OF ILLUMINATING LIGHTHOUSES. 
221 
illuminating power, a long duration of the brightest period, and a large angle of ver- 
tical divergence. 
The comparative qualities of the French system of lenses, and a first class light 
with reflectors, such as that at Beechy Head, as to their application to the above 
duties, will be seen in the following statement. 
Oil consumed in 3| hours .......... 
Illuminating power without the instruments expressed 
in Argand lamps 
Illuminating power with the instruments expressed in 
reflectors with Argand lamps ........ 
Maximum horizontal divergence per side . . . . . 
Minimum horizontal divergence per side . . . . . 
Number of sides ............. 
Degrees of horizon illuminated ........ 
Degrees of horizon with the full power ...... 
Maximum vertical divergence ......... 
First Class light with 
ten Reflectors on a 
side 21 in. diameter. 
4 qts. Ipt. 
French Lens with 
its additional ap- 
paratus. 
2 qts. | pt 
30 
10-4 
10 
10-4 
18° 45' 
5° 15' 
4° 46' 
4° 4' 
3 
8 
56° 15' 
42° 0' 
* 
O 
00 
32° 32' 
31° 54' 
3° 0' 
From the above it will be seen that for a beacon light, when not more than 3° of 
vertical divergence is required, the lens is far the cheapest and best instrument, on 
account of its greater power and longer duration of the brightest period, although 
the illuminating power without the instrument is obtained at a greater consumption 
of oil than in the Argand burners ; and that the reflectors as at present constructed 
are best adapted for leading or guiding lights, where a very large angle of vertical 
divergence is required. But in all those situations where more than 3° of vertical 
divergence are wanted, the lens is inapplicable, and where less than 30° are required 
there would be a useless waste of light in the reflectors. Now this is undoubtedly by 
far the most numerous class of lighthouses, we shall therefore examine with what 
advantage an alteration may be effected in the vertical divergences of the above in- 
struments ; but as many circumstances, such as the accuracy with which the instru- 
ment is made and fixed, atmospheric refraction, and a little variation in the height of 
the flame of the lamp would all tend to render variable the range of the angle of ver- 
tical divergence, we shall not attempt to employ very great exactness in this respect. 
Considering these circumstances, the natural vertical divergence of the lens is the 
least that ought to be employed in any situation ; and as the height of the flame which 
oil is capable of supporting to burn with advantage is nearly the same whatever may 
be the dimension of the lamp, the only way in which this divergence can be increased 
is by shortening the focal distance, to do which we must either reduce the area of 
each lens or the number of sides ; the one would produce a loss of illuminating power, 
and in the others the figure of the instrument would become more distorted, and the 
greater obliquity with which the rays would strike and leave the lens would also be 
a cause of loss of light. Gas might perhaps be applied here with advantage, for by 
2 g 2 
