Barometers arc usually divided into two classes—cistern barometers, and siphon barometers. The simplest form of the cistern B. is that shown in fig. 1, which only requires to be set properly in a frame, and provided with a scale, to make ft complete. Fig. 2 presents another form of that class, being that generally seen in weather.glasses or ordinary barometers. The tube is bent at the bottom, and the cistern is merely an expansion of the lower end. Very generally, the cistern is hidden from view, and protected from injury bv a wooden cover in front. There aro two causes of inaccuracy in cistern barometers—one being the larity, which tends to lower the column. and the other being the difference of level in the cistern caused by the ations in the tube, which renders the readings on the•fixed scale above at one time too great, and at another too small, according as this level rises above or falls below the original level from which the scale was measured. The effect of be avoided by uSing tubes of more than half an inch in bore, in which the depression comes so small, that it may be left out of account; and in smaller tubes it may bo estimated from tables constructed for the purpose. Wide tubes have the tional advantage, that atmospheric changes are seen earlier in them than in narrow tubes, there being less friction in the former than in the latter. It is worthy of notice that the capillary depression is lesS in boiled than in unboiled tubes, in consequence of the admixture of a minute quantity of the oxide of mercury; formed in the process of boiling, which lessens the repulsion between the mercury and glass. With ence to the error of level, it must be borne in mind that tlio height of the column sustained by the atmosphere is always to bo reckoned from the lower level. This error becomes all the less the larger the capacity of the cistern is compared with that of the tube, for then a very considerable rise or fall in the tube, when spread over the surface of the cistern, makes only a slight difference of level in it. Care must be taken, then, in ordinary barometers, to make the cistern as large as ble. The only B. in which the error of level is completely obviated, is that invented by Fortin, which, from its being in every respect the- most perfect cistern B., deserves particular notice. The cistern, and the lower portion of the tube of this B., is shown in fig. 3. The cistern is made of boxwood, with a movable leather bottom, bb, and a glass cylinder is inserted into it. above, all except the glass being incased in brass. In the bottom of the brass box a screw works, on the upper end of which the leather rests, so that by the sending in or taking out of the screw, the bottom of the cistern, and with it the cistern level of the mercury, can be raised or depressed at will. A small ivory pin, p, ending in a fine point, is fixed to the upper frame of the cistern ; and when an observation is made, the surface of the mercury is made to coincide with the point Of the pin as the standard level from which the barometric column is to be measured. The tube of the B.—the upper part of which is shown in fig. 4—is inclosed in one of brass, which has two directly opposite slits in it for showing the height of the column, and on the sides of these the uation is marked. A. brass collar, cc, slider; upon the tube with a vernier (q.v.), Dr, marked on it for reading the height with the greatest exactness, and in which two oblong holes are cut, a little wider than the slits in the brass tube. When a reading 'is taken, the collar is so placed that the last streak of light is cut off by the two upper edges of the holes, or until they form a gent to the convex mercurial curve. By this means, the observer is sure that his eye is min level with the top of the column, and that the reading is taken exactly for this point. This is the contrivance usually adopted to prevent the error of parallax, or that caused by the eve being slightly above or below the top of the column, by which the scale and the top of the column are projected too high or too low, the one upon the other, as the case may be. The only other arrangement worthy of mention for effecting the same object is that by Weber, who etches the scale on of silverized glass placed over one side of the tube; and when—the mirror arid tube being vertical—the image of the eve appears along with the vertex of the column, the eye is in the same horizontal line with it. Fortin's B. is generally arranged so as to be portable, in which case the screw, 8, is sent in until the mercury fills the whole cistern, by which the air is kept from entering the tube during transport, the leather yielding sufficiently at the same time to allow for expansion from increase of temperature. It packs in a case, which serves as a tripod when the instru
ment is mounted for use. On this tripod it is suspended about the middle, swinging upon two axes at right angles to each other, so that the cistern may act the part of a plummet in keeping the tube vertical—the position essential to all correct measurements.
The siphon B. consists of a tube bent in the form of a siphon, having the same diameter at the lower as at the upper end. 'Fig. 5 represents a simple form of it. The tube travels along the board on which it is placed by passing easily through fixed rings or collars of brass. A scale, divided in inches and parts of an inch, is fixed on the upper part of the board; and when an observation is taken, the tube is adjusted by the screw 8, working below it, so that the top of the lower mercurial column may be on a level with the fixed mark, a, which is the point from which the fixed scale is measured. In the best forms of the siphon 13., both tube and scale are fixed, the latter being graduated upwards and downwards from a zero-point near the middle of the tube, and the height of the column is ascertained by adding the distances from it of the upper and lower levels. The siphon B. is in many respects a more perfect instrument than the cistern barometer. In the first place, the bore at the upper and lower ends of the tu0e being the same, the depression aris ing from capillarity is alike forboth, and the error from this cause disappears in taking the difference of the heights. In the second place, since the final reading is got from a refer ence to both upper and lower surfaces, the error in the cistern B. produced by the different capacities of the tube and cistern, is effectually avoided. On the other hand, the taking of two readings, one for each column, is a serious addition to the labor of observation. Gay-Lussac's siphon B. (fig. 6) is bent near the bottom, so as to allow of the lower branch being placed in the same straight line as the upper one—a position highly favorable to accurate observation. When constructed for transport, the tube at the bend is nar rowed, as in the figure, to a capillary width, which effectually excludes the air; and when the tube is inverted (fig. 7), being the position in which it is carried, the is nearly all held in the longer branch. Such a tube when mounted, like Fortin's B., makes an excellent traveling instrument, and is comparatively light, from the small quantity of mercury it contains. See ANEROID BAROMETER.
The wheel B., originally invented by Hook, and generally seen as a parlor ornament, has little to recommend it as a trustworthy instrument. Fig. 8 shows the main features of its construction. It is essentially an ordinary B. like the siphon 13. below, but having a cistern above, to increase the amount of variation in the lower branch. A small piece of iron or glass, f, floats on the open surface, and a thread is attached to it, and passed over a small wheel, a,-fixed to a horizontal axis, to which it is kept tight by a small weight, c. hanging at the other end. A pointer, p, is fixed to the other extremity of the horizontal axis, which moves to the right or left of the dial, dd, according as the mercury falls or rises in the lower branch. The great sweep which the index takes, as compared oith the comparatively minute variations of the mercurial column, is the only merit of this instrument. It is easy to see, that with so flinch intervening between the mercury and the index, the chances of error from friction and other causes are very considerable.
The correction of the B. for temperature is of importance. 'Mercury expands of its bulk for every degree of Fahrenheit's thermometer; consequently, a column of ao in. at 32° F., or the freezing-point, would, at F., for instance, be times 80 in., or nearly *of an inch longer, for in. of mercury at 60' produce the same pressure as 30 in. of it at 32°. In order, therefore, that all observations may be compared correctly with each other, the observed heights are reduced to what they would be at 32' F. as a standard temperature. The rule for reduction is very simple: Multiply the number of degrees above or ',elow 32° F. by the observed height, divide the product by 9990, and subtract or add the qmtient from or to the observed height .for the reduced height. Tables for this purpose have been published by the royal society; from which the corrections are found at once.