PLATE XXIII. 
slip projects beyond its roller, as seen at 5 , 6 , that when fallen into the cavity they may lap over 
each other, as at 7, 8 the cavity is placed midway the house, and a little deeper than the semi¬ 
diameter of the roller, that it may not be pulled out of its place by the weights; as they will then act 
nearly at right angles on it, (the roller and slip are together called the runner) the ledge, which keeps 
the rollers in their places is removed, that the whole may be seen distinctly : to close the house, the 
runners are pulled forward (by the contrivance beforementioned) until they fall into the cavity, 
where they remain. To uncover it, the lower runner is pushed up, and both return to their places at 
■ 9 > 9 . by the descending of the weights. That the canvas and roller may be entirely taken out of 
the cavity,(it should run through the front, but be stopped only when wanted), lest they should 
rot; the canvas has holes worked in it, and the edge of the runner buttons on it, so that they 
may be readily disengaged from one another; at the bottom of the grooves, where the weights 
descend, should besmallholes stopped with a loose brick (plastered over), to take out the weights 
if the cords should break. At D, is shewn the plan of a furnace ; E, its length section ; and F, its 
elevation in perspective. Having the bottom of the flue on a level with the bottom of the furnace, 
may be justly esteemed an error, even when coals are used, but a very great one where wood or peat 
is the fuel burned ; for the ashes being then carried in by the draught, will with the soot, soon choke 
the flues; to avoid this, the flue should ascend directly from the furnace, (at the very least) in an angle 
of thirty degrees, and may be so much as sixty ; the flue should be narrower at its throat than else¬ 
where, and then, though the bottom of each revolution was on a level, or such dips as seen at G, 
continued round the stove, the heated air would be forced through them ; at the end of the furnace 
the sides are all inclined to the flue, that none of the rays may be reflected, but directed to it as a 
