According to the experinients of Mi. Dalton, mercury does not boil till it has acquired a temperature equal to 660° of Fahrenheit's scale; and it does not freeze till it is subject to a degree of cold expressed by 39 divisions below the zero of that scale, of below the freezing-point of water. Pure alcohol, on the other hand, has never been frozen, though it has been exposed .to a degree of Cold exceed ing that which is expressed by 91° below the zero of Fahrenheit; and therefore a spirit thermometer is to be preferred to one of mercury when it is intended to ascertain the temperature of the air in high nOrthern or southern latitudes brit since the spirit boils in air with a degree of heat expressed by of Fahrenheit, it is unfit for many of the pur poses for which a thermometer is required. For instruments capable of measuring very high temperatures, see PYttomtiEn.
Register Therriameters.—It is of great im portance hi meteorology that the observer should be able to ascertain the highest or lowest point of a thermometer scale at which the column of mercury may hrtite stood during his absence ; and several contrivances have been adopted by artists in order to obtain this end. Of these, one, which is still preferred, was invented by Mr. Six, Whose name the instrument bears ; II is described in the Philosophical Transactions' for 1782. It consists of a long tube bent se as to form three parallel branches, of which the central branch is art elongated bulb, and the rest of the tube has a capillary bore. The lcoVer portion of the bent tube contains mercury, which rises in the two side branches to Certain points, and the bulb is filled with spirit of Which passing over a bend at the top descends to the upper extremity of the mercury in one of the branches ; the upper end of the other branch is also filled with spirit, and this is hermetically sealed. Two small indices of
steel, coated with glass, are introduced in the branches, and are capable of being forced upwards by the rising of the column of mercury in either tube, and they have about their a fine wire or a thread of glass ; so that they will remain stationary where they happen to be when the heads of the columns recede from them. Their lower extremities conse quently indicate the points at which the ends of the columns may have stood before such recess.
Differential Thermometer.—This instrument, which was invented by M. Sturmius, of Altdorf, before the year 1676, and was revived by Professor Leslie in 1801, consists of two thermometer tubes, terminating, at one extre mity of each, in a holloW glass ball, and con taining coloured sulphuric acid : the opposite extremities are united by the flame of a blow pipe, and an enlargenient of the bore is made, at the place of junction. The tube is then bent so as to form three sides of a rectangle, the two balls, which are of equal diameter forming the upper extremities of two sides ; and'the instrument is on a stand with the branches of the tube hi vertical positions. When the temperatnre Of the air in the two balls is the same, the acid occupies one side and the base, and rises a little way tip the other side of the rectangle. To the latter side is attached a gtadtiated scale, with the ierti Of Which the upper extremity of the acid in that branch sbetild coincide. In the event of this adjustment being deranged, it may be restored by causing a small quantity of air to pass from one ball to the other, which is done simply by the warmth of a hand applied to that ball from whence the air is to be driven.
Radiating Thermometer.--[AcTnioarETER.]