Fig. 17.
Description of the Instrument.—The farmer’s barometer, as figured in the margin, consists of an upright tube of mercury inverted in a cistern of the same fluid; this is secured against a strong frame of wood, at the upper end of which is fixed the scale, divided into inches and tenths of an inch. On either side of the barometer, or centre tube, are two thermometers—that on the left hand has its bulb uncovered and freely exposed, and indicates the temperature of the air at the place of observation; that on the right hand has its bulb covered with a piece of muslin, from which depend a few threads of soft lamp cotton; this cotton is immersed in the small cup situated just under the thermometer, this vessel being full of water; the water rises by capillary attraction to the muslin-covered bulb, and keeps it in a constantly moist state.
These two thermometers, which we distinguish by the names “Wet Bulb” and “Dry Bulb,” form the Hygrometer; and it is by the simultaneous reading of these two thermometers, and noting the difference that exists between their indications, that the humidity in the atmosphere is determined.
Admiral FitzRoy’s words (seep. 22) are placed upon the scale of the barometer, as the value of a reading depends, not so much on the actual height of the mercury in the tube, as it does on whether the column is rising, steady, or falling.
The moveable screw at the bottom of the cistern is for the purpose of forcing the mercury to the top of the tube when the instrument is being carried from place to place, and it must always be unscrewed to its utmost limit when the barometer is hung in its proper place. After this it should never be touched.
The manner in which the Hygrometer acts is as follows: It is a pretty well-known fact that water or wine is often cooled by a wet cloth being tied round the bottle, and then being placed in a current of air. The evaporation that takes place in the progressive drying of the cloth causes the temperature to fall considerably below that of the surrounding atmosphere, and the contents of the bottle are thus cooled. In the same manner, then, the covered wet bulb thermometer will be foundinvariablyto read lower than the uncovered one; and the greater the dryness of the air, the greater will be the differencebetween the indications of the two thermometers; and the more moisture that exists in the air, the more nearly they will read alike.
The cup must be kept filled with pure water, and occasionally cleaned out, to remove any dirt. The muslin, or cotton-wick, should also be renewed every few weeks. The hygrometer may be had separate from the barometer, if the combined instruments cannot be sufficiently exposed to the external air, this being essential for the successful use of the hygrometer.
This farmer’s weather-glass, then, consists of three distinct instruments: the barometer, the thermometer, and the hygrometer. He has thus at command the three instrumental data necessary for the prediction of the weather. And now to describe—
How to Use the Instrument.—The observations should be taken twice a day, say at 9A.M.and 3P.M.; and should be entered on a slip of paper, or a slate hung up by the barometer. The observer will then be able to see the different values of the readings from time to time, and to draw his conclusions therefrom.
The thermometer on the left hand should first be read, and a note made of its indication, which is the temperature of the air. The wet bulb thermometer should now be read, and also noted; and the difference should be taken of these two readings. Next read the barometer by moving the small index at the side of the tube until it is on a level with the top of the mercury. Having noted the number of inches at which the column stands, compare with the last observation, and see immediately whether the barometer is rising, steady, or falling.
Now, having taken the observations as above, we naturally ask the question,What are we to predict from them?
And, probably, the best way of answering this query will be by giving an example. We will suppose that our readings yesterday were as follows:—Temperature, 70°; Wet Bulb, 69°; Difference, 1°; =very moist air. Barometer, 29·5, and that rain has fallen.
To-day, we read:—Temperature, 60°; Wet Bulb, 55°; Difference, 5°; =dryer air. Barometer, 30. We may safely predict that the rain will cease, and probably we may have wind from the northward.
In spring or autumn, if the barometric height be steady any where between 29·5 and 30 inches, with the temperature about 60°, fresh to moderate south-westerly winds, with cloudy sky, will probably characterize the weather; the indications of the hygrometer being then specially serviceable in enabling us to foretell rain; but if the mercury become steady at about 30·5 inches, with temperature about 40°, north-easterly winds, dry air, and clear sky, may be confidently expected.
Many cases will doubtless suggest themselves to the observer where these figures do not occur, and where he might find a difficulty in interpreting the indications of his instruments. We have, therefore, drawn up some concise rules for his guidance; and although they will not prove absolutely infallible guides to this acknowledged most difficult problem, still, they will be found of much service in foretelling the weather, when added to an intelligent observation of ordinary atmosphericphenomena, as force and direction of wind, nature of any particular season, and the time of year.
26. RULES FOR FORETELLING THE WEATHER.
A RISING BAROMETER.
A “Rapid” rise indicates unsettled weather.A “Gradual” rise indicates settled weather.A “Rise,” with dry air, and cold increasing in summer, indicates wind from northward; and if rain has fallen, better weather is to be expected.A “Rise,” with moist air and a low temperature, indicates wind and rain from northward.A “Rise,” with southerly wind, indicates fine weather.
A “Rapid” rise indicates unsettled weather.
A “Gradual” rise indicates settled weather.
A “Rise,” with dry air, and cold increasing in summer, indicates wind from northward; and if rain has fallen, better weather is to be expected.
A “Rise,” with moist air and a low temperature, indicates wind and rain from northward.
A “Rise,” with southerly wind, indicates fine weather.
A STEADY BAROMETER,
With dry air and a seasonable temperature, indicates a continuance of very fine weather.
With dry air and a seasonable temperature, indicates a continuance of very fine weather.
A FALLING BAROMETER.
A “Rapid” fall indicates stormy weather.A “Rapid” fall, with westerly wind, indicates stormy weather from northward.A “Fall,” with a northerly wind, indicates storm, with rain and hail in summer, and snow in winter.A “Fall,” with increased moisture in the air, and the heat increasing, indicates wind and rain from southward.A “Fall,” with dry air, and cold increasing (in winter), indicates snow.A “Fall,” after very calm and warm weather, indicates rain with squally weather.
A “Rapid” fall indicates stormy weather.
A “Rapid” fall, with westerly wind, indicates stormy weather from northward.
A “Fall,” with a northerly wind, indicates storm, with rain and hail in summer, and snow in winter.
A “Fall,” with increased moisture in the air, and the heat increasing, indicates wind and rain from southward.
A “Fall,” with dry air, and cold increasing (in winter), indicates snow.
A “Fall,” after very calm and warm weather, indicates rain with squally weather.
27. Causes which may bring about a Fall or a Rise in the Barometer.[3]—As heat produces rarefaction, a sudden rise of temperature in a distant quarter may affect the weight of the atmosphere over our heads, by producing an aerial current outwards, to supply the place of the lighter air which has moved from its former position; in which case the barometer will fall. Now such a movement in the atmosphere is likely to bring about an intermixture of currents of air of different temperatures, and from this intermixture rain is likely to result.
On the other hand, as cold produces condensation, any sudden fall of temperature causes the column of air over the locality to contract and sink to a lower level, whilst other air rushes in from above to supply the void; and, accordingly, the barometer rises. Should this air, as often happens, proceed from the north, it will contain in general but little moisture; and hence, on reaching a warmer latitude, will take up the vapour of the air, so that dry weather will result.
It is generally observed, that wind causes a fall in the instrument; and, indeed, in those greater movements of the atmosphere which we denominate storms orhurricanes, the depression is so considerable as to forewarn the navigator of his impending danger. It is evident, that a draught of air in any direction must diminish the weight of the column overhead, and consequently cause the mercury in the barometer to sink.
The connection, therefore, of a sinking of the barometric column with rain is frequently owing to the wind causing an intermixture of the aerial currents which, by their motion, diminish the weight of the atmosphere over our heads; whilst a steady rise in the column indicates the absence of any great atmospheric changes in the neighbourhood, and a general exemption from those causes which are apt to bring about a precipitation of vapour.
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.”
SYPHON TUBE BAROMETERS.
30. Principle of.—If some mercury, or any other fluid, be poured into a tube of glass, bent in the form of∪, 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 Glasses” 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 glass; to this a string is attached and passed over and around a brass 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 passes 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 quantity in the short tube, and pushes up the float, the counterpoise falls, and thus moves the hand or pointerto the left. When the pressure increases, the pointer is drawn in a similar manner to the right.
The dials are generally made of metal silvered over or enamelled, but porcelain may be used. If the circumference 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 circumference, 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 glass in a brass rim, similar to a clock face. A brass index, working over the dial, moveable by a key or button, 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 localitiesat 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 illustrations are those of ordinary designs.
Dial barometers required for transmission to distant parts, as India and the Colonies, are furnished with a steel stop-cock, to render them portable more effectually than can be done by the method ofpluggingthe tube.
Fig. 24.
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 lateralpuncture is made ata, 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 (seep. 17), and here illustrated (fig. 25).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 quantity of airmaypass; 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 illustration, 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.
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 quantity 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 glass tube, or on brass or wood frames, are published.
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 passes 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 glass 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 hitherto 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.”
Fig. 26.Larger Image
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 degreemove 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 illustration, 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 illustrating the arrangement and principles. The instrument has a large syphon barometer tube, in which the mercurial column is represented. On the mercury atA, in its open end, rests a glass 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 space in the short limb of the tube, andvice 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 brass 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 brass 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 adot 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 space traversed by the marker is precisely equal to the range of the barometric column.
Fig. 27.
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.
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 passes 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 largetubes, 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 illustrate 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 Association 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 theGreenwich 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 11⁄10inch in diameter. The glass 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 prong 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’sPractical Meteorology, will enable the above description to be better understood:
Fig. 28.
“Q eis a lever whose fulcrum ise, the counterpoisefnearly supporting it;sis an opaque plate of mica, with a small aperture atp, through which the light passes, having before been refracted by a cylindrical lens into a long ray, the portion only of which opposite the aperturepimpinges on the paper;dis a wire supported by a float on the surface of the mercury;G His the barometer;p, the vertical cylinder charged with photographic paper;r, the photographic trace;I, the timepiece, carrying round the cylinder by the projecting armt. It is evident that the respective distances of the float and the aperturepfrom the fulcrum may be regulated so that the rise and fall of the float may be multiplied to any extent required.” Whenonlythe 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 theshorttube; but this in so short a column will rarely be sensible.
MOUNTAIN BAROMETERS.
37. The Syphon Tube Mountain Barometer, on Gay Lussac’s principle, constructed as described atpage 31, and fixed in a metallic tubular frame, forms a simple and light travelling instrument. The graduations are made upon the frame, and it is suspended for reading by a ring at the top, from beneath an iron tripod stand, which is usually supplied with it. Considerable care is requisite in adjusting the verniers, so as to keep the instrument steady and vertical. A drawback to the convenience of this barometer is the movement of the mercury in the short limb, which is generally not confined, and hence has every facility for becoming quickly oxidised in travelling. To remedy this, Messrs. Negretti and Zambra so construct the Mountain Syphon Barometer that by a simple half turn of a screw the mercury can be confined for portability, while the lower limb can be taken out for cleaning whenever found requisite.
38. Mountain Barometer on Fortin’s principle.—This barometer, with Fortin’s cistern, as arranged by Messrs. Negretti and Zambra, is an elegant, manageable, and very accurate instrument for travelling purposes, and well adapted for careful measurement of heights. The cistern is made large enough to receive all the mercury that will fall from the tube at the highest attainable elevation. The screw at the bottom confines the mercury securely for carriage, and serves to adjust the surface of the mercury to the zero of the scale when making an observation. The vernier reads to ·002 of an inch, and slides easily on the brass frame, which is made as small in diameter as is compatible with the size of the tube. The tube in this barometer should be altogether without contractions, so that the mercury will readily fall when it is set up for observation. It must be carefully calibrated, and its internal diameter ascertained, in order that correction may be made for capillarity. This correction, however, should be combined with the error of graduation, and form a permanent index error, ascertainable at any time by comparison with an acknowledged standard barometer.
The barometer is supported in the tripod stand (furnished as part of the instrument) when used for observation. It is suspended by placing two studs, in the ring on the frame, in slots formed on the top of the stand, so that it hangs freely andvertically in gimbals. To the metal top of the stand, mahogany legs are hinged. To make the barometer portable, it must be lifted out of the stand, sloped gently until the mercury reaches the top, turning the screw at the bottom meanwhile; then invert and screw until the mercury is made tight. The inverted instrument packs in the stand, the legs being formed to fit round the frame; and receptacles are scooped out for the cistern, thermometer, gimbals, and vernier; so that the instrument is firmly surrounded by the wooden legs, which are held fast together by brass rings passed over them.
Fig. 29.
39. Newman’s Mountain Barometer.—Fig. 29 is an illustration of the mountain barometer known as Newman’s. The cistern consists of two separate compartments;—the top of the lower and the bottom of the upper, being perfectly flat, are pivoted closely together at the centres, so that the lower can move through a small arc, when turned by the hand. This movement is limited by two stops. The top of the lower compartment and the bottom of the upper have each a circular hole, through which the mercury communicates. When the instrument is required for observation, the cistern is turned close up to the stop marked “open” or “not portable.” When it is necessary to pack it for travelling, the mercurial column must be allowed to fill the tube by sloping the barometer gently; then invert it, and move the cistern to the stop marked “shut” or “portable.” In this condition, the upper compartment is completely filled with mercury, and consequently that in the tube cannot move about, so as to admit air or endanger the tube. Nor can the mercury pass back to the lower compartment, as the holes are not now coincident, and the contact is made too perfect to allow the mercury to creep between the surfaces. The tube does not enter the lower compartment, which is completely full of mercury when the instrument is arranged for observation. The spare capacity of the upper cistern is sufficient to receive the mercury which descends from the tube to the limit of the engraved scale, which in these barometers generally extends only to about 20 inches. A lower limit could of course be given by increasing the size of the cisterns, which it is not advisable to do unless for a special purpose. This barometer may be had mounted in wood, or in brass frame. If in wood, it has a brass shield, which slides round the scale part of the frame, so as to be easily brought in front of the tube and scale as a protection in travelling; the vernier screw, in this case, being placed at the top of the instrument. When the scale is graduated with true inches, the neutral point, the capacity and capillarity corrections should be marked on the frame. The graduated scales, however, placed on these barometers in brass frames, are usually artificial inches, like the Kew plan of graduation; the advantage being that one simple correction only is required, viz. one for index error and capillarity combined, which can always be readily determined by comparison with a standard barometer; moreover, as no adjustment of cistern isrequired in reading, the instrument can be verified by artificial pressure throughout the scale, by the plan practised at Kew, Liverpool, &c., and already described (seep. 18).
40. NEGRETTI & ZAMBRA’S PATENT MOUNTAIN AND OTHER BAROMETERS.
Fig. 30.
This invention is intended to make mountain and other barometers of standard accuracy stronger, more portable, and less liable to derangement, when being carried about, than heretofore, by dispensing with the ordinary flexible cistern containing the mercury at the bottom of the instrument, and adapting in lieu thereof a rigid cistern constructed of glass and iron. The cistern is composed of a glass cylinder, which is secured in a metallic tube or frame. In order to render the cistern mercury-tight at top and bottom, metal caps are screwed into the tube or frame, and bear against leather washers placed between them and the edges of the glass cylinder. The upper cap of the cistern is tapped with a fine threaded screw to receive the iron plug or socket, into which the barometer tube is securely fixed. The whole length of this plug has a fine screw cut upon it by which the cistern can be screwed up or down. At the side of this plug or socket, extending from the lower end to within half an inch of the top, is cut a groove for admitting the air to the surface of the mercury within the cistern when the barometer is in use. An ivory point is screwed into the under surface of the plug, carrying the barometer tube. This ivory point is very carefully adjusted by measurement to be the zero point of the instrument, from which the barometer scale of inches is divided. The surface of the mercury in the cistern is adjusted to the zero point by screwing the cistern up or down until the ivory point and its reflected image are in contact.
The instrument (fig. 30) is shown in a state of adjustment, ready to take an observation; butwhen it is desired to render it portable, it must be inclined, until mercury from the cistern fills the tube; the cistern must then be screwed up on the socket, so as to bring the face of the upper cap against the under side of the shoulder of the cover immediately above it; the instrument may then be carried without being liable to derangement.
Precautions necessary in using the Mountain Barometer.—On removing the barometer from its case after a journey, allow it to remain with its scale end downward, whilst the cistern is unscrewed to the extent ofone turn of the screw, after which slightly shake the cistern; the mercury in it will then completely fill the end of the barometer tube, should any portion of it have escaped therefrom.
The barometer is then inverted, and if it be desired to make an observation, suspend it vertically from its stand by the ring at top. The cistern must then be unscrewed, until the surface of the mercury is brought just level with the extreme end of the ivory or zero point fixed to the iron plug on which the glass cistern moves up and down.
Should the elevation of the place where the barometer is to be used be considerably above the sea level, it will be well—after suspending it from the stand—to unscrew the cistern several turns,holding the barometer in an oblique position, as at great heights the mercury will fall considerably quicker than the cistern can be unscrewed, thereby filling it to overflowing; but by partly unscrewing the cistern first, room is given for the reception of a fall of mercury to the extent of several inches.
The cistern must not be unscrewed when theInstrument isINVERTEDmore thantwo turns of the screw, otherwise the mercury will flow out through the groove.
It is found safer when travelling to carry the barometer in a horizontal position, or with its cistern end uppermost.
To clean the Barometer.—Should at any time the mercury in the cistern become oxidised, and reading from its surface be difficult, it can be readily cleaned by removing the cistern and its contained mercury from the barometer frame by unscrewing itwhen in a horizontal position; this precaution is necessary that the mercury in the tube may not escape, and thereby allow air to enter. The cistern must then be emptied, and with a dry clean leather, or silk handkerchief, well cleaned.
The operation of cleaning being performed, return the cistern to the frame, and screw it until the face is brought up against the under side of the shoulder, still keeping the instrumenthorizontal. The cistern is now ready for re-filling, to do which stand the barometer on endhead downwards, and remove the small screw at bottom; through the aperture thus opened, pour in mercury, passing it through a paper funnel with a very small aperture. It is well to pass the mercury through a very small funnel two or three times before returning it to the barometer cistern, as by this process all particles of dust or oxide adhere to the paper, and are effectually removed.
Should any small quantity of the mercury be lost during the operation of cleaning, it is of no importance so long as sufficient remains to allow of adjustment to the zero point. This latter constitutes one of the great advantages of this new instrument over the ordinary barometer; for, in the majority of cases, after an instrument has been compared carefully with a standard, should mercury be lost, there is no means of correcting the error unless a standard barometer be at hand; the new barometer is, in this respect, independent, a little mercury more or less being unimportant.
41. Short Tube Barometer.—This is simply a tube shorter, as may be required, than that necessary to show the atmospheric pressure at the sea level. It is convenient for balloon purposes, and for use at mountain stations, being of course a special construction.
42. Method of Calculating Heights by the Barometer.—The pressure of the atmosphere being measured by the barometer, it is evident that as the instrument is carried up a high mountain or elevated in a balloon, the length of the column must decrease as the atmospheric pressure decreases, in consequence of a stratum of air being left below. The pressure of air arises from its weight, or the attraction of gravitation upon it, and therefore the quantity of air below the barometer cistern cannot influence the height of the column. Hence it follows that a certain relation must exist between the difference of the barometric pressure at the foot and at the top of a hill or other elevation, and the difference of the absolute heights above the sea. Theoretical investigation, abundantly confirmed by practical results, has determined that the strata of air decrease in density in a geometrical proportion, while the elevations increase in an arithmetical one. Hence we have a method of determining differences of level, by observations made on the density of the air by means of the barometer. It is beyond our purpose to explain in detail the principles upon which this method is founded, or to give its mathematical investigation. We append Tables, which will be useful to practical persons,—surveyors, engineers, travellers, tourists, &c.,—who may carry a barometer as a travelling companion.
Table I. is calculated from the formula, height in feet = 60,200 (log. 29·922 - log. B) + 925; where 29·922 is the mean atmospheric pressure at 32° F., and the mean sea-level in latitude 45°; and B is any other barometric pressure; the 925 being added to avoid minus signs in the Table.
Table II. contains the correction necessary for the mean temperature of the stratum of air between the stations of observation; and is computed from Regnault’s co-efficient for the expansion of air, which is ·002036 of its volume at 32° for each degree above that temperature.
Table III. is the correction due to the difference of gravitation in any other latitude, and is found from the formula,x= 1 + ·00265 cos. 2 lat.
Table IV. is to correct for the diminution of gravity in ascending from the sea-level.
To use these Tables: The barometer readings at the upper and lower stations having been corrected and reduced to temperature 32° F., take out from Table I. the numbers opposite the corrected readings, and subtract the lower from the upper. Multiply this difference successively by the factors found in Tables II. and III. The factor from Table III. may be neglected unless precision is desired. Finally, add the correction taken from Table IV.
Table I.
Approximate Height due to Barometric Pressure.
Table I.—continued.
Approximate Height due to Barometric Pressure.
TableII.
Correction due to Mean Temperature of the Air.