1826.] Royal Society. 451 



Collimator to be applicable to any circle, without introducing 

 any errors whatever of its own. 



May 4. — Colonel Mark Wilks was admitted a Fellow of the 

 Society ; and the following paper was read : 



On the means of facilitating the Observation of Distant Sta- 

 tions, in Geodesical Operations ; by Lieut. T. Drummond, Roy. 

 Eng.: communicated by Lieut. CoL T. Colby, FRS. 

 t A comniittee of the House of Commons, having recom- 

 mended to his Majesty's government inl824,theaccomplishment 

 of a new survey of Ireland, which circumstances rendered it 

 expedient to eflect as speedily as was consistent with accuracy, 

 Lieut. Col. Colby, whilst engaged in active preparations for 

 the survey, entrusted to the autlior the contrivance of means 

 for obviating the delay which usually occurs in connecting the 

 stations in triangulation in this country, from the frequently 

 unfavourable state of the weather not permitting the ordinary 

 signals to be seen from distant stations. The delay from this 

 cause, which had been experienced in the Western Isles of 

 Scotland, gave reason to apprehend that such would be the case, 

 to a still greater extent, in Ireland. 



To remove this inconvenience, as far as day-observations 

 were concerned, Lieut. Drummond had recourse, in preliminary 

 trials, to tin-plates, as substitutes for regular Heliostats ; and the 

 advantages derived, from applying, even in this rough way, the 

 principle of reflection, as suggested by Prof. Gauss, led to the 

 invention of an instrument described in the paper, which was 

 used with much benefit last season, in the survey of Ireland. 



Some method of connecting the stations during the night 

 was also desirable, in order still further to expedite' the prose- 

 cution of the survey. For this purpose, Bengal and white- 

 lights had formerly been employed by General Roy, but the 

 use of them had given way to that of Argand lamps, their 

 light being concentrated, and reflected towards the observers, 

 by a parabolic mirror. These, however, had been found to 

 answer but imperfectly; and Col. Colby and Capt. Kater, 

 when connecting the meridians of Greenwich and Paris, in 

 1821, with M.M. Matthieu and Arago, employed the light of 

 an Argand lamp with four concentric wicks, concentrated by a 

 lens. 



This apparatus, however, though well-adapted to lip-ht- 

 houses, for which purpose it had been originally devised by'M. 

 Fresnel, was in many respects objectionable in geodesic 

 operations ; and the parabolic reflector still appeared to be the 

 most eligible means of concentrating the light, from whatever 

 source that might be obtained. 



With the view of obtaining a povverfuUight, Lieut. Drummond 

 first tried various pyrotcchnical preparations, and afterwards 

 the combustion of phosphorus in oxygen gas, but he found 



2 g2 



