changeable weather happened when it ranged from 30 to 29 inches, and when rainy or stormy weather occurred it was even lower. Hence, it became the practice to place upon barometer scales words indicatory of the weather likely to accompany, or follow, the movements of the mercury; whence the instruments bearing them obtained the name "Weather Glasses." =2. Construction of Barometers.=--In order that the instrument may be portable, it must be made a fixture and mounted on a support; and, further, to render it scientifically or even practically useful, many precautions are required in its construction. The following remarks apply to the construction of all barometers:--Mercury is universally employed, because it is the heaviest of fluids, and therefore measures the atmospheric pressure by the shortest column. Water barometers have been constructed, and they require to be at least 34 feet long. Oil, or other fluids, might be used. Mercury, however, has other advantages: it has feeble volatility, and does not adhere to glass, if pure. Oxidised, or otherwise impure mercury, may adhere to glass; moreover, such mercury would not have the density of the pure metal, and therefore the barometric column would be either greater or less than it should be. The mercury of commerce generally contains lead; sometimes traces of iron and sulphur. It is necessary, therefore, for the manufacturer to purify the mercury; and this is done by washing it with diluted acetic, or sulphuric acid, which dissolves the impurities. No better test can be found for ascertaining if the mercury be pure than that of filling a delicate thermometer tube; if, on exhausting the air from this thermometer, the mercury will freely run up and down the bore, which is probably one thousandth of an inch in diameter, the mercury from which this thermometer was made will be found fit for any purpose, and with it a tube may be filled and boiled, not only of one inch, but even of two inches diameter. In all barometers it is requisite that the space above the mercurial column should be completely void of air and aqueous vapour, because these gases, by virtue of their elasticity, would depress the column. To exclude these the mercury is introduced, and boiled in the tube, over a charcoal fire, kept up for the purpose. In this manner the air and vapour which adhere to the glass are expanded, and escape away. One can tell whether a barometer has been properly "boiled," as it is termed, by simply holding the tube in a slanting direction and allowing the mercury to strike the top. If the boiling has been well performed, the mercury will give a clear, metallic sound; if not, a dull, flat sound, showing some air to be present.