328 Lieut. Drummond on the means of facilitating the 
has been employed on the survey. When packed for travel- 
ling, the mirror mm is detached, and the bar gg turned till it 
coincides with bf. The instrument once directed, its manage- 
ment was usually confided to one of the non-commissioned 
officers. 
To combine, with the heliostat now described, a means of 
exhibiting a bright light at night, that no opportunity might 
be lost of effecting our purpose, was the next consideration. 
In the beginning of the survey, General Roy had, on seve- 
ral occasions, but especially in carrying his triangles across 
the Channel to the French coast, made use of Bengal and 
white lights prepared at the Royal Arsenal : for these, 
parabolic reflectors, similar to those with which our light- 
houses are supplied, and illuminated by argand burners, 
were afterwards substituted, as more convenient ; but they 
have been gradually discontinued, the advantages derived 
from them proving inadequate, from their want of power, to 
the trouble and expense incident to their employment. In 
the trigonometrical operations of 1821 carried on by Colonel 
Colby and Captain Kater, conjointly with MM. Arago and 
Mathieu, for connecting the meridians of Greenwich and 
Paris, an apparatus of a very different kind was employed for 
the first time. A large plano-convex lens 0.76 metre 
square being substituted for a parabolic reflector, and the 
illuminating body an argand lamp with four concentric wicks. 
The lens was composed of a series of concentric rings, re- 
duced in thickness and cemented together at the edges. This 
apparatus resulted from an inquiry into the state of the 
French light-houses, and was prepared under the direction 
of MM. Fresnel and Arago. Its construction and advan- 
