GENERAL REMARKS ON HEATING. 145 
Two other serious objections to the flue system of heating 
are the constant danger of cracks or flaws in the masonry 
occurring, when the smoke and sulphurous fumes will 
escape into the house, and may do serious damage. The 
young growth of maidenhair and other ferns, as well as 
some other plants, is so delicate as to be quickly destroyed 
by even a slight escape of this kind. The second objection 
is the danger of the draught becoming stopped by contrary 
winds—for all furnaces and flues draw better with the 
wind in certain directions than others—or by the chimney, 
which is usually at a distance from the furnace, becoming 
cold. When this occurs the fumes must go somewhere, 
and however sound the brickwork may be, a portion is 
sure to find its way through into the interior of the house, 
to the certain injury and possibly total destruction of the 
contents, in whole or part. This difficulty is however 
obviated by employing the double flue described farther on, 
which possesses several important advantages over the old- 
fashioned or single flue. 
On the other hand, a fire contained in the brick furnace 
of a flue will (if not allowed to burn away too rapidly) 
remain alight much longer than one of the same size and 
character in a boiler furnace and surrounded by iron and 
water only, and farther, when once the mass of brickwork 
becomes thoroughly heated it will retain its warmth and 
prevent the ingress of frost to the house for some hours. 
Neither does such a fire require such frequent attention as 
those in the boilers of a small hot water apparatus. In fact, 
it may be safely affirmed that a well-constructed flue is a 
more dependable and less troublesome method of heating 
small structures than any of the usual type of hot water 
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