Thermometer is an instrument for measuring variations of temperature. The volume of a given quantity of air or other gas increases uniformly as its temperature rises, provided its pressure remains constant, and this expansion is made use of in the construction of the air-thermometer. A glass tube of small bore terminates at one end in a glass bulb, and in this tube is an index consisting of a drop of mercury. If the air in the bulb is brought to the temperature which is to be measured, it will expand or contract, as the case may be, and its change in volume will be indicated by the motion of the index; or, the tube being vertical, with the bulb at the upper end, the free end of the tube may dip under the surface of liquid, which will be forced up the tube by atmospheric pressure to a greater or less extent as the temperature of the enclosed air is varied. Owing to the large coefficient of the expansion of gases, air-thermometers may be made very sensitive, but it is necessary always to take account of any variation of pressure. For most purposes a mercury-thermometer is more convenient. Here again a capillary tube has a bulb at one end, and the bulb and part of the tuhe are filled with mercury. The mercury is carefully boiled in the tube, so that all air may be carried away by the mercury-vapour and the upper end is then hermetically sealed. Coloured alcohol is sometimes substituted for mercury, but is in general less satisfactory. Thermometers may be graduated either by comparison with a standard instrument, or by the following method:- The instrument is surrounded by melting ice, which has a fixed temperature, and a mark is made on the tube at the point at which the mercury comes to rest - this is freezing-point, 0° on the Centigrade and Reaumur scales, 32° on the Fahrenheit. It is then immersed in the steam from water boiling at the normal atmospheric pressure, which again is at a fixed temperature, and a second mark is made - this is boiling-point, 100° C., 80° R., and 212° F. The interval between these marks is then divided into 100,80, or 180 divisions, according to which scale is adopted, and the graduation is continued above and below as far as necessary. These graduations are preferably etched on the glass stem, but may be marked upon a paper, or other scale attached thereto. A thermometer is more sensitive as the size of the bulb is greater and the bore of the tube smaller; and in an instrument required to indicate quickly the bulb must be made as small as will give the requisite sensitiveness, and the glass of the bulb should be thin. The glass undergoes a molecular change in course of time, whereby the volume of the bulb is diminished and the zero displaced, and a thermometer should, therefore, not be graduated until it has been made for some considerable time. In registering thermometers a small index is placed in the tube, and is pushed along by the mercury; when the liquid sinks this index is left, and indicates the maximum or minimum temperature which has occurred since the index was set. The indices may be made of iron, in order that they may be replaced with the aid of a small magnet. Metallic thermometers are occasionally used, and consist of a strip composed of two ribands of metal fastened together. These metals are selected so that their coefficients of expansion are as different as possible, and by their differential expansion and contraction the strip bends with changes of temperature. In Immisch's thermometer the tube of a small Bourdon pressure-gauge is filled with ether. The attractions of pressure and volume with change of temperature thus move a pointer over a scale.