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.
To any one about to build, and who wants a really good plant-house, I would suggest one of similar size and construction to that shown in the accompanying section, p. 274. What is worth doing at all is worth doing well. There is a general want of room in a great majority of plant-houses. They are very often built too narrow, under the impression that they will cost ever so much less than if they were wider. This, I think, is more imaginary than real, as any one will find out by a little calculation, or by submitting plans of a narrow and wide house for an estimate to any horticultural builder, and compare the extra cost with the extra accommodation. Plants, like animals, require some body of atmosphere to breathe, and if a lot of plants are crowded into a small house, the air will become vitiated in the same way as it would when animal life is shut up. To enclose a good body of atmosphere without having high side-lights and a high-pitched roof, a considerable width of house is necessary. High side-sashes are objectionable (except for very large specimens), on account of their placing the roof so far from the plants.
And I think high-pitched roofs are also objectionable, because in hard weather heat is more apt to be driven from them by the force of the wind than in roofs of a lower pitch. In houses that are too high in proportion to their width, the ventilation in windy weather approaches to something like a draught or blast of cold air, which generally makes the atmosphere of the house anything but favourable.
Most gardeners are ready to admit the desirability of a regular temperature being maintained where they have their choice plants; and I am sure there are few men who have not had their patience tried in endeavouring to do this in small houses. They are quickly heated by the sun, and as rapidly cooled by frost and cold winds - span-roofed houses, of course, much more so than lean-to.
Our section represents a house 20 feet wide inside. Opposite the pipes in the side wall are a series of ventilators 18 inches by 9 inches, closed by wooden shutters fitted to slide backward and forward on the inside of the wall, so that they can be used in all weathers. A lath or iron rod can be attached to each shutter, and by means of a level the whole can be opened together. The side sashes and those of the lantern may be worked by the usual rod-and-lever so commonly adopted. A heavy roof like this would require some support, and that might be supplied by iron pillars similar to that represented in our fig. at convenient distances along the centre of the house. The side stages I have represented rather lower than we generally see them. To see a plant to advantage, place it on a low stage, or on the ground. It is quite a mistake, I think, to have plant-stages 2 feet 6 inches or 3 feet high. If the above stages were made 2 feet 6 inches high, the pots would be upon a level with the top of the wall-plate, and so will be continually subjected to the draught from the side lights when these are used, and will therefore be continually getting dry, not so much through what the plant absorbs as through the drying effect of the air (and in many cases through the direct action of the sun) on the pots.
If the stage is a foot below the level of the wall, the air will circulate amongst the plants without having that objectionable tendency to dry the soil in which they are growing. To avoid the dry heat from the pipes, the side benches should be covered with an inch of fine gravel or rough sand. Under the centre stage is a convenient place for a tank for rain-water. The pipes shown in plan will be sufficient for a stove.