In the case of small green-houses, where a regular gardener is not kept, a provision for preventing the house from becoming over-heated, which will act independently of human assistance, is very desirable. There are many contrivances of this kind; some, such as Kewley's automaton gardener, calculated for regulating the temperature to the greatest nicety; and others for merely preventing great extremes of heat. The simplest and most economical mode is to have a hinged pane in the upper part of one of the upright sashes, opening inwards. To the lower part of this pane one end of a cord is attached, and the other is joined to a netting which encloses a bladder nearly filled with air, and air-tight A string from the other end of the bladder is attached to any fixed point near at hand. The bladder should be moderately distended, by filling it with air in the house, when the air of the house is at the highest temperature required; and, consequently, when any increase of temperature takes place, the air in the bladder will expand and completely distend it, so as to change its shape from that of an oval to that of a globe; shortening the long axis and lengthening the short one. The effect of shortening the long axis is to pull in the hinged pane of glass, and thus to admit the external air.

This is, without doubt, a very rude mode, and will not regulate the temperature with any degree of nicety; but for ordinary purposes, and where economy is a great object, it is quite sufficient. Another mode consists in applying a hollow brass tube, of 5 or 6 feet in length, against the lower part or valve of a window that opens outwards, or against the lower end of the vertical rod of a cap which opens upwards. Thus, in fig, 361., if we imagine the rod f to be a hollow brass tube, touching the floor (g) at one end, and the spindle (h) of the ventilating cap at the other; and that, when the rod is in this state of contact with both the floor and the spindle, the house is at the maximum temperature required; it is evident that any increase of heat, by expanding, and consequently lengthening, the brass rod, would raise the ventilating cap; and the extent to which this might be done, would be greatly increased by placing the brass rod on the end of a lever. Brass rods, when required to open the sashes or ventilators of a green-house, may also be applied in a great variety of other ways, which any ingenious mechanic will readily discover.

The last mode which we shall mention is one which has been adopted with success by an eminent horticulturist, John Williams, Esq., of Pitmatson. This ventilator operates by the expansion and contraction of air in an air-tight vessel, (fig. 362. a), communicating with a cylinder and piston (b, c, d), which, by means of a rod (g), operates on the hinged pane or sash to he opened. The use of the water or other fluid is to confine the air; and, by that means, when the air expands or contracts, it operates upon the piston. By means of an adjusting screw, the register may he made to open at any required degree of heat. The air-vessel should contain several gallons, according to the size of the valve or register to he opened. When first used, the vessel roust he heated sufficiently to expand the internal air; water is then to he poured in at the top of the cylinder, so as to give the required motion to the float; and about half an inch of fine oil must be laid on the top of the water, to prevent evaporation.

In a plant cabinet, such an instrument may he conveniently placed under the stage, so as to have the rod (g) directly under the ventilator or sash to be opened.