2(54 Mr Galbraith on the Enrjlish Arc of the Meridian. 



railway from Paris to Lille, with tlic branches to Valenciennes, Dunkirk, 

 Ciilais, and Boulog-ne, derived great advantages from the new Survey of 

 France, both with regard to economy in time and diminution of expense. 

 Tlic Marine Surveys are also of immense benefit for the purpose of en- 

 abling our vessels to navigate the ocean in safety, and also to facilitate 

 the selection of the proper points for the erection of piers and harbours, 

 either by the nation or by patriotic individuals, — a noble example of 

 which has been set by his Grace the Duke of Buccleuch and Mr Glad- 

 stone in the erection of the piers at Granton and Burntisland. It is much 

 to be regretted that our Trigonometrical Survey has proceeded so slowly. 

 The English Trigonometrical Survey commenced in earnest in 1701, the 

 French Description Geometrique de la France, similar to ours, after the 

 peace of 1815, and, considering their relative progress, the latter, though 

 begun a quarter of a century after the former, it appears probable will be 

 finished before it.* 



William Galbraith. 



51 SoDTH Bridge, Novcraber 1842. 



Ill the Astronomische Nachrichten, conducted by Professor 

 Schumacher of Altona, there are several papers by M. Bcssel, 

 the justly celebrated astronomer of Konigsberg, relative to the 

 figure and magnitude of the earth. In this new determination, M. 

 Bessel.has thought proper to reduce almost all the astronomical 

 observations afresh, and to introduce every necessary correc- 

 tion relative to the true places of the stars, the instruments 

 employed in making the observations, and the latest correc- 

 tions of the measured arcs. In general, however, he has not 

 re-examined the trigonometrical calculations, but merely ap- 

 plied such corrections to the arcs as had been suggested by 

 recent examinations of the standard scales from which the 

 fundamental bases were obtained. This he has done with 

 regard to therarc of the meridian in England, between Dun- 

 nose and Clifton, and it will be seen by the following remarlis 



* It is now generally admitted, that, by a deflection of the plumb-line from the 

 vertical in the direction of the meridian, an irregularity of 5" of latitude has oc- 

 curred at Arburyhill. But a deflection of the plumb-line in an arc at right 

 angles to the meridian would cause a like irregularity in the azimuth there ; and 

 if that azimuth be determined from the pole star, as obtained by Ramsdon's theo- 

 dolite, the irregularity from this cause must be multiplied by the secant of the 

 altitude, or 1.68. Hence an error of 5'' committed at the height^of the pole star 

 becomes 5'' x 1.G8 or 8".4 at the horizon. 



This hypothesis may be offered as a solution of the inconsistency pointed out 

 in page 270 of this paper, independent of errors of collimation arising from a ne- 

 glect to reverse the axis of the tolescoiic. 



1 



