81 



Where tlie velocity is moderate, no diminution is observed, and 

 the managers of gas lights find that f of an inch of wa- 

 ter afford sufficient pressure to send the coal gas throuo-h 

 many miles of pipe, while the flames are of equal magnitude 

 near the gasometer, or at the remote extremity. Angles do not 

 seem to affect mvich the progress of air, so that it is unnecessary 

 to raise the chimney from the top of the furnace directly. But 

 though it is useless to be very careful in smoothing the interior 

 of the tube, we must take another precaution, that it shall be 

 constructed of materials which are bad conductors of caloric. 

 Lavoisier's proposal of a double tube of iron, with the interval 

 filled with pounded charcoal, is very good while the iron 

 lasts, but it would be soon destroyed at high heats, particularly 

 by the sulphurous fuel of this country. On the large scale a 

 similar contrivance is used with success, and has the additional 

 advantage of preventing the ruin of the structure. In the im- 

 mense fires of large steam engines, distilleries, &c. the interior 

 of the tube is in a state of strong ignition through its whole 

 extent, and if the tier of bricks, in immediate contact with the 

 flame, be firmly connected with the remainder of the fabric, the 

 whole must be disjointed and shaken by their unequal expansion : 

 but when it is constructed with a double wall, separated by an 

 interval of some inches, the interior one is seen to rise, when 

 heated, above the other, and as it cools, gradually to subside. 

 For the chemist, a nine inch wall will generally be sufficient, 

 and if he fears its expansion, a few iron hoops will give it the 

 necessary strength. 



We have already seen, that the draught increases with the 



VOL. XIII. N 



