface of the earth to be straight lines, from which in mode- 

 rate distances they do not much differ ; or if great nicety be 

 required, let them be reduced to such by the usual methods, 

 and supposing meridians to pass through these points, and 

 also the place of observation, — then by the principles of sphe- 

 rical trigonometry, the distances of these three points may be 

 computed, and consequently by the problem of Townley, also 

 the distances of the same points from the place of observation. 

 Now by spherical trigonometry the co-latitude and difference 

 of longitude of the place of observation from all of the three 

 given positions, of which the latitudes and longitudes are 

 known, may be found by computation. The results, however, 

 would be rather more accurate, were a correction for the sphe- 

 roidal figure of the earth applied to the latitudes, though the 

 difference, even near 45° where it is greatest, only amounts 

 to a very few seconds. In this case, unless very great preci- 

 sion be required, the correction may be neglected. 



I shall illustrate this method of proceeding, which may be 

 useful where a proper opportunity accompanied with the ne- 

 cessary advantages for a superior cannot be obtained. 



It may be remarked, however, that many of the poles put 

 up by the trigonometrical surveyors have been wantonly de- 

 stroyed; so that without the trouble of replacing them, very 

 correct intersections cannot, as was the case in the present 

 instance, be obtained. 



The observations were made with a repeating theodolite 

 placed near the old entrance from the east, into the grounds 

 surrounding the Observatory. 



The angle between North Berwick Law, in latitude 

 56° 3' 8"N., longitude 2° 42' 11" W., and the Isle of May 

 Light, in latitude 56'' 1 1' 22" N., longitude 2° 32' 47" W., was 

 13° 48' 48"; and the angle between the Isle of May and the 



West 



