Mr. Ward said—Having been engaged in horticultural pursuits, he had found that it was desirable to have some instrument for regulating the ventilation and temperature of hothouses, without requiring the close attention of the gardener. He then described the invention which he calls a Thermostat, and which consists of metallic vessels, in which the vapour of a volatile liquid is used to give motive power for opening or shutting ventilators, or dampers, or regulating the flow of hot or cold water, or similar applications for regulation of temperatures. The vapour of aether has been used for such purposes when confined in tubes by mercury, but in such apparatus the amount of power to be obtained is much limited by the weight and consequent expense of mercury required. He makes use of vessels of thin metal, of which the ends or sides being discs of metal, are so thin and flexible as to be changeably either convex or concave, and in which the vapour of sulphuric aether (or of suitable volatile liquid), along with a portion of such liquid, is enclosed; the discs or ends of such vessels acting as diaphragms, separating the vapour from the external atmosphere, will, at temperatures under the boiling point of the fluid employed, be pressed inwards by the atmosphere, but with a pressure varying according to the temperature. He therefore attaches a counterpoise of sufficient weight partially to counteract the pressure of the atmosphere, so that the differential pressure of the aethereal vapour above ...