228 
PLANS OF RESIDENCES 
and six feet will be a sufficient clear width inside. For an alley 
fifteen feet wide, the arch should be eighteen feet long. The steps 
down to it, and their flanking walls, would make a length of ten to 
fifteen feet more on each side — depending on the manner of the 
descent, and the nature of the superincumbent improvements — and 
likely in any case to make the entire excavation upwards of forty 
feet in length, including the slopes for the steps. The side-walls 
throughout should be double or hollow walls; the inner one of 
brick, nicely pointed, the outer one of stone, and both made water- 
tight with water-lime cement. The arch over the tunnel proper 
should be made with great care to render it perfectly water-tight 
also ; and if the entire filling above the arch, and on the outside of 
the side-walls, is made with good gravel, broken stone, or coarse 
sand, so as to let all surface water soak down directly to the drain 
below the floor of the tunnel, there will be little liability to excessive 
dampness or dripping water in the tunnel. The arch for the main 
tunnel on this plan is to have the springing points five feet from 
the floor, and to be that segment of a circle which will make the 
centre seven feet high. For stairs, broad solid stone steps are of 
course the best in the long run, but some expense for such work 
may be saved by having the slope down to the tunnel floored with 
a smooth water-lime cement, and a flight of plank steps put in, 
supported at the ends only, and high enough above the sloping 
cement floor to allow the latter to be readily brushed and kept 
clean under the plank steps. These, having the air circulating 
freely all around them, will not be liable to quick decay. 
In the plan under consideration, the walk leading directly from 
the rear arcade of the double-house to the grape-house is to de- 
scend gradually for about twenty feet, so that at the front line of the 
latter it will be two feet below the general surface, and a step on 
the same line will drop eight inches more to a stone landing, from 
which four steps up on each side lead to the two sides of the grape- 
house, and ten steps down, to the floor of the tunnel. On the side 
towards the mansion, the inclosed porch and roof of the entrance 
to the tunnel being made in the construction of the grape-house, 
cannot be considered a part of the cost of the former, but the flank- 
ing walls, the steps, the tunnel itself, and the necessary covered 
