Warming and Ventilating Buildings. 4S 
exercise, than the polygonal figures of many of our new prisons, 
and, perhaps, quite as favourable for other objects. Where a pri- 
son is in a low and unhealthy situation, it would be desirable to 
adapt a machine for changing the air of the prison to the tread- 
wheel to work, when there was a deficiency of more profitable 
employment for the power. This would surely be better than 
either working vanes against the wind for no purpose, or work- 
ing against the friction of a brake-wheel. 
The atmosphere of London is truly a problematical subject ; 
but it is important that it should be studied. It contains up- 
wards of a million of human beings, each of which consumes 32 
cubic inches of oxygen per minute, and ejects an equal bulk of 
carbonic acid gas in the same time ; there is also an immense 
number of animals, all tending to vitiate the atmosphere. The 
greater part of the carbon of nearly two million chaldrons of 
coals is also converted into carbonic acid gas in one year, at the 
ex pence of an equal bulk of oxygen. But the evolution of so 
much carbonic acid gas, immense as it is, almost always takes 
place at a temperature, and under circumstances, very favour- 
able for its diffusion in the atmosphere ; while the power of car- 
bon to absorb animal eflluvia, very probably renders it an im- 
portant agent in improving the quality of the air of the metro- 
polis. We must, however, regret, that the ascending currents 
of smoke are almost always charged with considerable quantities 
of soot ; and that of the ingenious methods which have been 
tried to remedy this inconvenience, very few have been, in any 
material degree, successful. There are two principles which 
may be resorted to ; the one consists in causing the soot to pre- 
cipitate from the smoke before it ascends, or during its ascent 
up the chimney ; the other consists in providing the means to 
consume the soot ; and whichever of these principles be acted 
upon, the draught of the chimney will be impaired. Hence, for 
all operations which require a strong fire, there must either be 
a very high chimney, or the neighbourhood must be annoyed 
by smoke. A well managed fire will afford very little sooty 
smoke when it is properly constructed ; but how difficult it is to 
get a fire well managed ; and, therefore, while every exertion 
to reduce the quantity of sooty smoke should be encouraged, 
we can scarcely expect more than a slight amelioration of the 
