160 ARTIFICIAL ILLUMINATION. 
and the other considerably larger, the water flows slowly out of the 
larger hole while it rushes rapidly in a thin stream out of the smaller 
one. It is just so with gas. Under ordinary pressures in the pipe the 
gas flows out of the two tiny holes in the small burner at a rapid rate 
and mixes itself so intimately with the air that it is almost like a 
Bunsen burner, and therefore gives very little light. On the contrary, 
the flow of gas from the larger holes of this No. 6 burner before you 
is a gentler one, and there is less mixing with the air, and we get a 
satisfactory illumination. The larger burner is about four times as 
efficient as the smaller one, because while it consumes only double the 
amount of gasit is giving about eight times as much light. It appears, 
therefore, that small lights are relatively extravagant, and this is really 
so, excepting when the gas is exceptionally rich, which is never the case 
with ordinary London gas, or when the pressure is extremely low. If 
we can by any device succeed in checking the velocity of this out-rush 
through the small holes or the thin slit of a small burner, we may con- 
siderably improve the illuminating power. The little cap which I 
hold in my hand, and which can be bought for a penny, is simply a 
burner with a large slit and is adapted to fit over the small Bray 
burner. I place it over this extravagant little “ No. 1” and it quite 
doubles the amount of light, as you see, without increasing the quantity 
of gas burnt. The theory of its action is simply this:—the gas rushing 
violently out of the small holes of the little burner is caught in the cap 
above from which it flows slowly out of the larger aperture; thus the 
air-mixing is largely prevented or the “‘ Bunsen” effect is reduced, and 
a much better light obtained. A fitting like this cannot, of course, 
produce anything like the same increase of illumination if applied to a 
large burner from which the flow of gas is already slow enough. 
There is another reason why small burners give relatively less light in 
proportion to the quantity of gas they burn: it is because their flames 
offer comparatively more radiating surface, which keeps their temper- 
ature lower, and, you know that for intense illumination we want as high 
a temperature as possible. A curious illustration of this I will now show 
you. Here I have two small lights burning; they are from No. 1 Bray 
burners. lJach by itself gives, as you see, a very poor light, but when 
I put them close together, making a duplex burner, allowing the two 
flames to coalesce into one thicker flame, you see that the total illumi- 
nation is more than doubled. Burners have been constructed on this 
principle and put on the market under the title of duplex burners ; 
they are simply two little burners combined on a single mount and are 
really fairly efficient, but the principle of their construction is based 
on a fallacy. It does not follow that because allowing two flames to 
coalesce gives increased illumination, it is therefore wise to construct a 
burner by putting together two small ones which are each terribly 
inefficient. If we burn the same amount of gas in a well constructed 
single burner, such for instance as a “ Bray Special,” we get quite as 
much light per cubic foot of gas as we can on this, once much advertised, 
duplex system. 
I have told you, I think, that the efficiency of a good sized ordinary 
Bray burner is about 2 candle power for every cubic foot of London gas 
