24< Drs Christison and Turner on the Construction of 
ply of air increases in a greater ratio than the flow of gas. Thus, the expen- 
diture of the four burners mentioned above, is in the ratio of 100 to 177, 280, 
and 366; while the supply of air is in the ratio of 100 to 255, 447, and 631. 
Hence, for reasons formerly mentioned, it might be of advantage to lessen the 
supply of air, by shaping the air-canal of the larger burners like an inverted 
truncated cone. When they are to be used by the customers of a great com- 
pany, however, other circumstances, which must be also taken into account, 
render it advisable to have the air-hole somewhat larger than is required for 
complete combustion of the gas. These circumstances will be taken notice of 
immediately, under the head which we shall now proceed to consider, namely, 
the influence on the relation between the light and expenditure exercised by 
the glass chimney. 
3. Two objects are served by the glass chimney. It renders the flame 
steady, and enlivens the combustion. 
From what was formerly said concerning the effect of enlivening the com- 
bustion, there must evidently be a certain medium, beyond which its activity 
cannot be increased, without loss of light. This medium is to be attained, by 
adapting to each other the interval between the jet-holes, the diameter of the 
air-aperture, and the form of the glass chimney ; and, consequently, as none 
of these can be made to vary, without necessitating some alteration in the rest, 
no form or proportion of chimney can be pointed out, which will answer for 
every kind of burner. 
If the burner is so constructed, that the gas is perfectly consumed without 
a glass at all, its light cannot be increased by any form of chimney. This is 
the case with all burners of which the air-aperture is large, and the jet-holes 
far apart. For example, the five-holed burner of Edinburgh gives as much 
light with a naked flame as with any kind of chimney. Its holes are so far 
apart, that the jets rise an inch or more before they meet, the air plays freely 
round them, and is therefore supplied in sufficient quantity to burn the gas 
thoroughly ; consequently, the only use of a glass chimney for such a burner 
is to render the flame steady. In order to effect this, without rendering the 
combustion too vivid, and causing a loss of light, the chimney must be very 
wide. That in common use, which is 6 inches long, and 1.6 in diameter, an- 
swers very well. If its diameter be diminished to 1.3 or 1.2, the flame be- 
comes shorter and more brilliant ; but, at the same time, the light is diminish- 
ed in the ratio of 100 to §0 and 66. 
The ten-holed gas-burner of Edinburgh is likewise so constructed as to give 
nearly as much light without as with a chimney, and therefore it requires a 
very wide one. That generally used, which is 1.9 or 2 inches in diameter, is 
scarcely wide enough. If the light with this chimney be 100, it is diminished 
to 81 with a chimney 1.6 wide, and to 66 when the diameter is 1.3. But the 
glass with which the greatest light and the steadiest flame are procured, is 
one contracted to a narrow tube at the top of the flame, the lower cylinder 
being 4 inches long, and 1.7 wide, the upper 3 by 1.1. With such a glass, the 
light is increased to 115, and the flame is perfectly steady. 
When the holes of a burner are increased in number, so that the jets unite 
at the very bottom of the flame, and the air-aperture is at the same time small, 
