=60. The method of testing Thermometers= for meteorological purposes is very simple. Such thermometers are seldom required to read above 120 deg. In these the freezing point having been determined, the divisions of the scale are ascertained by careful comparisons, with a standard thermometer, in water of the requisite temperature. "For the freezing point, the bulbs, and a considerable portion of the tubes of the thermometers, are immersed in pounded ice. For the higher temperatures, the thermometers are placed in a cylindrical glass vessel containing water of the required heat: the scales of the thermometers intended to be tested, together with the Standard with which they are to be compared, are read through the glass. In this way the scale readings may be tested at any required degree of temperature, and the usual practice is to test them at every ten degrees from 32 deg. to 92 deg. of Fahrenheit."--_FitzRoy._ =61. Porcelain Scale Plates.=--Thermometer scales of brass, wood, or ivory, either by atmospheric influence or dipping in sea-water, are very liable to become soiled and discoloured, so much so that after a very little time the divisions are rendered nearly invisible. To obviate this inconvenience, Messrs. Negretti and Zambra were the first to introduce into extensive use thermometer and barometer scale-plates made of porcelain, having the divisions and figures engraved thereon by means of fluoric acid, and permanently burnt-in and blackened, so as always to present a clear legible scale. That these scales have been found superior to all others, may be inferred from the fact that all the thermometers now supplied to the various government departments are provided with such scales. They can be adapted to replace any of the old forms of brass or zinc scales, the divisions and figures of which have become obliterated or indistinct. =62. Enamelled Tubes.=--Nearly all thermometer tubes are now made with enamelled backs. This contrivance of enamelling the backs of the tubes enables the makers to use finer threads of mercury than had before been found practicable; for were it not for the great contrast between the dark thread of mercury and the white enamel on the glass, many of the thermometers now in use would be positively illegible. The enamelling of thermometers is an invention of Messrs. Negretti and Zambra. It is necessary to state this, as many persons, from interested motives, are anxious to ignore to whom the credit of the invention is due.