392 LIEUT. DRUMMOND ON THE ILLUMINATION OF LIGHT-HOUSES. 
The lime requires 4 cubic feet of hydrogen and 2 of oxygen per hour, and 
the probable expense is 5 d. per hour. 
In a revolving light of the first class, containing 30 reflectors, the expense 
per hour would therefore be about 2s. Id. If the French method were em- 
ployed, the increase of light would be ith, and the expense only Is. 2\d. per 
hour. If six reflectors illuminated with lime-balls were used, which would 
probably be sufficient, the probable expense would be 2s. Qd. per hour, and the 
increase of light 26 times. 
If this estimate be erroneous, I think it will prove to be so in excess : ad- 
mitting, however, that the expense should, in the first instance, somewhat ex- 
ceed what has been stated, it may in this as in every similar instance be 
expected that after a little experience a considerable reduction would be effected. 
This is a new source of artificial light, differing from every other at present in 
use, and the materials by which it is produced are among the most abundant 
products of nature ; but never having yet been applied on a great scale to any 
practical purpose, it has not hitherto been an object to obtain them in a sepa- 
rate state at a small expense. When this is effected, it will no doubt receive 
many useful and important practical applications. 
Meanwhile, however, the case in question may perhaps be regarded as one 
where expense ought not to be a primary object of consideration. On all ordi- 
nary occasions, the preference of one mode of illumination to another is a 
question of convenience, luxury or economy; but in this it assumes a more im- 
portant character, for it involves to a great extent the preservation of life and 
property. 
To complete the preceding account, it only remains to add a description of 
the appearances presented by the different lights when exhibited at a distance ; 
and to those who have entered with any degree of interest into the above de- 
tails, such a description, it is hoped, cannot fail of proving acceptable. 
The experiments at the Trinity House being concluded, the whole of the ap- 
paratus was removed to Purfleet, where on a knoll of chalk about 100 feet 
above the river a temporary light-house had been erected, and being fitted with 
the requisite machinery, the different lights were made to revolve in succession, 
and the appearance which they presented, as well as the duration of the light, 
were observed from the Trinity Wharf at Blackwall, a distance in a straight 
line of 10£ miles. 
