The barometer had been but a short time invented before it was observed that the height of the mercurial column is subject to variations connected in some way with the changes of weather. But the variations are confined within a limit ed range, scarcely exceeding 3 inches in all, and often, for many days together, do not exceed a few hundredths of an inch. It therefore was considered de sirable to render these minute oscilla tions more apparent by increasing their range ; and accordingly, of the numerous forms which the barometer has received, or which have been suggested, the greater part have been proposed with a view to this purpose. The most remarkable or useful constructions are the following, the descriptions of which will be readily understood with the assistance of the diagrams Fig. 1, is the Cistern Barometer, and is merely the inverted tube of Torricelli already described. The tube must be about 84 inches long. When placed in the cistern, the mercury sinks till the column between the two surfaces in and a just counterbalances the pressure of the air. The space above the mercury, a m, is or ought to be a perfect vacuum, or only filled with the vapor of mercury. In this barometer, as the diameter of the cistern is generally very much greater than that of the tube, almost the whole effect of the rise or fall is perceived in the variation of the upper eurfitee at M. For supposing the section of the cistern 20 times greater than that of the tube, and that the height of the column m n suffers a diminution of one inch ; it is evident that as all the mercury which goes out of the tube passes into the cis tern, when it falls at in it must rise at a, but less in proportion as the section of the cistern exceeds that of the tube. In the ease supposed, therefore, the altera tion of the level at m will be 20 times greater than at n; that is to say, there will be a fall of of an inch at m, and a rise of of an inch at n.
Fig. 2, is the Siphon Barometer, which was also proposed by Torricelli, as being more convenient than the former. It is merely a tube hermetically sealed at the upper end, having the lower or open end bent upwards in the form of a siphon. The variations in this are only half as great as in the cistern barometer • for the tube being of the same width through out, a diminution of the column m n amounting to one inch will be marked by a fall of half an inch at m and a rise of half an inch at n. This inconvenience may, however, be remedied by having the lower branch blown into a wide bulb; but as it is very into a perifectly regular shape, this enlargement of the bulb is found to give rise togriaccuracies.
Fig. 3, is the Wheel Barometer, pro posed by Hooke. A small weight floats on the surface of the mercury in a siphon barometer, which is very nearly counter poised by another weight, w, connected with the former by a string passing over a pulley, p. When the mercury rises at n the weight w descends, and turns the An index attached to the axle of the pulley shows on a dial the quantity of revolution. This barometer, though very commonly met with, is a mere toy; and indicates neither the absolute height of the mercurial column, nor its varia tions, with sufficient accuracy to be of the slightest use for any philosophical purpose whatever. Even as a weather glass, it is the worst of all the common forms of the barometer.
Sir Samuel Moreland proposed to en large the scale, by inclining the upper part of the tube so as to form a consider able angle with the perpendicular. By this contrivance the scale is increased in the proportion of radius to the cosine of the angle of inclination ; but the friction on the sites of the tube is greatly in creased, and it is very difficult to deter mine the exact plane of the top of the column which requires to be read off on a vertical scale. This construction is
easily conceived without a diagram.
We shall notice two other forms of the barometer, proposed with a different view from that of enlarging the scale. Fig. 4, is a modification of the siphon barometer proposed by Gay, Lessee. It differs from the common form in this respect, that, after the tube has been filled, the short branch is hermetically closed at the top, and the communication with the atmos phere takes place through a small capil lary hole drilled laterally through the tube at o, so fine that though it admits the air to pass freely, it prevents the passage of the mercury. The barometer is thus rendered very convenient for car riage; but notwithstanding the promising appearance of this barometer it has been found, particularly in travelling, that a portion of air will frequently insinuate itself through the mercury. In order to prevent the possibility et the accident, an ingenious modification has been made by M. Bunten, a Parisian artist. It con sists in causing the part of the tube a b to terminate in a very fine point, and to penetrate to some depth into the other part c o, to which it is joined at a, in the manner represented in Fig. 5. Now if an air bubble from the end o, which com municates with the atmosphere, should find its way through the bent capillary tube, it will pass along the sides of the bulging part, and instead of penetrating to the vacuum at a, will be arrested at c, whence it is easily expelled by reversing the barometer.
None of the contrivances which have been described for increasing the range of the oscillations have been found to succeed well in practice. It is found to be decidedly better to apply minute divi sions, than to attempt to enlarge the scale ; accordingly, experimenters now adhere to one or other of the two ancient forms, the cistern barometer and the siphon barometer. The height of the column in the siphon barometer is con veniently measured by means of a mov able scale attached to the frame which supports the tube ; by means of a tangent screw, the scale is raised or lowered till its zero coincides exactly with the surface of the mercury in the lower branch ; and with the assistance of a vernier, the height can be read off to the hundredth or two-hundredth of an inch with suffi cient precision. The scale of the cistern barometer is usually fixed, and the bot tom of the cistern is raised or lowered by a screw till the surface of the mercury in it coincides with the zero of the scale ; but the scale may be movable, and its zero brought to coincide with the surface of the mercury in the basin, as in the former case, In order to determine when this coincidence takes place, various ex pedients may be had recourse to. The most usual is to place on the surface of the mercury a float carrying a vertical needle, some point on which answers to a fixed point on the scale, and the coin cidence obtains when the two points are brought into the same level. Another contrivance to effect the same purpose was employed by Fortin, a celebrated French artist. An ivory needle is at tached to the scale, pointing downwards, and having its point exactly in the same level with the zero of the scale. The image of the needle is clearly reflected from the surface of the mercury in the cistern, and the cistern is raised or lower ed till the point of the needle and its image precisely coincide.