LIEUT. DRUMMOND ON THE ILLUMINATION OF LIGHT-HOUSES. 393 
The four faces of the revolving machine were thus occupied : 
No. 1 . A single reflector 21 inches diameter, 3 inches focal distance, with 
an Argand lamp. 
No. 2. Seven reflectors with ditto. 
No. 3. French lens, with its lamp. 
No. 4. Single reflector with lime-ball. 
The respective lights were accurately placed in focus. 
On the evening of the 10th of May, the machine performing one revolution 
in eight minutes. Captain Pelly of the Trinity House made the following ob- 
servations on the different lights from the Trinity House Wharf, Blackwall, 
10^ miles distant. 
No. 
Duration. 
Divergence. 
Computed 
maximum 
Divergence. 
min. 
sec. 
1 
0 
25 
17-40 
17-81 
2 
0 
25 
17-40 
17-81 
3 
0 
7 
5.17 
5.18 
4 
0 
9 
6.12 
6.7 
When No. 4, the reflector lighted with the lime-ball, was turned towards 
the Wharf, the light was so great that the shadow of the hand and fingers was 
distinctly visible even on a dark brick wall, while no such effect was discern- 
ible when the other lights were turned in the same direction. 
In order more justly to estimate their comparative effects, No. 4. was removed 
to a temporary tent about twenty-five yards to the right of the light-house, as 
far as the edge of the cliff would permit, and on the evenings of the 25th and 
31st May regular series of experiments were made. Being engaged at Pur- 
fleet, directing these exhibitions, I never had an opportunity of witnessing their 
effects at Blackwall ; but Captain Basil Hall, R.N., who from the interest 
which he took in these experiments was an attentive observer of all that oc- 
curred, has at my request kindly favoured me with the following interesting 
account : 
4, St. James’s Place, 1st June 1830. 
“ My dear Sir, 
“You wished me to take particular notice of last night’s experiments with 
the different kinds of lights exhibited at Purfleet, and observed at the Trinity 
3 e 2 
