277 



the index corrections as determined by the collimator. Fourthly, 

 the index corrections as determined by reversion from several stars. 



On the Means of facilitating the Observation of distant Stations in 

 Geodeetical Operations. By Lieutenant Thomas Drummond, of the 

 Royal Engineers. Communicated April 14, 1826, by Lieut. -Co- 

 lonel H. Colby, F.R.S. Read May 4, 1826. [Phil. Trans. 1826, 

 p. 324.] 



In consequence of the Report of a Select Committee of the House 

 of Commons in June 1824, it was resolved that a new survey of Ire- 

 land should be undertaken ; and the author was directed by Colonel 

 Colby to consider the most effective means of rendering distant sta- 

 tions observable in the prevalent hazy state of that country. 



After adverting to some of the usual means resorted to in such 

 cases, and showing their inefficiency for his present purpose, and 

 after noticing some unsatisfactory trials of brilliant pyrotechnical 

 preparations, and of the combustion of phosphorus in oxygen gas, 

 the author attempted applying to the purpose in view the brilliant 

 light emanating from intensely heated quicklime. To obtain the 

 requisite temperature, he passed a stream of oxygen through the 

 flame of alcohol ; and this jet being directed upon a small spherical 

 piece of quicklime placed in the focus of a proper reflector, the light 

 which it emitted was found to have 83 times the intensity of the 

 brightest part of the flame of an Argand burner. Other substances, 

 such as zirconia, magnesia, and oxide of zinc, were tried as substi- 

 tutes, but were found very inferior as sources of light, when ignited, 

 to quicklime from chalk, which moreover admits of being conveni- 

 ently turned in a lathe into focal balls of any requisite dimensions. 

 Mr. Drummond proceeds to detail the necessary application of this 

 system to a case of considerable difficulty that occurred at the end of 

 last season. Slieve Snaght, the highest hill of Innishowen (2 100 feet), 

 and 15 miles North of Londonderry, forms an important point in the 

 triangulation connecting the North of Ireland with the Western Isles 

 of Scotland. On the 23rd of August a conspicuous object was placed 

 upon its summit, that it might be observed from Divvis Hill, near 

 Belfast ; but till the 26th of October it remained so enveloped in fog, 

 as to frustrate all efforts at observation, the distance between the 

 stations being 66 miles. On the 27th of October the author pro- 

 ceeded to the hill ; and after some delay from tempestuous weather, 

 brought the new instrument into use, which was brilliantly visible at 

 the other station, and thus successfully terminated the observations. 



This paper concludes with notices of some other proposed appli- 

 cations of these sources of intense light, more especially to certain 

 lighthouses, in which the author thinks that the additional expense 

 would be amply counterbalanced by the advantages of so powerful a 

 source of illumination. 



In a note appended to this paper by Mr. Herschel, he observes, 

 that red, yellow, and green, appear to be the predominant rays in 



