I observed the instrument three times a day during the whole summer and autumn of 1881, and there was never an increase in the height of the precipitate without a corresponding fall of temperature, or a decrease without a corresponding rise of temperature. It will thus be seen that the instrument is not a " storm glass" or a "chemical barometer," but a chemical thermometer, and a very poor thermometer at that.
Do its indications bear any relation to weather changes? Yes; because temperature changes are closely related to other weather changes. In the summer under an extended but moderately high pressure area, or anti-cyclone, there usually exists beautiful, fair, warm, settled weather, and the top of the precipitate will be even with the division marked "Fair." When showers come on, the air turns cool, and the precipitate rises to the division marked "Change." Whenever an area of decided low pressure passes over the country, the air is warm in front of it; and as the centre passes over, there is usually a sudden fall of temperature. This causes the precipitate to rise up to the division marked "Storm," and the wind in such cyclonic systems is always high, especially in their rear. In the winter the top of the precipitate is nearly always up in the division marked "Change," and as the weather is eternally changing, I suppose it may be considered about right. On the contrary, a decided anti-cyclone may in summer bring cool weather; and the precipitate will rise up into the division marked "Storm," accompanied by the clearest skies and delightful breezes. It is also common for rains to pass over without any material changes in temperature.
In such cases the height of the precipitate will remain unchanged. The indications of the instrument are then sometimes right, sometimes wrong, like the negro's dumb watch, which was always right twice a day, but, as it gave no indications of the time when it was right, it was not worth much as a practical instrument. (H. Helm Clayton.)
(12) Glycerine Barometer
The marked influences of the variations in the pressure of the atmosphere upon the disengagement of carbureted gases in coal mines has led engineers to devise a new barometer that will not only indicate the most minute variation of atmospheric pressure, but will indicate it so plainly that miners and others not experienced in making barometric observations can readily detect the variations.
Among the instruments of this class one of the most interesting is the large water barometer constructed for the Royal Society by Prof. Daniell, in 1830, which, however, was not a success, as the effects of the pressure were annulled by the effect of the temperature upon the vapour found in the Torricellian vacuum.
B. Jordan, a mermber of the office of the English mining archives, has spent several years in studying the different liquids that might possibly be applicable in constructing an accurate and highly sensitive barometer, and finally found that glycerine produced the best results. A glycerine barometer constructed by Jordan, 1870, is still in use. The glycerine is very pure, and has a specific gravity of 1.26, and on account of its high point of ebullition the vapours have no perceptible tension at the ordinary temperature, and it will only congeal at a very low temperature. The height of a column of glycerine is 26 ft. 9 in., and a variation of -1/16 in. of mercury corresponds to a variation of about 1 in. in the column of glyeerinc. As glycerine is very apt to absorb the moisture of the air, it is covered with a thin layer of prepared thickened petroleum in the cistern of the barometer. Jordan has constructed barometers for the South Kensington and Jermyn Street Museums; both have given perfect satisfaction, and to show the scientific value of the instrument the Royal Society has built one at the Kew Observatory.
This instrument is shown in Fig. 3GG, and consists of a cylindrical cistern of tinned copper, about 6 in. high and 10 in. diameter, provided with a screw cover or cap, having a email opening leading into a recess containing cotton to act as filter and keep out the dust. The large barometric tube is made of ordinary gas pipe, about 3/4 in. diameter, and is rigidly attached to the cylindrical cistern or cup. The upper end of this tube fits into a piece of bronze, into which a glass tube, 3/4 in. diameter and about 4 ft. high is securely cemented. This tube terminates in a cnp inclosing a rubber packing. Graduated scales provided with indicators are placed at ench side of the glass tube, the one on the left side indicating the inches and tenths of inches, and the right-hand scale shows the equivalent measure of a corresponding column of mercury. The scales are attached to an oaken plank, which is fastened to the wall of one of the upper storeys of the observatory, and the large tube passes down to a 'room situated 26 ft. 8 In. lower. The glycerine in the barometer is coloured, with aniline red. Before putting the glycerine in the tube, it Is bailed at a air and to make it purer.
The air is exhausted from the barometer tube by at the Kew Observatory, where it is considered to be n scientific instrument of the greatest precision.
(13) How To Make A Glycerine Barometer
A bottle about a quarter filled with glycerine, coloured red with magenta or crimson aniline, has a glass tube of about the diameter of a pencil passing airtight through the cork, which is inserted airtight into the bottle. The lower end of the tube dips beneath the surface of the glycerine. The bottle is made to contain compressed air by blowing into the upper end of the tube. On removing the mouth, part of the glycerine will rise in the tube until the weight of the liquid column in the tube and the atmosphere balance the internal air pressure on the surface of the glycerine. The column in the tube will tend to rise when the pressure of the atmosphere diminishes, or the temperature of the compressed air rises, and to fall when the atmospheric pressure increases or the temperature of the compressed air diminishes. So far as the variation in the height of the column is due to changes in atmospheric pressure, the column moves in the opposite direction from that in a mercurial barometer.