A Treatise on Meteorological Instruments / Explanatory of Their Scientific Principles, Method of Construction, and Practical Utility

by Enrico Angelo Lodovico Negretti; Joseph Zambra

CHAPTER V.

SECONDARY BAROMETERS.

43. Desirability of Magnifying the Barometer Range.—The limits within which the ordinary barometric column oscillates, do not exceed four inches for extreme range, while the ordinary range is confined to about two inches; hence it has often been felt that the public utility of the instrument would be greatly enhanced if by any means the scale indications could be increased in length. This object was sought to be obtained by bending the upper part of the tube from the vertical, so that the inches on the scale could be increased in length in proportion to the secant of the angle it made with the vertical. This was called “the diagonal barometer.” The upper part of the tube has also been formed into a spiral, and the scale, placed along it, is thus greatly enlarged.

But these methods of enlarging the indications cannot be so successfully accomplished, nor so cheaply nor so elegantly, as is done by the principle employed in the dial barometer. Hence they are not in use.

Fig. 31.

44. Howson’s Long Range Barometer.—Very recently quite a novel design has been patented by Mr. Howson, for a long range barometer. The construction requires neither distortion of the tube, nor mechanism for converting a short scale into a long one; but the mercury itself rises and falls, through an extended range, naturally, and in simple obedience to the varying pressure of the atmosphere. The tube is fixed, but its cistern is sustained by the mere pressure of the atmosphere. Looking at the instrument, it seems a perfect marvel. It appears as though the cistern with the mercury in it must fall to the ground. The bore of the tube is wide, about an inch across. A long glass rod is fixed to the bottom of the glass cistern, where a piece of cork or some elastic substance is also placed. The tube is filled with mercury; the glass rod is plunged into the tube as it is held top downwards, until the cork gets close up to the tube and fits tightly against it. The pressure against the cork simply prevents the mercury from coming out while the instrument is being inverted. When it is inverted, the mercury partly falls, and forms an ordinary barometric column. When the top is held, the cistern and glass rod, instead of falling away, remain perfectly suspended. There is no material support to the cistern; the tube only is fixed, the cistern hangs to it. Glass is many times lighter than mercury. When the glass rod is introduced, it displaces an equal volume of mercury. The glass rod, being so much lighter than mercury, floats and sustains the additional weight of the cistern by its buoyancy. In the mean time, the atmosphere is acting upon the mercury, keeping up the ordinary barometric column. Supposing there is a rise in the ordinary barometer, the atmosphere presses some more mercury up the tube. This mercury is taken out of the cistern, which of course becomes lighter, and therefore the rod and cistern float up a little higher, which thus causes the column of mercury to rise still more. The increased pressure and buoyancy thus acting together, increase the ascent in the barometric column, as shown by the fixed scale. One inch in the barometer might be represented by two or more inches in this instrument, according to construction. Supposing there was a decrease of pressure, the mercury would fall, come into the cistern, make it heavier, and increase the fall somewhat. Friction guides, at the top of the rod, prevent it coming into contact with the side of the tube when vertically suspended. The illustration, Fig. 31, shows the appearance of the instrument as framed in wood by the makers, Messrs. Negretti and Zambra.

45. McNeild’s Long Range Barometer.—A barometer designed by a gentleman named McNeild is on a directly opposite principle to the one just described. The tube is made to float on the mercury in the cistern. It is filled with mercury, inverted in the usual manner, then allowed to float, being held vertically by glass friction points or guides. By this contrivance, the ordinary range of the barometer is greatly increased. One inch rise or fall in the standard barometer may be represented by four or five inches in this instrument, so that it shows small variations in atmospheric pressure very distinctly. As the mercury falls in the tube with a decrease of pressure, the surface of the mercury in the cistern rises, and the floating tube rises also, which causes an additional descent in the column, as shown by fixed graduations on the tube. With an increase of pressure, some mercury will leave the cistern and rise in the tube, while the tube itself will fall, and so cause an additional ascent of mercury. This barometer is identical in principle with King’s Barograph (see [p. 34]).

The construction of Howson’s and McNeild’s Barometers has been assigned to Messrs. Negretti and Zambra. These instruments are usually made for domestic purposes with a scale of from three to five, and for public use from five to eight times the scale of the ordinary standard. Their sensitiveness is consequently increased in an equal proportion, and they have the additional advantage of not being affected by differences of level in the cistern. However, these novelties have not been sufficiently tried to determine their practical value for strictly scientific purposes; but as weather-glasses, for showing minute changes, they are superior to the common barometer.