=28. Use of the Barometer in the management of Mines.=--The inflammable and suffocating gases, known to coal-miners as fire-damp and choke-damp, are specifically heavier than air; and as they issue from the fissures of the mine, or are released from the coal, the atmospheric pressure tends to drive them into the lowest and least ventilated galleries. Consequently a greatly reduced atmospheric pressure will favour a sudden outflow or advance of gas; whence may result cases of explosion or suffocation. It has been found that these accidents occur for the most part about the time of a low barometric column. A reliable barometer should, therefore, be systematically consulted by those entrusted with the management or control of coal-mines, so that greater vigilance and caution may be enjoined on the miners whenever the mercury falls low, especially after it has been unusually high for some days.

=29. Use of the Barometer in estimating the Height of Tides.=--The pressure of the atmosphere affects the height of the tide, the water being in general higher as the barometer is lower. The expressions of seamen, that "frost nips the tide," and "fog nips the tide," are explained by the high barometer which usually accompanies frost and fog. M. Daussy, Sir J.

C. Ross, and others, have established that a rise of one inch in the barometer will have a corresponding fall in the tide of about one foot.

Therefore navigators and pilots will appreciate the following suggestion of Admiral FitzRoy:--

"Vessels sometimes enter docks, or even harbours, where they have scarcely a foot of water more than their draught; and as docking, as well as launching large ships, requires a close calculation of height of water, the state of the barometer becomes of additional importance on such occasions."

CHAPTER II.

SYPHON TUBE BAROMETERS.

=30. Principle of.=--If some mercury, or any other fluid, be poured into a tube of gla.s.s, bent in the form of =U=, and open at both ends, it will rise to the same height in both limbs, the tube being held vertically. If mercury be poured in first, and then water upon it at one end, these liquids will not come to the same level; the water will stand much higher than the mercury. If the height of the mercury, above the line of meeting of the fluids, be one inch, that of the water will be about thirteen-and-a-half inches. The explanation of this is, that the two columns balance each other. The pressure of the atmosphere in each limb is precisely similar; but the one column stands so much higher than the other, because the fluid of which it is composed is so much lighter, bulk for bulk, than the other. If one end of the tube be hermetically closed, the other limb be cut off within a few inches of the bend, and the tube carefully filled with mercury; by placing it in a vertical position, the mercury will fall, if the closed limb be long enough, until it is about thirty inches higher than that in the exposed limb, where it will remain.

Here the atmosphere presses upon the short column; but not upon the long one. It is this pressure, therefore, which maintains the difference of level. In fact, it forms a barometer without a cistern, the short limb answering the purpose of a cistern. The first barometers on this principle were devised by the celebrated philosopher, Dr. Hook, as described in the next section.

31. DIAL, OR WHEEL BAROMETERS.

The familiar household "Weather Gla.s.ses" are barometers on the syphon principle. The portions of the two limbs through which the mercury will rise and fall with the varying pressure of the atmosphere are made of precisely the same diameter; while the part between them is contracted. On the mercury, in the exposed limb, rests a round float of ivory or gla.s.s; to this a string is attached and pa.s.sed over and around a bra.s.s pulley, the other end carrying another lighter weight. The weight resting on the mercury rises and falls with it. On the spindle of the pulley, which pa.s.ses through the frame and centre of the dial-plate, is fixed a light steel hand, which revolves as the pulley turns round. When the mercury falls for a decrease of atmospheric pressure, it rises by the same quant.i.ty in the short tube, and pushes up the float, the counterpoise falls, and thus moves the hand or pointer to the left. When the pressure increases, the pointer is drawn in a similar manner to the right.

[Ill.u.s.tration: Fig. 18.]

[Ill.u.s.tration: Fig. 19.]

[Ill.u.s.tration: Fig. 20.]

The dials are generally made of metal silvered over or enamelled, but porcelain may be used. If the circ.u.mference of the pulley, or "wheel," be two inches, it will revolve once for an alteration of level amounting to two inches in each tube, or four inches in the height of the barometric column; and as the dial may be from twenty to thirty-six inches in circ.u.mference, five to nine inches on the graduated scale corresponds to one inch of the column; and hence the sub-divisions are distinctly perceptible, and a vernier is not necessary.

The motion of the pointer alone is visible; and a mahogany, or rosewood, frame, supports, covers, and renders the instrument ornamental and portable. In the back of the frame is a hinged door, which covers the cavity containing the tube and fixtures. The dial is covered by a gla.s.s in a bra.s.s rim, similar to a clock face. A bra.s.s index, working over the dial, moveable by a key or b.u.t.ton, may be applied, and will serve to register the position of the hand when last observed. These instruments are usually fitted with a thermometer, and a spirit level; the latter for the purpose of getting the instrument perfectly vertical. They sometimes have, in addition, a hygrometer, a sympiesometer, an aneroid, a mirror, or a clock, &c., singly or combined. The frame admits of much variety of style and decoration. It may be carved or inlaid. The usual adjustment of scale is suited for localities at no considerable elevation above the sea. Accordingly, being commercial articles, they have been found frequently quite out of place. When intended for use at high elevations, they should have a special adjustment of scale. As household instruments they are serviceable, and ornamental. But the supply-and-demand principle upon which they are sold, has entailed upon those issued by inferior makers a generally bad adjustment of scale. The ill.u.s.trations are those of ordinary designs.

[Ill.u.s.tration: Fig. 21.]

[Ill.u.s.tration: Fig. 22.]

[Ill.u.s.tration: Fig. 23.]

Dial barometers required for transmission to distant parts, as India and the Colonies, are furnished with a steel stop-c.o.c.k, to render them portable more effectually than can be done by the method of _plugging_ the tube.

32. STANDARD SYPHON BAROMETER.

Fig. 24 represents the most accurate form of the Gay Lussac barometer. The short limb is closed at the top, after the mercury is introduced, and a small lateral puncture is made at _a_, which is covered over with a substance which permits the access of air, but prevents the escape of any mercury when the instrument is packed for travelling. The bent part of the tube is contracted to a capillary bore; and just above this, in the long limb, is placed the air-trap, already described (see p. 17), and here ill.u.s.trated (fig. 25). When reversed, as it must be for portability, the capillary attraction keeps the mercury in the long branch. Should the mercury of the short column get detached, some small quant.i.ty of air _may_ pa.s.s; but it will be arrested at the pipette, and will not vitiate the length of the barometric column. It can be easily expelled by gently shaking or tapping the instrument before suspending it for observation. In the ill.u.s.tration, the zero of the scale is placed at Z, near the middle of the tube; and the graduations extend above and below. In making an observation, it is necessary to take the reading ZA on the long branch, and ZB on the short one. The sum of the two gives the height of the barometer. The zero of the scale in some instruments is placed low down, so as to require the difference of the two readings to be taken. A thermometer is attached to the frame as usual.

[Ill.u.s.tration: Fig. 24.]

[Ill.u.s.tration: Fig. 25.]

These instruments can be very accurately graduated, and are very exact in their indications, provided great care has been exercised in selecting the tubes, which must be of the same calibre throughout the parts destined to measure the variations of atmospheric pressure. They should be suspended so as to insure their hanging vertically.

The syphon barometer does not require correction for capillarity nor for capacity, as each surface of the mercury is equally depressed by capillary attraction, and the quant.i.ty of mercury which falls from the long limb of the tube occupies the same length in the short one. The barometric height must, however, be corrected for temperature, as in the cistern barometer.

Tables containing the temperature corrections to be applied to barometer readings for scales engraved on the gla.s.s tube, or on bra.s.s or wood frames, are published.

CHAPTER III.

BAROGRAPHS, OR SELF-REGISTERING BAROMETERS.

=33. Milne's Self-Registering Barometer.=--For a long time a good and accurate self-recording barometer was much desired. This want is now satisfactorily supplied, not by one, but by several descriptions of apparatus. The one first to be described was the design of Admiral Sir A.

Milne, who himself constructed, in 1857, we believe, the original instrument, which he used with much success. Since that time several of these instruments have been made, and have performed satisfactorily. The barometer tube is a syphon of large calibre, provided with a Gay Lussac pipette, or air-trap; and fitted with a float, a wheel, and a pointer, as in the "Dial" barometer. The float is attached to a delicate watch-chain, which pa.s.ses over the wheel and is adequately counterpoised. Behind the indicating extremity of the pointer or hand is a projecting point, which faces the frame of the instrument, and is just within contact with the registering paper. A clock is applied, and fitted with auxiliary mechanism, so as to be able to move the mounted paper with regularity behind the pointer, and at designed equal intervals of time to release a system of levers and springs, so as to cause the marker to impress a dot on the paper, either by puncture or pencil-mark. The paper is ruled with horizontal lines for the range of the mercurial column, and parallel arcs of circles for the hours. Thus the barometer is rendered self-recording, by night or day, for a week or more; hence the great value of the instrument. The clock, index, and registering mechanism are protected from dust and interference by a gla.s.s front, hinged on and locked. As the temperature of the mercury is not registered, there is fixed to the frame a Sixe's thermometer to record the maximum and minimum temperatures, which should be noted at least every twenty-four hours.

Admiral FitzRoy has suggested the name "Atmoscope" for Admiral Milne's barometer; and he has also termed it a "Barograph." This latter word appears to be applicable to all kinds of self-registering barometers. .h.i.therto designed. Of the arrangement under consideration Admiral FitzRoy writes:--"It shows the alterations in tension, or the pulsations, so to speak, of atmosphere, on a large scale, by hourly marks; and the diagram expresses, to a practised observer, what the 'indicator card' of a steam-cylinder shows to a skilful engineer, or a stethescope to a physician."

[Ill.u.s.tration: Fig. 26.]

=34. Modification of Milne's Barometer.=--The great difficulty to be overcome in Milne's barometer, is to adjust the mechanism for obtaining registration so that the action of the striker upon the pointer should not in the slightest degree move it from its true position. A different mode of registration, capable of recording accurately the least appreciable movement of the mercurial column, has been effected. In this instrument the registering paper is carried upon a cylinder or drum. By reference to the ill.u.s.tration, Fig. 26, the details of construction will be readily understood. It should, however, be mentioned, that it is not a picture of the outward appearance of the instrument. The position of the barometer should be behind the clock; it is represented on one side merely for the purpose of clearly ill.u.s.trating the arrangement and principles. The instrument has a large syphon barometer tube, in which the mercurial column is represented. On the mercury at _A_, in its open end, rests a gla.s.s float, attached to a watch-chain, or suitable silken cord, the other end of which is connected to the top of the arched head on the short arm of a lever-beam. The long arm of the beam is twice the length of the short arm, for the following reason. As the mercury falls in the long limb, it rises through an equal s.p.a.ce in the short limb of the tube, and _vice versa_. But the barometric column is the difference of height of the mercury in the two limbs; hence the rise or fall of the float through half-an-inch will correspond to a decrease or an increase of the barometric column of one inch. In order, then, to record the movements of the barometric column, and not those of the float, the arm of the beam connected with the float is only half the radius of the other arm. Both arms of the beam carry circular-arched heads, which are similar portions of the complete circles, the centre of curvature being the fulcrum, or axis. This contrivance maintains the leverage on each extremity of the beam always at the same distance from the fulcrum. From the top of the large arched head a piece of watch-chain descends, and is attached to the marker, _B_, which properly counterpoises the float, _A_, and is capable of easy movement along a groove in a bra.s.s bar, so as to indicate the barometric height on an ivory scale, _C_, fixed on the same vertical framing. On the opposite side of the marker, _B_, is formed a metallic point, which faces the registration sheet and is nearly in contact with it. The framing, which carries the scale and marker, is an arrangement of bra.s.s bars, delicately adjusted and controlled by springs, so as to permit of a quick horizontal motion, in a small arc, being communicated to it by the action of the hammer, _E_, of the clock, whereby the point of the marker is caused to impress a dot upon the paper. The same clock gives rotation to the hollow wooden cylinder, _D_, upon which is mounted the registering paper. The clock must be rewound when a fresh paper is attached to the cylinder, which may be daily, weekly, or monthly, according to construction; and the series of dots impressed upon the paper shows the height of the barometric column every hour by day and night. The s.p.a.ce traversed by the marker is precisely equal to the range of the barometric column.

=35. King's Self-Registering Barometer.=--Mr. Alfred King, Engineer of the Liverpool Gas-light Company, designed, so long ago as 1854, a barometer to register, by a continuous pencil-tracing, the variations in the weight of the atmosphere; and a highly-satisfactory self-recording barometer, on his principle and constructed under his immediate superintendence, has quite recently been erected at the Liverpool Observatory.

[Ill.u.s.tration: Fig. 27.]

Fig. 27 is the front elevation of this instrument. _A_, the barometer tube, is three inches in internal diameter, and it floats freely (not being fixed as usual) in the fixed cistern, _B_, guided by friction-wheels, _W_. The top end of the tube is fastened to a peculiar chain, which pa.s.ses over a grooved wheel turning on finely-adjusted friction rollers. The other end of the chain supports the frame, _D_, which carries the tracing pencil. The frame is suitably weighted and guided, and faces the cylinder, _C_, around which the tracing paper is wrapped, and which rotates once in twenty-four hours by the movement of a clock. Mr. Hartnup, Director of the Liverpool Observatory, in his Annual Report, 1868, says:--"For one inch change in the mercurial column the pencil is moved through five inches, so that the horizontal lines on the tracing, which are half an inch apart, represent one-tenth of an inch change in the barometer. The vertical lines are hour lines, and being nearly three-quarters of an inch apart, it will be seen that the smallest appreciable change in the barometer, and the time of its occurrence, are recorded."

"It has been remarked by persons in the habit of reading barometers with large tubes, that, in squally weather, sudden and frequent oscillations of the mercurial column are sometimes seen. Now, to register these small oscillations must be a very delicate test of the sensitiveness of a self-registering barometer, as the time occupied by the rise and fall of the mercury in the tube in some cases does not exceed one minute." Mr.

Hartnup affirms that the tracing of this instrument exhibits such oscillations whenever the wind blows strong and in squalls.

As the barometer in this instrument is precisely similar to the "Long Range Barometer" invented by Mr. McNeild (and which will be found described at page 48), it may be desirable to quote the following, from Mr. Hartnup's Report:--"Mr. King constructed a small model instrument to ill.u.s.trate the principle. This instrument was entrusted to my care for examination, and it was exhibited to the scientific gentlemen who visited the Observatory in 1854, during the meeting of the British a.s.sociation for the Advancement of Science."

=36. Syphon, with Photographic Registration.=--A continuous self-registering barometer has been constructed, in which photography is employed. Those who may wish to adopt a similar apparatus, or thoroughly to understand the arrangements and mode of observation, should consult the detailed description given in the _Greenwich Magnetical and Meteorological Observations_, 1847. As the principles are applicable to photographic registration of magnetic and electric as well as meteorologic variations in instrumental indications, it would be beside our purpose to describe fully the apparatus.

The barometer is a large syphon tube; the bore of the upper and lower extremities, through which the surfaces of the mercury rise and fall, is 1-1/10 inch in diameter. The gla.s.s float in the open limb is attached to a wire, which moves a delicately-supported light lever as it alters its elevation. The fulcrum of the lever is on one side of the wire; the extremity on the other side, at four times this distance from the fulcrum, carries a vertical plate of opaque mica, having a small aperture. Through this hole the light of a gas-jet shines upon photographic paper wrapped round a cylinder placed vertically, and moved round its axis by a clock fixed with its face horizontal. The cylinder is delicately supported, and revolves in friction rollers. A bent wire on the axis is embraced by a p.r.o.ng on the hour hand of the time-piece; therefore the cylinder is carried round once in twelve hours. It might be arranged for a different period of rotation.

As the cylinder rotates, the paper receives the action of the light, and a photographic trace is left of the movements of the barometer four times the extent of the oscillations of the float, or twice the length of the variations in the barometric column. Certain chemical processes are required in the preparation of the paper, and in developing the trace. The diagram which we give on the next page, with the explanation, taken from Drew's _Practical Meteorology_, will enable the above description to be better understood:

[Ill.u.s.tration: Fig. 28.]

"_Q e_ is a lever whose fulcrum is _e_, the counterpoise _f_ nearly supporting it; _s_ is an opaque plate of mica, with a small aperture at _p_, through which the light pa.s.ses, having before been refracted by a cylindrical lens into a long ray, the portion only of which opposite the aperture _p_ impinges on the paper; _d_ is a wire supported by a float on the surface of the mercury; _G H_ is the barometer; _p_, the vertical cylinder charged with photographic paper; _r_, the photographic trace; _I_, the timepiece, carrying round the cylinder by the projecting arm _t_.

It is evident that the respective distances of the float and the aperture _p_ from the fulcrum may be regulated so that the rise and fall of the float may be multiplied to any extent required." When _only_ the lower surface of the mercury in a syphon barometer is read, as in the instrument just described, a correction for temperature is strictly due to the height of the quicksilver in the _short_ tube; but this in so short a column will rarely be sensible.