hes with
panelling above, carved with the salamander and other armorial devices.
In the Roman palaces they are sometimes masked by the balustrades, and
(when shown) take the form of sepulchral urns, as if to disguise their
real purpose. Though not of a very architectural character, the chimneys
at Venice present perhaps the greatest variety of terminations, and as a
rule the smoke comes out on the sides and not through the top.
(R. P. S.)

_Factory Chimneys_.--Chimneys, besides removing the products of
combustion, also serve to provide the fire with the air requisite for
burning the fuel. The hot air in the shaft, being lighter than the
cold air outside it, tends to rise, and as it does so air flows in at
the bottom to take its place. An ascending current is thus established
in the chimney, its velocity, other things being equal, varying as the
square root of the height of the shaft above the grate. The velocity
also increases with increase of temperature in the gas column, but
since the weight of each cubic foot grows less as the gases expand,
the amount of smoke discharged by a chimney does not increase
indefinitely with the temperature; a maximum is reached when the
difference in temperature between the gases in the shaft and the
outside air is about 600 deg. F., but the rate of increase is very slow
after the difference has passed about 300 deg. F. In designing a chimney
the dimensions (height and sectional area) have to be so proportioned
to the amount of fuel to be burnt in the various furnaces connected
with it that at the temperature employed the products of combustion
are effectively removed, due allowance being made for the frictional
retardation of the current against the sides of the flues and shafts
and in passing through the fire. The velocity of the current in actual
chimneys varies widely, from 3 or 4 to 50 or 60 ft. a second.
Increased velocity, obtainable by increasing the height of the shaft,
gives increased delivering capacity, but a speed of 10 or 12 ft. a
second is regarded as good practice. Ordinary factory chimneys do not
in general exceed 180 or 200 ft. in height, but in some cases,
especially when, as in chemical works, they are employed to get rid of
objectionable vapours, they have been made double that height, or even
more. In section they are round, octagonal or square. The circular
form offers the least resistance to wind pressu