observation of distant stations in geodcetical operations. 329 
tages are detailed in a “ Memoire sur un nouveau systeme 
d’eclairage” by M. Fresnel. The light which it gave is 
stated to possess 3^ times the intensity of that given by a 
reflector. It was employed, during the operations alluded 
to, at Fairlight Down and Folkestone Hill on the English 
coast ; at Cape Blancnez and Montlambert on the French 
coast : the greatest distance at which it was observed being 
48 miles ; and its appearance, I have understood from Colonel 
Colby, was very brilliant. 
But valuable as this apparatus may be when employed in 
a light-house, the purpose for which it was indeed invented 
and constructed, the properties of the simple parabolic re- 
flector appeared still to give it a preference for the service of 
the Trigonometrical Survey, provided a more powerful light 
could be substituted in its focus, instead of the common 
argand lamp. 
With this object in view, I at first endeavoured to make 
use of some of the most brilliant pyrotechnical preparations, 
then phosphorus burning in oxygen, with a contrivance to 
carry off the fumes of phosphoric acid was tried : but the 
first attempts with these substances promising but little suc- 
cess, they were abandoned. In all cases the flames, besides 
being difficult and troublesome to regulate, were large and 
unsteady ; little adapted to the nature of a reflecting figure, 
which should obviously, when used to the utmost advantage, 
be lighted by a luminous sphere, the size being regulated by 
the spread required to be given to the light. This form of 
the focal light it was manifest, either could not be obtained 
or preserved when combustion was the source of light ; and 
it was chiefly this consideration which then led me to attempt 
