Lieut . Drummond on the means of facilitating , &c. 325 
the triangulation of Ireland, and in connecting that country 
with the western shores of Scotland and England. The con- 
necting triangles are large, many of the sides being 60, 70, 
and 80 miles. The means resorted to for rendering these 
distant stations visible in ordinary states of the weather, and 
their successful application on some occasions of difficulty 
that presented themselves at the close of last year’s operations, 
form the subject of the communication which I have now the 
honour of submitting to this Society. 
The reflection of the sun from a plane mirror, as affording 
a point of observation that might be seen at remote distances, 
was suggested and employed by Professor Gauss in 1822, 
while engaged with a trigonometrical measurement in Han- 
over ; and the result of the first trials made at Inselberg and 
Hohenhagen, rendered it highly probable that it might be 
applied with much advantage to this purpose. 
The principle was adopted in this country when Colonel 
Colby and Captain Kater were engaged, in 1822, in verifying 
General Roy’s triangulation connecting the meridians of 
Paris and Greenwich. At their concluding station on 
Shooter’s Hill, seven or eight days had elapsed, during which 
Hanger Hill Tower, though only 10 miles distant, having re- 
mained completely obscured by the dense smoke of London, 
tin plates were attached to the signal post, so as to reflect the 
sun towards the station at stated times on a certain day. 
At the hours for which they had been calculated these 
plates became visible, and the observations were in conse- 
quence immediately and easily completed. In the subsequent 
operations of 1823, recourse was again had on two important 
occasions to the same method, and with equal success, I 
Uu 
MDCCCXXVI. 
