An instrument by which the pressure or elasticity of the air is ascertained. It consists essentially of a glass tube, not less than 34 inches in length, closed at one end. The tube is completely filled with mercury, and then inverted in a cup or box of the same metal. By inverting the tube, a portion of the mercury will fall out, but a column varying in height from 28 to 31 inches above the surface of the metal will still remain, and which, being supported by the pressure of the air, will be an indication of its amount at the time. The barometer has assumed a variety of forms, the construction of the most important of which we shall endeavour to explain; but we deem it necessary, previously, to enter a little more minutely into the nature and operation of that form of the instrument to which we have just referred. The common barometer was the discovery (for it can scarcely be called an invention) of Torricelli, a disciple of the celebrated Galileo. It is, from this circumstance, frequently called the Torricellian tube; and the vacuous space above the surface of the mercury in the tube, is called the Torricellian vacuum.
In the construction of the barometer, the principal object to be attained is a perfect vacuum in the upper part of the tube.
To effect this, we must first make the tube perfectly free from moisture by exposing it gradually to the heat of a charcoal fire. The tube must have a bore sufficiently large to render the effect of capillary attraction insensible. The mercury employed to fill the tube must be rendered as pure as possible, by pressing it through the pores of chamois leather, and afterwards distilling it. A small portion of the purified mercury may now be put into the tube, and heat gradually applied to it, till the mercury boils. Another portion may then be added, and boiled in a similar manner; and this process continued till the tube is completely filled. By these means, the mercury itself will be purged of any air it may have acquired, and the air which adheres to the sides of the tube will also be effectually expelled. If the tube be now inverted into a cup or other vessel of mercury, the barometer, as far as its construction is concerned, will be complete. In the annexed cut, a c represents the barometric tube inverted in its cup. The mercury has subsided to the point f; and the next object is, to ascertain the precise length of the column from c to f.
For this purpose, a scale a b is attached to the barometer near the top.
In the ordinary use of this instrument as a weather-glass, the rise and fall of the mercury seldom exceeds a range of 3 inches, the height of the column varying with the atmospheric pressure from 28 to 31 inches. If, therefore, a scale of about 4 inches long, carefully graduated to tenths of an inch, be attached at the height that accords with the number of inches we have mentioned, it will be sufficient to determine the ordinary changes of atmospheric pressure. There is, however, one circumstance to which it is necessary to attend, in measuring the height of the mercurial column. It will be seen, that as the column f c falls, it will necessarily raise the level of the fluid in the cistern; and contrariwise, when the column rises, it will depress the level of the cistern. On this account, the initial point of the scale is perpetually changing its position, and, consequently, the numbers on the scale cannot indicate the true distance between the surface of the mercury in the cistern and that of the supported column.
To obviate this inconvenience, the horizontal sectional area of the cup is made considerably greater than a like section of the tube, by means of which a considerable fall in the tube produces but an inconsiderable alteration in the level of the fluid in the cup.
For example, if the diameter of the tube be 1/4 of an inch, and that of the cup be 2 inches, the respective areas will be as 1/16 to 4, or as 1 to 64; hence a fall of l inch in the tube will cause a rise of g of an inch in the cup. In ordinary observations, this degree of accuracy would be sufficient; but in the nicer operations of science, a greater degree of precision is essential; hence different contrivances have been employed for the purpose of ascertaining more exactly the difference of levels. The most general mode of effecting this object is to make the bottom of the cistern movable, so as to be raised or depressed by means of a screw e. An ivory index d may be attached to the top of the cup, and which being finely pointed at the lower end, will serve to indicate a fixed level. Before an observation is made with the barometer, the screw e is turned until the surface is brought exactly to coincide with the point of the index, by raising or depressing the bottom according as the surface was below or above that point.
By this means, the surface of the mercury always stands at the same level, and hence the divisions on the scale a b will represent the actual changes in the height of the barometric column.
Having attained a method of fixing the initial point of the scale, the next object to be attained is a means of reading off its indications with the utmost precision. In our description, we have supposed the scale divided into tenths of an inch, but this is not sufficiently accurate for experimental research. It would therefore be necessary to divide each of these tenths into hundredths of an inch, and adapt a small microscope to the instrument, in order to obtain the precise height of the column. The usual mode, however, of obtaining these smaller subdivisions of the scale, is by means of a vernier, or small graduated plate, which is movable by a screw, or otherwise, on the divided scale of the barometer. The principle of the vernier will be seen by reference to the accompanying engraving. Let a b represent a portion of the scale divided into tenths of an inch; let c d be the sliding scale, or vernier, equal in length to 11 divisions on the principal scale, but divided into only 10 equal parts.