The thermometer is an instrument for determining the temperature of the air or any other fluid into which it may be introduced. The thermometers in general use contain mercury, but some contain colored spirit; yet, as mercury is most generally used, it will be only necessary to say of spirit thermometers, that they act on the same principle. A thermometer consists of a glass ball having a long thin hollow tube rising out of it and attached to a graduated scale--the bore or hollow of the tube is very small, scarcely sufficient to admit a piece of sewing cotton. The ball or bulb and part of the stalk are filled with mercury by holding a lamp to the ball till the air is nearly all expelled by its expansion--for heat expands air very greatly--and putting the end of the stalk into a vessel of the fluid. When the lamp is removed the air in the bulb cools and therefore contracts, by which means the mercury is forced up the fine tube, very nearly filling the bulb. The bulb is then held downwards and the mercury so heated that it expands, as did the air, till it fills the whole of the bulb and stalk up to the very top; the top is then melted with the blow-pipe (see “Blowpipes”), and the glass, running together, closes up the bore at the end. As the mercury cools it contracts, and consequently, occupying less space, falls down in the stalk pretty close to the bulb, the space above it is therefore empty, and forms a “vacuum.” Now, therefore, we have an instrument, consisting of a bulb and stalk half-filled with mercury (fig. 1). Upon any amount of heat being applied to the bulb, the mercury in it expands, and rises in the stalk in proportion to the amount of heat applied, or shrinks and sinks down again as it cools. The next thing to be done is to form a “scale” by which the height of the mercury in the stalk may indicate some known or recognised temperature. There are three scales in use, “Fahrenheit’s,” “Reaumur’s,” and the “centigrade.” The scale universally used in England is Fahrenheit’s, although both this and Reaumur’s are sometimes marked on the same thermometer (fig. 2). Fahrenheit’s scale is formed thus:--The bulb of the thermometer is placed in boiling water, and the height to which the mercury rises is marked by a scratch on the stalk; it is then put into snow or ice in the act of melting, and another scratch is made where the mercury has descended to. The space between these two marks is divided into 180 equal parts called degrees, and these divisions are carried upwards to nearly the end of the stalk and downwards to near the bulb; the upper scratch, indicating the heat of boiling water, is marked 212, and the lower one, which marks the freezing point of water, being 180 divisions lower, will be 32; and of course, 32 degrees lower will be 0, and is called “zero.” On the scale of Reaumur’s thermometer the zero or point marked 0, is at the freezing point of water, and the boiling point is marked 80 (fig. 2).