HOW TO BUILD A CHIMNEY. 83 
slopes downwards however, as it does where the house 
is on the side of a hill, over which the wind is 
blowing; or even if the house be on the edge of a 
precipice, down which the current sweeps,—of course 
the wind will blow slanting downwards into the upper 
mouth of the vent; and, if it is at all strong, will 
easily send down the current of smoke. To avoid 
this, a sufficient protection is needed at the top. 
Pigeon-holes—as one is always (and two often) turned. 
to the wind, and so wind allowed to enter, which may 
go down, or at best occupy the opposite hole for 
egress—are not good in any difficult case. The sim- 
plest and best in ordinary circumstances is an iron 
pipe, say of 9 inches diameter, with a pyramidal 
cover fixed over it, sufficiently high and wide to allow 
of the easy exit of the smoke, and reaching far enough 
down to keep the downward slanting wind from get- 
ting into the pipe. In very bad situations, an ‘‘old 
wife” may be necessary, i.e., a movable bent tube 
over the other tube, the mouth of the bent tube turned | 
to one side, and, by the aid of a vane, kept turned 
from the wind. Or better still a ‘‘sailor’s hat,” ie. 
2 pyramidal cover like the firet-mentioned, but, instead. 
of being fixed, turning on a universal ball joint in 
the apex. Hither of these has the advantage of 
keeping out the current that would blow down the 
vent, and also another advantage which will be no- 
ticed hereafter. The only other source of annoyance 
requiring to be noticed is the interruption, by any 
cause, of the pressure of the external air. At home 
where doors and windows are so close as to allow 
scarcely any opening for air to enter, the greater part 
of the few feet of extra pressure of the air without 
is expended, in forcing the requisite current of air into 
the room through the key-hole, and sending it whi:tling 
through other chinks and crannies, so that often not 
enough pressure is left to carry up the smoke. The 
opening of a door, or window, or other hole to carry 
the air below the fire, is the easy remedy. In this 
country where al! is so open no such cause operates. 
But an interruption may also be caused by an eddy. 
If we put a board into a swift running stream, and 
hold it firm with the end on the bottom and its breadth 
across the current, it will be found that the water, 
although it bends in its current round the edges of 
the board to fill the space behind it, yet does not 
stand so deep behind the board as elsewhere. If the 
current be strong and the wat-r say a foot deep gener- 
ally, behind the board it will be only 9, or 8, or 
6 inches, according to the force of the current. So 
with a stone, even when the water flows over it. 
Behind the board or stone where the eddy is, there 
