114 GREENHOUSE CONSTRUCTION AND HEATING. 
the flow in front, or a flow and return of the same size 
is sometimes placed along the front or lower side. 
Pits may be constructed with either lean-to or span roofs, 
and in some cases the three-quarter span form is adopted. 
As a general rule the span roof is employed in the case 
of long pits running north and south, and the lean-to, or in 
some cases the three-quarter span, where they trend east 
and west, or face south. The simple lean-to roof consists 
merely of ordinary lights, the usual size being 6ft. by 4ft., 
made to slide up and down on suitable bearing-bars, 
or rafters, but the three-quarter or full span forms are 
generally made with shorter lights on each side, hung to the 
ridge by means of suitable hinges or joints so as to lift up 
on the outside when necessary. 
A usual and convenient height for the walls of such a 
lean-to pit of the usual (6ft.) width, is about 1 foot in 
front, and 2ft. 3in. or 2ft. 6in. at the back. A greater 
depth than this is occasionally afforded, but as a rule, where 
depth is required it is obtained by sinking the interior below 
the ground level. In a span-roofed pit both sides are of 
equal height, of course, and are as a rule comparatively low, 
seldom exceeding a foot or so. 
The walls are usually constructed of brick—generally 
44in. work—which should preferably be put together with 
cement mortar, but concrete or stout boards are frequently 
employed, and where a very cheap affair is required they 
may be formed of turf-sods neatly stacked together. 
Slight plates, usually of 3in. by Qin. or 4in. by 2in. stuff 
must be placed on the walls, in the same manner as directed 
on p. 51. In order to make a good job these should 
be bevelled on their upper surfaces to the same angle 
