heated with a flow-and-return four-inch pipe on each side, and openings in each wall must be provided to allow the heat to escape into the pits to keep the inmates safe. There is no objection to the pits being heated with a separate system of pipes, from the same boiler, of course. Two-inch pipes will be large enough for the pits; and one passing down one pit and up the other will be quite sufficient for ordinary purposes, especially as the lights can be covered with mats in severe weather. The greenhouse floor should be laid with white and black tiles, unless stone can be procured at a cheap rate; but for the floor of the pits a bed of ashes will suffice. The openings in the wall should be provided with shutters, in case the frames should be occupied with hardy plants, that merely require protection from too much moisture; if side-lights are adopted, they should be made to slide, instead of pushing out, and, of course, be made to slide from the inside.
The next example is an improvement on the foregoing. The pits are removed, the space is wider, and the angle at ridge more sharp, and more ample staging is provided, the centre stage having the advantage of a course of pipes on each side, connected with the flow near the boiler. The width is seventeen feet, the height to the ridge nine feet. This would make a good vinery, and a number of plants could be grown beneath the vines, though, to be sure, some caution would be required in selecting them, and it would not be prudent to clothe the roof densely with the ample foliage of the vines.
SECTION OF SPAN-ROOF GREENHOUSE WITH STAGING AND HOT WATER PIPES.
Now, we propose to go to work and build a cheap span-roof
