Thermometer
An instrument used for measuring the intensity of heat; one of the most useful and essential in gardens, particularly in glass houses, where the occupants have to be subjected to temperatures regulated to meet their several requirements. An ordinary Thermometer consists of a fine glass tube, with a bulb at one end. In this either some mercury or spirit is placed, and, as it expands or contracts in the tube, the amount of rise or fall in the temperature is indicated. When a Thermometer is made, all air is expelled from the tube and the end is hermetically sealed. The space not filled with the mercury or spirit is thus rendered vacuous, or nearly so, and there is no resistance to the expanding substance when the temperature rises. Every Thermometer tube is provided with a frame of some sort, on which the scale of degrees is marked, to denote the temperature which the expanding or contracting substance indicates. Fahrenheit's scale is in general use in this country; there are two others, known respectively as Reaumur's and the Centigrade; the last-named is generally adopted on the Continent. Fahrenheit's scale fixes the freezing-point at 32 deg., and the point at which water boils at 212 deg. In Reaumur's scale, freezing-point is Zero, and boiling-point 80 deg. In the Centigrade scale, the freezing-point is Zero, as in Reaumur's, but boiling-point is 100 deg. The three are, therefore, quite different to read, because of the spaces on the scales between freezing and boiling-points being differently divided.
There are various kinds of Thermometers in use where meteorological observations are taken; many of these are made in special ways, for special purposes: thus, a Maximum registers the highest, and a Minimum the lowest, temperature which has been experienced since the indicators in either of the instruments were last set; a Solar Radiation is specially made for indicating the highest temperature in the sun's rays; and there is also a Minimum, for laying on the grass. In all gardens, a minimum registering Thermometer is most desirable, for ascertaining the lowest degree reached in frosty weather during any given period; it should be suspended in a position with north aspect, where there is a fair exposure. Ordinary Thermometers, if correctly graduated to the scale, are well adapted for all horticultural purposes under glass. There are also special ones made for use in Mushroom-beds, or other places where there is bottom heat.