The Garden Under Glass

264 THE GARDEN UNDER GLASS the case of a span-roofed house the ventilators on the lee-ward side of the house should be opened, with those to the windward usually closed. Ventilating a Frame There are various methods of ventilating frames, such as pushing the lights alternately up and down, propping them up at the top or at the bottom, tilting them at the side or drawing them off entirely. Another method of affording a free circulation of air is to stand the frame on four bricks —one at each corner—thus admitting air through the space left between the bottom of the frame and the ground. When propping up the frame either at top, bottom, or side, it is essential that the “ prop ” be placed centrally. If set more on one side it will cause the light to warp. Methods of Ventilation It would, of course, be impossible to give readers an idea of the many methods of ventilation, for nearly every horticultural engineer has ideas of his own on the subject. But they all centre round a few main principles. The old-fashioned method of giving ventilation to a house was to have the lights slide up and down. This is now well-nigh obsolete, and I do not remember ha ving seen any houses of recent erection supplied with this method. The plan was to form lean-to houses against a convenient wall, and to have the top lights slide down and the bottom ones up, something after the style of a casement window. The great drawback to this plan is that the lights must always be closed during rain, or the plants will get wet and the house and its occupants a prey to damp. Undoubtedly the best system is that by which the lights at the top of the