Two Greek words - , a weight, and , a measure - are enlisted to give the title to this most valuable instrument, which accurately demonstrates the variability of the pressure of the air. Mercury is about thirteen times heavier than water ; consequently a column of mercury 1 in. square and about 30 in. in height will counterbalance a column of water 34 ft. high and 1 in. square at the base, or both will hold in equipoise a column of air of its natural height from the earth, or that aerial mixture which is supposed to be included within a distance of 45 miles of the earth's surface - i.e., starting from the level of the sea.

The specific gravity of air is taken as unity or one, and it is the standard with which the density of all gaseous bodies is compared. Air at 30 Bar. and 320 Fahr. is 769.4 lighter than water, and 10,462 times less heavy than mercury.

A barometer, for rough purposes, is soon made. A clean, dry tube of stout glass, called barometer-tube, is hermetically sealed at one end; pure mercury is then poured in until the tube is filled within one inch of the open end ; the thumb is now held tightly over the latter, and the air included in the small space already alluded to is slowly passed up and down the tube, in order to collect all the smaller bubbles of air which adhere to the inside of the glass tube. The tube, filled with mercury, is left standing upright, and is gently tapped (say daily for a week), in order to assist the escape of any bubbles of air. It is then inverted in a basin of clean mercury, and, supposing the tube to be 36 in. in length, the mercury may fall to 30 in., and the space between 30 in. and 36 in. is called the Torricellian vacuum.

The barometer thus made, if placed under the receiver of an air-pump, indicates, by the falling of the mercury, the amount of vacuum procurable by any pump that the operator may wish to test; but there is always a fractional portion left behind, however excellent the air-pump may be. A pump that will remove 329 volumes out of 330 may be regarded as a very good one.

The Barometer in the Vacuum of an Air-Pump.

The more refined instruments required for meteorological purposes are made in the same manner, with the additional precaution of boiling the quicksilver in the tube, so as to get rid of the last bubbles of air. It is this which renders them more costly, as many tubes are sometimes broken in the process of boiling. There are many good barometer-makers in London.

The annexed figure represents a refined instrument, made by Negretti and Zambra, who give the following instructions: -

A Standard Barometer, on Fortin's principle, reading from an ivory point in the cistern, to insure a constant level - with mercury boiled in the tube. The barometer-tube, which is 4/10 of an inch diameter, 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, 1/500 of an inch; on the opposite side is sometimes divided a scale of French millimetres, reading also by a vernier to 1/10 of a millimetre; the reservoir or cistern of the barometer is of glass, closed at bottom by means of a leather bag, acted upon by a thumb-screw passing through the bottom of an arrangement of brass-work, by which it is protected. A delicate thermometer is attached to the brass tube.

Directions For Fixing The Barometer

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 cistern three or four turns, 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 quite perfect. If the tap be dull, or not heard at all, there is air above the mercury, which must be driven into the cistern by inverting the instrument, and gently tapping it with the hand. Supposing the barometer to be in perfect condition, it is next suspended on the brass bracket, its cistern passing through the ring at bottom of the mahogany board, and allowed to find its vertical position, after which it is firmly clasped by means of the three thumb-screws.

Before making an observation, the mercury in the cistern must be raised or lowered, by means of the thumb-screw, until the ivory point and its reflected image 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.

A Standard Barometer.

A very excellent and moderate-priced instrument is that made by Davis and Co., 163, Fenchurch Street. (See cut).

Changes in the weather are indicated by the barometer, or weather-glass, which was invented by Torricelli, a pupil of Galileo, in 1643.

The principle by which it is made to indicate changes of weather is by measuring the weight of air. When the atmosphere is filled with vapour the air is lighter than usual, and the column of mercury in the barometer stands low. When the air is dry and free from vapour it is heavy and the mercury stands high.

To sailors the barometer is of the highest value; it forewarns them of wind, rain, and storms, and thus enables them to prepare their vessel to meet it. An unusual and sudden fall in the barometer has been found frequently to precede a hurricane.