Standard Barometer
Fig. 3 represents a Standard Barometer on Fortin’s principle. The barometer tube is enclosed and protected by a tube of brass extending throughout its whole length; the upper portion of the brass tube has two longitudinal openings opposite each other; on one side of the front opening is the barometrical scale of English inches, divided to show, by means of a vernier, 1â„500th of an inch; on the opposite side is sometimes divided a scale of French millimetres, reading also by
As received by the observer, the barometer will consist of two parts, packed separately for safety in carriage,—1st, the barometer tube and cistern, filled with mercury, the brass tube, with its divided scale and thermometer; and 2nd, a mahogany board, with bracket at top, and brass ring with three adjusting screws at bottom.
Directions for fixing the Barometer.—In selecting a position for a barometer, care should be taken to place it so that the sun cannot shine upon it, and that it is not affected by direct heat from a fire. The cistern should be from two to three feet above the ground, which will give a height for observing convenient to most persons. A standard barometer should be compared with an observatory standard of acknowledged accuracy, to determine its index error; which, as such instruments are graduated by micrometrical apparatus of great exactitude, will be constant for all parts of the scale. It should be capable of turning on its axis by a movement of the hand, so that little difficulty can ever be experienced in obtaining a good light for observation. Having determined upon the position in which to place the instrument, fix the mahogany board as nearly vertical as possible, and ascertain if the barometer is perfect and free from air, in the following manner:—lower the screw at the bottom of the cistern several turns, so that the mercury in the tube, when held upright, may fall two or three inches from the top; then slightly incline the instrument from the vertical position, and if the mercury in striking the top elicit a sharp tap, the instrument is perfect. Supposing the barometer to be in perfect condition, as it is almost sure to be, it is next suspended on the brass bracket, its cistern passing through the ring at bottom, and allowed to find its vertical position, after which it is firmly clamped by means of the three thumb-screws.
To Remove the Instrument when fixed to another Position.—If it should be necessary to remove the barometer,—first, by means of the adjusting screw, drive the mercury to the top of the tube, turning it gently when it is approaching the top, and cease directly any resistance is experienced; next, remove from the upper bracket or socket; lift the instrument and invert it, carrying it with its lower end upwards.
Directions for taking an Observation.—Before making an observation, the mercury in the cistern must be raised or lowered by means of the thumb-screw, F, until the ivory point, E, and its reflected image in the mercury, D, are just in contact; the vernier is then moved by means of the milled head, until its lower termination just excludes the light from the top of the mercurial column; the reading is then taken by means of the scale on the limb and the vernier. The vernier should be made to read upward in all barometers, unless for a special object, as this arrangement admits of the most exact setting. In observing, the eye should be placed in a right line with the fore and back edges of the lower termination of the vernier; and this line should be made to form a tangent to the apex of the mercurial column. A small reflector placed behind the vernier and moving with it, so as to assist in throwing the light through the back slit of the brass frame on to the glass tube, is advantageous; and the observer’s vision may be further assisted by the aid of a reading lens. The object is, in these Standard Barometers, to obtain an exact reading, which can only be done by having the eye, the fore part of the zero edge of the vernier, the top of the mercurial column, and the back of the vernier, in the same horizontal plane.
Uniformity of Calibre.—The diameter of that part of the tube through which the oscillations of the mercury will take place is very carefully examined to insure uniformity of calibre, and only those tubes are used which are as nearly as possible of the same diameter throughout. The size of the bore should be marked on the frame of the barometer in tenths and hundredths of an inch. A correction due to capillary action, and depending on the size of the tube, must be applied to the readings.
5. Correction due to Capillarity.—When an open tube of small bore is plunged into mercury, the fluid will not rise to the same level inside as it has outside. Hence, the effect of capillary action is to depress the mercurial column; and the more so the smaller the tube. The following table gives the correction for tubes in ordinary use:—
| Diameter of tube. | Â | Depression, in boiled tubes. | Â | Depression, in unboiled tubes. |
| INCH. | Â | INCH. | Â | INCH. |
| 0·60 |  | 0·002 |  | 0·004 |
| 0·55 |  | 0·003 |  | 0·005 |
| 0·50 |  | 0·003 |  | 0·007 |
| 0·45 |  | 0·005 |  | 0·010 |
| 0·40 |  | 0·007 |  | 0·015 |
| 0·35 |  | 0·010 |  | 0·021 |
| 0·15 |  | 0·044 |  | 0·029 |
| 0·10 |  | 0·070 |  | 0·041 |
| 0·30 |  | 0·014 |  | 0·058 |
| 0·25 |  | 0·020 |  | 0·086 |
| 0·20 |  | 0·029 |  | 0·140 |
This correction is always additive to the observed reading of the barometer.
6. Correction due to Temperature.—In all kinds of mercurial barometers attention must be given to the temperature of the mercury. As this metal expands and contracts very much for variations of temperature, its density alters correspondingly, and in consequence the height of the barometric column also varies. To ascertain the temperature of the mercury, a thermometer is placed near the tube, and is sometimes made to dip into the mercury in the cistern. The freezing point of water, 32°F., is the temperature to which all readings of barometers must be reduced, in order to make them fairly comparable. The reduction may be effected by calculation, but the practical method is by tables for the purpose; and for these tables we refer the reader to the works mentioned at the end of this book.
7. Correction due to Height above the Half-tide Level.—Further, in order that barometrical observations generally may be made under similar circumstances, the readings, corrected for capacity, capillarity, and temperature, should be reduced to what they would be at the sea-level, by adding a correction corresponding to the height above the mean level of the sea, or of half-tide. For practical purposes of comparison with barometric pressure at other localities, add one-tenth of an inch to the reading for each hundred feet of elevation above the sea. For scientific accuracy this will not suffice, but a correction must be obtained by means of Schuckburg’s formula, or tables computed therefrom.
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This apparatus, the invention of the celebrated Dr. W. Whewell, registers the horizontal motion of the air with the direction. Its mechanism may be described in general terms, as follows:—</p> <p>A horizontal brass plate is attached to a ver... Athrioscope
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<p>Since the increased attention paid to the signs of forthcoming weather of late years, and the good which has resulted therefrom to farmers, gardeners, civil engineers, miners, fishermen, and mariners generally, by forewarning of impending wet or sto...