390 LIEUT. DRUMMOND ON THE ILLUMINATION OF LIGHT-HOUSES. 
Argand lamp £ inch in diameter, supplied with the finest spermaceti oil, and 
capable of supporting a flame If inch in height. The following results were 
obtained : 
French lamp, Mean of 5 observations by shadows . 
9 . 
6 by illuminated surfaces 12 
Wax candles, !■§ inch 
Standard. diam., the wick con- 
sisting of 40 threads. 
1 . . = 9.1 
8.7 
Oil gas burner, same 1 
diam. as standard.) 
-by shadows . 
0.95 
4*= 94.6 
= 8 ./ 
Lime-ball or cylin- 
der § inch diam. 
9.21 
10.4 ' 
-by illuminated surfaces 18.6 | 
16.0J 
>13.5 =122.9 
The light of the ball, depending upon the intensity of the heat, is very dif- 
ferent at different parts, being greatest opposite the jets, and diminishing 
towards the sides. The mean of the greatest and least intensity is taken in 
the above Table ; and moreover, though the greater number of observations 
might appear to warrant giving greater weight to their results, yet being 
made on the same day, and under the same circumstances, it was found that 
the results seldom differed, whatever might be the number of observations ; 
hence the arithmetical mean is taken, and we obtain this remarkable result, — 
that the light emitted by a lime-ball only § of an inch in diameter, heated by 
two jets, is equal to 13 Argand lamps. 
With respeet to the intensity or intrinsic brightness of the different lights, 
the property on which their utility in light-houses more immediately depends, 
we have the following results : 
The intensity of 
French lamp. Mean of 1 1 observations by shadows. . 
8 . . 
Oil gas 6 . . 
Lime G . 
3 by illuminated surfaces 
= 4,. | 
= 3.8 i 
5 - 
} 
4.1 
3 
0.85 f 
263.9 
264.4 
Mean. 
4.0 
264.1 
times 
the in- 
V tensity 
of the 
Stand- 
ard. 
* The result given by Fresnel, in the memoir quoted above, considerably exceeds this, being stated 
at 17 lamps of Carcel. I know not to what cause this difference is to be attributed, 
f Tl is low degree of intensity indicates impurity in the gas. 
