This section is from the book "The Gardener V1", by William Thomson. Also available from Amazon: The New Organic Grower: A Master's Manual of Tools and Techniques for the Home and Market Gardener.
Sir, - In the erection of pits, the conservation of heat by the means of "mother earth" is very often underestimated, if not ignored altogether. I think there is nothing that we can do with more advantage to our plants than endeavour to have them rather under ground than above it. The further a house or pit is raised above ground, the more it catches the bitter blast in winter. The roof we must have exposed; but why have the walls also exposed, when they can be built for less money, and heated at less cost afterwards, by having nothing exposed to the elements but the glass roof? And not only is it of advantage in heating in winter, but it is of great advantage in the maintenance of more genial moist atmosphere in hot dry weather in summer, as every one can testify who has had experience of such pits, or given the thing serious consideration. For a range of useful pits, I would suggest something like what is represented in the accompanying section. Supposing a a to be the ground-line, mark off and level the soil where the outside walls are to be, and run it hard so that there is no chance of its sinking. On this build your outside walls, placing at intervals of 6 or 8 feet under the wall a right-angle elbow 3-inch sanitary pipe, socket-end up, as shown at b b.
By placing three bricks on edge round its end, and breaking off the end of the brick just above this pipe, a connection with the inside of the pit is secured. Another pipe, placed in the socket at b, will rise above the eaves of the pit; and to prevent wet entering, a tin or zinc cover can be supported 3 inches above the pipe by three pieces of stout wire, to fit inside the sockets. These will form ventilators which may in most cases be left open, except in severe weather; but when desirable to have them at command, a small shutter to each inside can easily be applied. When the mortar is sufficiently set, the spaces between the walls d d and also e e may be filled up with the soil excavated for a footpath c, building a wall on each side in the usual way. The space between the pits should be in the form of a gutter, asphalted, and made to carry the water to tanks inside the pits. These gutters should be 18 inches or 2 feet wide, and if the ventilators are placed alternately there will then be plenty of room for cleaning out, attending to shading in summer, or applying mats or other coverings in the winter. A drain-pipe under the ashes in the beds will carry part of the water (otherwise wasted) back to the tanks.
The inside arrangement of this pit is specially adapted to the growing of decorative plants of dwarf growth, such as Cyclamens, Primulas, Cinerarias, Bouvardias, Achimenes, Begonias, Poinsettias, and dozens of other plants, which will do far better than in houses of any other description. But with a little modification of the arrangements, it can be made equally suitable for propagating, forcing winter-flowering plants, growing pot-Vines, Melons, Cucumbers, Tomatoes, etc. etc.
The great objection to these sunken pits is the necessity of having steps down to the doorways. This, however, is not always necessary. If they are built on sloping ground they may be so arranged as to be wholly under ground except the ends in which the doors are placed. In such a case the end walls would have to be built first, the mean height of the soil ascertained and levelled in the same way as you would form a terrace, and upon this level, properly consolidated, commence to build as on level ground. In building a number of such pits a large tank should occupy the opposite end to the door, and these should not only be connected with each other, but should be made one tank, so that the water will run direct from the gutter into it. In every such tank a flow and return hot-water pipe should be placed, for the use of cold water in watering plants works untold mischief wherever it is applied in heated structures. R. Inglis.