LIEUT. DRUMMOND ON THE ILLUMINATION OF LIGHT-HOUSES. 389 
sions partial shadows at the top and bottom ; still, however, the simplicity and 
certainty with which it may be renewed will probably entitle it to a preference 
even in this case. 
These different instruments were prepared by Mr. Simms of Fleet-street, to 
whom, for various ingenious suggestions, for the trouble which he has bestowed 
upon them, and the assistance which he contributed during the progress of 
the experiments, I am greatly indebted. 
The apparatus for supplying the lamps with gas is represented in fig. 6. It 
consists of two strong cylinders, A, 3 feet high, the one for oxygen, the other 
for hydrogen : the gas is compressed two or three times in each, the latter by 
being generated under pressure, the former by being pumped in. To each of 
these gas-holders, a governor, B, is attached, of one of which a section is shown ; 
by which means, whatever be the variation of pressure in the gas-holder, pro- 
vided it exceed that of the governor, the gas will issue at x with a uniform 
and constant stream ; in the present instance under a pressure of 30 inches of 
water. Although this apparatus was of great use in the experiments at the 
Trinity House, and subsequently at Purfleet, by enabling me to keep within a 
small compass a supply of gas sufficient for two hours consumption, and even 
to renew it without impeding the progress of the experiments, yet I may re- 
mark, that on a large scale the gasometers required would be much more 
simple, since compression would no longer be required. This apparatus was 
made for me by Mr. Samuel Crosley, the ingenious inventor of the gas- 
governor. 
Our first experiments were on the illuminating powers of the different lights, 
independently of the lenses or reflectors with which they are generally used. 
The method of shadows and that of equally illuminated surfaces, both depend- 
ing on the same principle, but requiring different instruments, were employed * ; 
the former after the manner of, and with all the precautions recommended by, 
Count Rumford -f- ; the latter according to the arrangements proposed by 
Mr. Ritchie who was kind enough on this occasion to make several experi- 
ments with his own instruments, and without being made acquainted with the 
results previously obtained by us. The standard we used was an excellent 
* Bouguer, Traite d’Optique. f Phil. Trans. 1794, Part I. page 67. 
f Phil. Trans. 1825, Part I. 
