HOW TO BUILD A CHIMNEY. St 
the rope draws the bucket, not the water, the bucket 
pushes the water all round and below. So you may 
draw a bucket of air, by the bucket pushing it, not 
otherwise. Smoke is lighter than common air, so the 
air gives the bigger (heavier) push, and pushes the 
smoke upwards. The hotter the smoke is, it is the 
lighter, consequently the greater the difference be- 
tween it and the air, and so the push of the air is 
so much greater, and the smoke must move off faster. 
It is to be noted, however, how very small this 
push is. Suppose you have a chimney a square foot 
in vent and twenty-two feet high, and the fire heated 
the smoke in it, till it expanded one-tenth more 
than the air outside. Then the twenty-two feet of 
smoke in the chimney would be only equal in weight 
(suppose smoke same weight as air, which it 1s not) 
to twenty feet of air: consequently there is equal to 
the weight of two feet of air less pressure inside 
than outside the chimney. Hence, if the bottom of 
the chimney is open to the air, the smoke is forced 
upwards by a pressure equal to the weight of two 
feet of air, an amount hardly to be detected by the 
finest aneroid. If heat be increased till the volume 
of smoke be increased to a fifth more, there would 
be nearly four feet of extra pressure outside. So if 
the chimney be doubled in height, and the heat 
maintained so as to increase the smoke by one-tenth 
throughout, we would have four feet of preseure extra 
outside, and the smoke be pushed upwards with corre- 
spondingly quickened speed. It is, however, a very 
small pressure at that. A windgauge would shew a 
very swall amount of pressure and speed at a chim- 
ney top. Therefore, this small pressure must be care- 
fully utilized. With pressure enough plenty of air 
might be driven through an inch pipe to supply a 
smelting furnace, and the smoke of an ordinary fire 
through a tobacco pipe, but to secure such a speed 
the pressure must be many atmospheres, not a few 
feet or irches of air. When we kindle a fire in the 
fire-place, we have then smoke (which is heated air, 
and bits of coal, &c.) pushed up gently by the heavier 
air around in a continuous current. To allow it to 
get up at the speed such small force causes, there 
must be room enough, say from 80 to 140 square 
inches (i.e. 8x 10 to 12x12) of avent. The straighter 
and smoother the vent, the easier the passage, just 
as in flowing water. All bends should be easy, and 
no sudden contractions, no jutting corners. To secure 
this, perhaps the best thing would be to make a 
block or frame the requisite size, and make the mason 
build round it, ¢rawing it up ashe raises his build- 
ing, thus securing a free and equal passage all the 
