As now constructed commercial greenhouses are built upon severely plain and simple lines. Dimensions vary considerably, but common ranges vary from 10 to 80 or more feet wide, 50 to 500 feet long and five to seven feet high at the walls inside. Great stress is laid upon the slope of the glass so as to take advantage of the inclination of the sun at various times of the year.
Until the closing quarter of the last cen tury wood was extensively used for the frame work, walls and many other parts of green houses. Objections to it are that in roof parts it is so bulky that it casts too heavy shade In walls and trenches, it quickly decays because of the excessive moisture and that for this rea son it demands frequent repair and renewal. Objections to metal in, roofs are its excessive contraction and expansion with changes of temperature and the consequent breakage of glass. Most modern greenhouses therefore combine concrete, brick or stone walls with metal posts, beams and purlins with wooden rafters upon which the glass rests. The most favored because most durable woods are South ern States cypress and California redwood. To increase durability, to lessen damage to glass and leakage into the interior, houses are gen erally built nowadays singly, that is, with an alley between rather than attached from end to end. This plan also favors drainage and con venience in filling the houses with soil or in emptying them. Thus the increased cost of walls is offset by the reduced cost of mainte nance. Where greenhouses are built °ridge and-furrow° style the gutters are generally made of metal and warmed from below by hot water or steam pipes to melt the snow and ice quickly and thus reduce or prevent leakage and damage.
Great care is exercised by greenhouse builders in selecting glass for the roof. Three qualities are sought for; least obstruction to sunlight, strength to withstand wind, storm and especially hail and freedom from defects that tend to form lenses and thus burn the foliage of plants grown beneath. Clear white single thick glass, while it allows 60 to 70 per cent of light to pass through is too frail to be safe. It is also dulled by exposure to the air. Common green glass (so called because green in cross section) allows only 52 to 55 per cent light passage, but it does not readily tarnish. Double thick green glass permits 50 to 52 per cent of light to pass. Because double thick
glass is stronger and because its chemical form ation is such that it is little affected by the atmosphere it is considered best for green house roofs.
The visible defects such as °stones, bubbles and are popularly believed to produce burning; but this is erroneous. J. C. Blair in 1895, after careful experiment, proved this a fallacy. (Bulletin 95, Cornell University Ag ricultural Experiment Station). Bubbles and blisters are generally thinner in the centre than at the circumference, hence they form concave, not convex, lenses and thus they actually spread the light rays instead of focusing them. While and asand stoneso may be convex lenses their foci are almost invariably very close to the glass —a few inches — hence they are pow erless to damage plants several feet below. The real cause of burning damage is the prismatic or lens-like effect due to variation in thickness in the pane as a whole or in large part thus focusing the light rays at 5 to 30 or more feet below the glass. Such defects may be found in all grades of glass, domestic or for eign alike. They can rarely be detected by the eye hut may be easily discovered by testing in sunlight or by a micrometer caliper.
Ample ventilation is essential to success in growing greenhouse crops. Usually about one tenth of the roof is arranged to be opened at the top. It is generally desirable also to have ventilation at the sides, more especially for summer use when fresh air is especially needed not only for the health of the plants but for the comfort of the workmen. It is highly desirable in large houses to provide ventilators on each side of the ridge, each set opening by independ ent apparatus. This will allow of ventilating on the lea side during stormy or windy weather. It is also well to have separate lifting apparatus for each 75 or 100 linear feet of ventilators, not only to make for ease of manipulation but to allow of ventilating one part independently of the others. Preference is given to the plan of hingeing the ventilators to the ridge pole so as to open from their lower sides, rather than to the plan of hingeing diem below so as to open at the peak. By the I orrner method the air enters with less draft and rain and snow are kept out better than by the latter plan.