Provision must be made for ample ventilation. The most approved plan is to have a line of vents on both sides of the ridge. If devices are used to prevent the ventilating sash from binding, there is no reason why they should not be continuous. If such devices are not used, at least one line of glass should separate the ventilators. They may be hinged on the ridge or on the headers of the roof bars. Both systems have earnest advocates. The ventilation is more free when the sash are hinged on the headers, opening at the ridge; while there is, on the other hand, greater danger of cold drafts striking the plants, and rain and snow are easily admitted with this form of ventilation. But when houses are used until midsummer or later, hinging on the header is probably the better plan. Side ventilators are often provided, but many growers regard them as of doubtful utility. They are most useful in warm weather. But whatever the method of ventilation, it is of the greatest importance that the ventilating machinery work easily.