222 
MR. W. H. BARLOW ON DIFFERENT MODES 
having five or six concentric burners the same intensity of light might be obtained, 
while the greater height of the flame would give a greater vertical diverging angle. 
But in the reflector where the vertical divergence admits of so much reduction, 
we are enabled to increase the focal distance, and consequently to employ a large 
reflector, by which means a considerable saving would be effected in the consump- 
tion of oil, or using the same oil, we might obtain a much greater illuminating power. 
If, for example, we employed a reflector 42 inches diameter, with six inches focal di- 
stance, the illuminating power depending on the area of the end would be equal to 
four of the usual reflectors, but the divergences would all be reduced one half, we 
should consequently require six sides instead of three to illuminate the same number 
of degrees in the horizon, so that by reducing the vertical divergence to 15° we obtain 
the same illuminating power with half the consumption of oil. Instead, however, of 
employing twice as many reflectors to produce the same durations, we might double 
the diameters of the lamps and use the same number as at present. 
Before proceeding further with this part of the subject, it will be necessary to say 
a few words regarding the consumption of oil to produce a given illuminating power 
in lamps of different sizes and constructions. In the French lamp the consumption 
of oil to produce the same illuminating power as the Argand burners is nearly as 1|- 
to I, but here there are four concentric wicks, the largest of which is not more than 
three inches in diameter, and there would in consequence be a great intensity of 
heat, which would cause a considerable portion of the oil to pass off in vapour ; but 
by increasing the diameter of a single wick, the intensity of heat would not be 
increased, we may therefore consider that the consumption would be exactly as the 
illuminating power. I am not aware of any experiments having been made to ascer- 
tain the consumption in lamps of two and three wicks, and it would perhaps be 
found to vary a little with their dimensions, but as we find the illuminating power to 
be obtained at 1 to 1 in the Argand lamp which has one wick, and as 1^ to 1 in the 
French lamp which has four, it will be sufficiently near for our purpose to assume 
that the consumption of oil to produce a given illuminating power will be as follows: 
In a lamp with one wick as 1 to 1. 
In a lamp with two concentric wicks as to 1. 
In a lamp with three concentric wicks as 1^ to 1. 
In a lamp with four concentric wicks as 1^ to 1. 
And as we find that a lamp with one wick has an intensity of 1, and that the French 
lamp which has four wicks has an intensity of 4, it appears that the intensity in- 
creases directly as the number of wicks. 
Calculating then as above, we should evidently obtain the illuminating power of ten 
reflectors at the cheapest rate, by employing a reflector the area of whose end should 
be equal to ten, with a lamp having only one wick ; but as this would lead to such 
unwieldy instruments, it would doubtless be preferable to diminish the size of the 
apparatus by increasing the intensity of the lamps, and if it be within practicable 
