Of the self-registering barometers, the best are those which accomplish this object by photography. This is done by concentrating the rays of a gas flame by means of a lens, so that they strike the top of the mercurial column. A sheet of prepared paper is attached to a frame placed behind a screen, with a narrow vertical slit in the line of the rays. The mercury being opaque throws a part of the paper in the shade, while abjve the mercury the rays from the flame pass unobstructed to the paper. The paper being carried steadily round on a drum at a given rate per hour, the height of the column of mercury is photographed continuously on the paper. From the photograph the height of the barometer at any instant may be taken. King s, Hardy s, Hough s, Hipps, and Thorell s self-register ing barometers may also be referred to as giving continuous records of the pressure. In all continuously registering barometers, however, it is necessary, as a check, to make eye-observations with a mercury standard barometer hang ing near the registering barometer from four to eight times daily.
In constructing the best barometers three materials are employed, viz.: (1) brass, for the case, on which the scale is engraved ; (2) glass, for the tube containing the mercury ; and (3) the mercury itself. Brass is the best material for ths case and scale, inasmuch as its co-efficient of expansion is well known, and is practically the same though the alloy be not in all cases exactly alike. It is evident that if the co-efficient of expansion of mercury and brass were the same, the height of the mercury as indicated by the brass scale would be the true height of the mercurial column. But this is not the case, the co-efficient of expan sion for mercury being considerably greater than that for brass. The result is that if a barometer stand at 30 inches when the temperature of the whole instrument, mercury and brass, is 32, it will no longer stand at 30 inches if the temperature be raised to 09; in fact, it will then stand at 30 - 1 inches. This increase in the height of the column by the tenth of an inch is not due to any increase of pressure, but altogether to the greater expansion of the mercury at the higher temperature, as compared with the expansion of the brass case with the engraved scale by which the height is measured. In order, therefore, to compare with each other with exactness barometric observations made at different temperatures, it is necessary to reduce them to the heights at which they would stand at some uniform temperature. The temperature to which such ob servations are now almost everywhere reduced is 32 Fahr.
The following is Schumacher s formula for computing the corrections for barometers, whose heights are noted in English inches, for temperature t, according to Fahrenheit s scale:—
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where h = height of barometer,
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m = expansion of mercury for 1 Fahr. = 0001001, s = expansion of brass for 1 Fahr. = 00001041.
The standard temperature of the English yard being 62 and not 32, it will be found in working out the correc tions from the above formula that the temperature of no correction is not 32 but 28 5. If the scale be engraved on the glass tube, or if the instrument be furnished with a glass scale or with a wooden scale, different corrections are required These may be worked out from the above formula by substituting for the co-efficient of the expansion of brass that of glass which is assumed to be - 00000498, or that of wood, which is assumed to be 0. Wood, how ever, should not be used, its expansion with temperature being unsteady, as well as uncertain.
If the brass scale be attached to a wooden frame and be free to move up and down the frame, as is the case with many siphon barometers, the corrections for brass scales are to be used, since the zero-point of the scale is brought to the level of the lower limb ; but if the brass scale be fixed to a wooden frame, the corrections for brass scales are only applicable provided the zero of the scale be fixed at (or nearly at) the zero line of the column, and be free to expand upwards. In siphon barometers, with which an observation is made from two readings on the scale, the scale must be free to expand in one direction. Again, if only the upper part of the scale, say from 27 to 31 inches, be screwed to a wooden frame, it is evident that not the corrections for brass scales, but those for wooden scales must be used. No account needs to be taken of the expansion of the glass tube containing the mercury, it being evident that no correction for this expansion is required in the case of any barometer the height of which is measured from the surface of the mercury in the cistern.
In fixing a barometer for observation, it is indispensable that it be hung in a perpendicular position, seeing that it is the perpendicular distance of the surface of the mercury in the cistern and that of the top of the column which is the true height of the barometer. Hence it is desirable that the barometer swing in position ; or if this be attended with risk or inconvenience, it must be seen that it be clamped or permanently fixed in a position exactly vertical. The surface of the mercurial column u convex, and in noting the height of the barometer, it is not the chord of the curve, an error not unfrequently made, but its tangent which is taken. This is done by setting the straight lower edge of the vernier, an appendage with which the barometer is furnished, as a tangent to the curve. The vernier is made to slide up and down the scale, and by it the height of the barometer may be read true to - 002 or even to O OOl inch. See Vernier.
In hanging a barometer the following points should be attended to : (1), That it be hung so that the mercurial column be quite perpendicular ; (2), that the scale be about 5 feet high, for facility of reading ; (3), that the whole instrument, particularly the scale and the cistern, be hung in a good light ; and (4), that it be hung in a position in which it will be exposed to as little fluctuation of tempera ture as possible. A wall heated by a flue, and positions which expose the instrument to the heat of the sun or to that of a fire, are very objectionable. It is to be kept in mind that no barometric observation can be regarded as good unless the attacked thermometer indicates a temperature differing from that of the whole instrument not more than a degree. For every degree of temperature the attached thermometer differs from the barometer, the observation will be faulty to the extent of about 003 inch, which in discussions of diurnal range, barometric gradients, lunar range, and many other questions, is a serious amount.
Before being used, barometers should be thoroughly examined as to the state of the mercury, the size of cistern (so as to admit of low readings), and their agreement with some known standard instrument at different points of the scale. The pressure of the atmosphere is not expressed by the weight of the mercury sustained in the tube by it, but by the perpendicular height of the column. Thus, when the height of the column is 30 inches, it is not said that the atmospheric pressure is 147 R> on the square inch, or the weight of the mercury filling a tube at that height whose transverse section equals a square inch, but that it is 30 inches, meaning that the pressure will sustain a column of mercury of that height.
inches in England and America. In France and most refu European countries, the height is given in millimetres, a millimetre being the thousandth part of a metre, which
equals 39 37079 English inches. Up to 1 SG9 the barometer