CHAPTER V.
SECONDARY BAROMETERS.
=43. Desirability of Magnifying the Barometer Range.=--The limits within which the ordinary barometric column oscillates, do not exceed four inches for extreme range, while the ordinary range is confined to about two inches; hence it has often been felt that the public utility of the instrument would be greatly enhanced if by any means the scale indications could be increased in length. This object was sought to be obtained by bending the upper part of the tube from the vertical, so that the inches on the scale could be increased in length in proportion to the secant of the angle it made with the vertical. This was called "the diagonal barometer." The upper part of the tube has also been formed into a spiral, and the scale, placed along it, is thus greatly enlarged.
But these methods of enlarging the indications cannot be so successfully accomplished, nor so cheaply nor so elegantly, as is done by the principle employed in the dial barometer. Hence they are not in use.
=44. Howson's Long Range Barometer.=--Very recently quite a novel design has been patented by Mr. Howson, for a long range barometer. The construction requires neither distortion of the tube, nor mechanism for converting a short scale into a long one; but the mercury itself rises and falls, through an extended range, naturally, and in simple obedience to the varying pressure of the atmosphere. The tube is fixed, but its cistern is sustained by the mere pressure of the atmosphere. Looking at the instrument, it seems a perfect marvel. It appears as though the cistern with the mercury in it must fall to the ground. The bore of the tube is wide, about an inch across. A long glass rod is fixed to the bottom of the glass cistern, where a piece of cork or some elastic substance is also placed. The tube is filled with mercury; the glass rod is plunged into the tube as it is held top downwards, until the cork gets close up to the tube and fits tightly against it. The pressure against the cork simply prevents the mercury from coming out while the instrument is being inverted. When it is inverted, the mercury partly falls, and forms an ordinary barometric column. When the top is held, the cistern and glass rod, instead of falling away, remain perfectly suspended. There is no material support to the cistern; the tube only is fixed, the cistern hangs to it. Glass is many times lighter than mercury. When the glass rod is introduced, it displaces an equal volume of mercury. The glass rod, being so much lighter than mercury, floats and sustains the additional weight of the cistern by its buoyancy. In the mean time, the atmosphere is acting upon the mercury, keeping up the ordinary barometric column. Supposing there is a rise in the ordinary barometer, the atmosphere presses some more mercury up the tube. This mercury is taken out of the cistern, which of course becomes lighter, and therefore the rod and cistern float up a little higher, which thus causes the column of mercury to rise still more. The increased pressure and buoyancy thus acting together, increase the ascent in the barometric column, as shown by the fixed scale. One inch in the barometer might be represented by two or more inches in this instrument, according to construction. Supposing there was a decrease of pressure, the mercury would fall, come into the cistern, make it heavier, and increase the fall somewhat. Friction guides, at the top of the rod, prevent it coming into contact with the side of the tube when vertically suspended. The illustration, Fig. 31, shows the appearance of the instrument as framed in wood by the makers, Messrs. Negretti and Zambra.
=45. McNeild's Long Range Barometer.=--A barometer designed by a gentleman named McNeild is on a directly opposite principle to the one just described. The tube is made to float on the mercury in the cistern. It is filled with mercury, inverted in the usual manner, then allowed to float, being held vertically by glass friction points or guides. By this contrivance, the ordinary range of the barometer is greatly increased. One inch rise or fall in the standard barometer may be represented by four or five inches in this instrument, so that it shows small variations in atmospheric pressure very distinctly. As the mercury falls in the tube with a decrease of pressure, the surface of the mercury in the cistern rises, and the floating tube rises also, which causes an additional descent in the column, as shown by fixed graduations on the tube. With an increase of pressure, some mercury will leave the cistern and rise in the tube, while the tube itself will fall, and so cause an additional ascent of mercury. This barometer is identical in principle with King's Barograph (see p. 34).
The construction of Howson's and McNeild's Barometers has been assigned to Messrs. Negretti and Zambra. These instruments are usually made for domestic purposes with a scale of from three to five, and for public use from five to eight times the scale of the ordinary standard. Their sensitiveness is consequently increased in an equal proportion, and they have the additional advantage of not being affected by differences of level in the cistern. However, these novelties have not been sufficiently tried to determine their practical value for strictly scientific purposes; but as weather-glasses, for showing minute changes, they are superior to the common barometer.
=46. The Water-glass Barometer.=--If a Florence flask, having a long neck, have a small quantity of water poured into it, and then be inverted and so supported that the open end dips into a vessel containing water, a small column of water will be confined in the neck of the bottle, the pressure of which, upon the surface of the exposed water, will be equal to the difference between the atmospheric pressure and the elasticity of the confined air in the body of the bottle. As the pressure of the atmosphere varies, this column will alter in height. But the elasticity of the confined air is also subject to variations, owing to changes of temperature. It follows, then, that the oscillations of the column are dependent on alterations of temperature and atmospheric pressure. Such an arrangement has been called "the Water-glass Barometer," and bears about the same relative value to the mercurial barometer, as an exponent of weather changes, that a cat-gut hygrometer bears to a thermometric hygrometer, as an indicator of relative moisture.
47. SYMPIESOMETER.
Nevertheless the instrument now about to be described, depending upon similar principles, but scientifically constructed and graduated, is a very useful and valuable substitute for the mercurial barometer. It consists of a glass tube, varying, according to the purposes for which the instrument is required, from six to twenty-four inches in length. The upper end is closed, and formed into a bulb; the lower is turned up, formed into a cistern, and open at top, through a pipette, or cone. A plug, moveable by a catch from below, can be made to close this opening, so as to render the instrument portable.
The upper portion of the tube is filled with air; the lower portion, and part of the cistern, with sulphuric acid, coloured so as to render it plainly visible. Formerly, hydrogen and oil were used. It was found, however, that, by the process known to chemists as _osmosis_, this light gas in time partially escaped, and the remainder became mixed with air, the consequence being that the graduations were no longer correct. They are more durable as at present constructed. The liquid rises and falls in the tube with the variations of atmospheric pressure and temperature acting together. If the pressure were constant, the confined air would expand and contract for temperature only, and the instrument would act as a thermometer. In fact, the instrument is regarded as such in the manufacture; and the thermometric scales are ascertained and engraved on the scale. A good mercurial thermometer is also mounted on the same frame. If, therefore, at any time the mercurial and the air thermometers do not read alike, it must evidently be due to the atmospheric pressure acting upon the air in the tube; and it is further evident that, under these circumstances, the position of the top of the liquid may be marked to represent the barometric pressure at the time. In this manner a scale of pressure is ascertained by comparison with a standard barometer, extending generally from 27 to 31 inches.
When made correctly, these instruments agree well with the mercurial barometer for a number of years, and their subsequent adjustment is not a matter of much expense.
For use at sea, the liquid column is contracted at the bend. The sympiesometer is very sensitive, and feels the alterations in the atmospheric pressure sooner than the ordinary marine barometer.
The scale is usually on silvered brass, mounted on a mahogany or rosewood frame, protected in front by plate glass. It is generally furnished with a revolving register, to record the observation, in order that it may be known whether the pressure has increased or decreased in the interval of observation.
Small pocket sympiesometers are sometimes fitted with ivory scales, and protected by a neat velvet-lined pasteboard or morocco case.
_How to take an Observation._--In practice, the indications of the atmospheric pressure are obtained from the sympiesometer by noting, first, the temperature of the mercurial thermometer; secondly, adjusting the pointer of the pressure scale to the same degree of temperature on the scale of the air column; thirdly, reading the height of the liquid on the sliding scale.
_Directions for Use._--The sympiesometer should be carried and handled so as to keep the top always upwards, to prevent the air mechanically mixing with the liquid. Care should also be taken to screen it from casual rays of the sun or cabin fire.
48. ANEROIDS.
The beautiful and highly ingenious instrument called by the name _Aneroid_, is no less remarkable for the scientific principles of its construction and action, than for the nicety of its mechanism. It is a substitute, and perhaps the best of all substitutes, for the mercurial barometer. As its name implies, it is constructed "without fluid." It was invented by M. Vidi of Paris. In the general form in which it is made it consists of a brass cylindrical case about four inches in diameter and one and a half inch deep, faced with a dial graduated and marked similarly to the dial-plate of a "wheel-barometer," upon which the index or pointer shows the atmospheric pressure in inches and decimals of an inch in accordance with the mercurial barometer. Within the case, for ordinary sizes, is placed a flat metal box, generally not more than half an inch thick and about two inches or a little more in diameter, from which nearly all the air is exhausted. The top and bottom of this box is corrugated in concentric circles, so as to yield inwardly to external pressure, and return when the pressure is removed. The pressure of the atmosphere, acting externally, continually changes, while the elastic pressure of the small quantity of air within can only vary by its volume being increased or decreased, or by change of temperature. Leaving out of consideration, for the moment, the effect of temperature, we can readily perceive that as the pressure is lessened upon the outside of the box, the elastic force of the air within will force out the top and bottom of the box; and when the outer pressure is increased they will be forced in. Thus with the varying pressure of the atmosphere, the top and bottom of the box approach to and recede from each other by a small quantity; but the bottom being fixed, nearly all this motion takes place on the top. Thus the top of the box is like an elastic cushion, which rises and falls according as the compressing force lessens or increases. To the eye these expansions and contractions would not be perceptible, so small is the motion. But they are rendered very evident by a nice mechanical arrangement. To the box is attached a strong piece of iron, kept pressed upon it by a spring at one extremity; so that as the top of the box rises, the motion is made sensible at the point held by the spring, and when the top descends the spring draws the piece of iron into close contact with it. This piece of iron acts as a lever, having its fulcrum at one extremity, the power at the centre of the box-top, and the other extremity controlled by the spring. Thus it is evident that the small motion of the centre of the box-top is much increased at the spring extremity. The motion thus obtained is communicated to a system of levers; and, by the intervention of a piece of watch-chain and a fine spring passing round the arbour, turns the index to the right or left, according as the external pressure increases or decreases. Thus, when by increase of pressure the vacuum box is compressed, the mechanism transfers the movement to the index, and it moves to the right; when the vacuum box bulges out under diminished pressure, the mechanical motion is reversed, and the index moves to the left. As the index traverses the dial, it shows upon the scale the pressure corresponding with that which a good mercurial barometer would at the same time and place indicate; that is, supposing it correctly adjusted.
A different and more elegant arrangement has since been adopted. A broad curved spring is connected to the top of the vacuum box, so as to be compressed by the top of the box yielding inward to increased pressure, and to relax itself and the box as the pressure is lessened. The system of levers is connected to this spring, which augments and transfers the motion to the index, in the manner already described. Increase of pressure causes the levers to slacken the piece of watch-chain connected with them and the arbour of the index. The spring now uncoils, winds the chain upon the arbour, and turns the index to the right. Decrease of pressure winds the chain off the barrel, tightens the spiral spring, which thus turns the index to the left. The graduations of the aneroid scale are obtained by comparisons with the correct standard reading of a mercurial barometer, under the normal and reduced atmospheric pressure. Reduced pressure is obtained by placing both instruments under the receiver of an air pump.
Fig. 33 represents the latest improved mechanism of an aneroid. The outer case and the face of the instrument are removed, but the hand is attached by its collet to the arbour. _A_ is the corrugated box, which has been exhausted of air through the tube, _J_, and hermetically sealed by soldering. _B_ is a powerful curved spring, resting in gudgeons fixed on the frame-plate, and attached to a socket behind, _F_, in the top of the box. A lever, _C_, joined to the stout edge of the spring, is connected, by the bent lever at _D_, with the chain, _E_, the other end of which is coiled round, and fastened to the arbour, _F_. As the box, _A_, is compressed by the weight of the atmosphere increasing, the spring, _B_, is tightened, the lever, _C_, depressed, and the chain, _E_, uncoiled from _F_, which is thereby turned so that the hand, _H_, moves to the right. In the mean while the spiral spring, _G_, coiled round _F_, and fixed at one extremity to the frame-work and by the other to _F_, is compressed. When, therefore, the pressure decreases, _A_ and _B_ relax, by virtue of their elasticity; _E_ slackens, _G_ unwinds, turning _F_, which carries _H_ to the left. Near _J_ is shown an iron pillar, cast as part of the stock of the spring, _B_. A screw works in this pillar through the bottom of the plate, by means of which the spring, _B_, may be so adjusted to the box, _A_, as to set the hand, _H_, to read on the scale according to the indications of a mercurial barometer. The lever, _C_, is composed of brass and steel, soldered together, and adjusted by repeated trials to correct for the effects of temperature.
A thermometer is sometimes attached to the aneroid, as it is convenient for indicating the temperature of the air. As regards the instrument itself, no correction for temperature can be applied with certainty. It should be set to read with the mercurial barometer at 32 deg. F. Then the readings from it are supposed to require no correction.
In considering the effects of temperature upon the aneroid, they are found to be somewhat complex. There is the effect of expansion and contraction of the various metals of which the mechanism is composed; and there is the effect on the elasticity of the small portion of air in the box. An increase of temperature produces greater, a diminution less elasticity in this air. The compensation for effects of temperature is adjusted by the process of "trial and error," and only a few makers do it well. It is very often a mere sham. Admiral FitzRoy writes, in his _Barometer Manual_, "The known expansion and contraction of metals under varying temperatures, caused doubts as to the accuracy of the aneroid under such changes; but they were partly removed by introducing into the vacuum box a small portion of gas, as a compensation for the effects of heat or cold. The gas in the box, changing its bulk on a change of temperature, was intended to compensate for the effect on the metals of which the aneroid is made. Besides which, a further and more reliable compensation has lately been effected by a combination of brass and steel bars."
"Aneroid barometers, if often compared with good mercurial columns, are similar in their indications, and valuable; but it must be remembered that they are not independent instruments, that they are set originally by a barometer, require adjustment occasionally, and may deteriorate in time, though slowly."
"The aneroid is quick in showing the variation of atmospheric pressure; and to the navigator who knows the difficulty, at times, of using barometers, this instrument is a great boon, for it can be placed anywhere, quite out of harm's way, and is not affected by the ship's motion, although faithfully giving indication of increased or diminished pressure of air. In ascending or descending elevations, the hand of the aneroid may be seen to move (like the hand of a watch), showing the height above the level of the sea, or the difference of level between places of comparison."
In the admiral's _Notes on Meteorology_, he says, "The aneroid is an excellent _weather glass_, if well made. Compensation for heat or cold has lately been introduced by efficient mechanism. In its _improved_ condition, when the cost may be about L5, it is fit for measuring heights as far as 5,000 feet with approximate accuracy; but even at the price of L3, as a _weather-glass_ only, it is exceedingly valuable, because it can be carried anywhere; and if now and then compared with a good barometer, it may be relied on sufficiently. I have had one in constant use for ten years, and it appears to be as good now as at first. For a ship of war (considering concussion by the fire of guns), for boats, or to put in a drawer, or on a table, I believe there is nothing better than it for use as a common weather-glass."
Colonel Sir H. James, R.E., in his _Instructions for taking Meteorological Observations_, says of the aneroid, "This is a most valuable instrument; it is extremely portable. I have had one in use for upwards of ten years, and find it to be the best form of barometer, as a "weather-glass," that has been made."
One of the objects of Mr. Glaisher's experiments in balloons was "to compare the readings of an aneroid barometer with those of a mercurial barometer up to five miles." In the comparisons the readings of the mercurial barometer were corrected for index-error and temperature. The aneroid readings, says Mr. Glaisher, "prove all the observations made in the several ascents may be safely depended upon, and also that an aneroid barometer can be made to read correctly to pressures below twelve inches." As one of the general conclusions derived from his experiments he states, "that an aneroid barometer read correctly to the first place, and probably to the second place of decimals, to a pressure as low as seven inches." The two aneroids used by Mr. Glaisher were by Messrs. Negretti and Zambra.
Aneroids are now manufactured almost perfectly compensated for temperature. Such an instrument therefore ought to show the same pressure in the external air at a temperature say of 40 deg., as it would in a room where the temperature at the same time may be 60 deg.; provided there is no difference of elevation. To test it thoroughly would require an examination and a comparison with barometer readings reduced to 32 deg. F., conducted through a long range of temperature and under artificially reduced pressure. A practical method appears to be to compare the aneroid daily, or more often, for a few weeks with the readings of a mercurial barometer reduced to 32 deg.; and if the error so found be constant, the object of the compensation may be assumed to be attained, particularly if the temperature during the period has varied greatly.
_Directions for using the Aneroid._--Aneroids are generally suspended with the dial vertical; but if they be placed with the dial horizontal, the indications differ a few hundredths of an inch in the two positions. Hence, if their indications are registered, they should be kept in the same position.
The aneroid will not answer for exact scientific purposes, as it cannot be relied upon for a length of time. Its error of indication changes slowly, and hence the necessity of its being set from time to time with the reading of a good barometer. To allow of this being done, at the back of the outer case is the head of a screw in connection with the spring attached to the vacuum box. By applying a small turnscrew to this screw, the spring of the vacuum box may be tightened or relaxed, and the index made to move correspondingly to the right or left on the dial. By this means, besides being enabled to correct the aneroid at any time, "if the measure of a height rather greater than the aneroid will commonly show be required, it may be _re-set_ thus: When at the upper station (_within its range_), and having noted the reading carefully, touch the screw behind so as to bring back the hand a few inches (if the instrument will admit), then read off and start again. _Reverse the operation when descending._ This may add some inches of measure _approximately_."--_FitzRoy._
=49. Small Size Aneroids.=--The patent for the Aneroid having expired, Admiral FitzRoy urged upon Messrs. Negretti & Zambra the desirability of reducing the size at which it had hitherto been made, as well as of improving its mechanical arrangement, and compensation for temperature. They accordingly engaged skilful workmen, who, under their directions, and at their expense, by a great amount of labour and experiment, succeeded in reducing its dimensions to two inches in diameter, and an inch and a quarter thick. The exact size and appearance of this aneroid are shown in fig. 34. The compensation is carefully adjusted, and the graduations of the dial ascertained under reduced pressure, so that they are not quite equal, but more accurate.
=50. Watch Aneroid.=--Subsequently the aneroid has been further reduced in size and it can now be had from an inch and a quarter to six inches in diameter. The smallest size can be enclosed in watch cases, fig. 35, or otherwise, so as to be adapted to the pocket. By a beautifully simple contrivance, a milled rim is adjusted to move round with hand pressure, and carry a fine index or pointer, outside and around the scale engraved on the dial, or face, for the purpose of marking the reading, so that the subsequent increase or decrease of pressure may be readily seen. These very small instruments are found to act quite as correctly as the largest, and are much more serviceable. Besides serving the purpose of a weather-glass in the house or away from home, if carried in the pocket, they are admirably suited to the exigencies of tourists and travellers. They may be had with scale sufficient to measure heights not exceeding 8,000 feet; with a scale of elevation in feet, as well as of pressure in inches, engraved on the dial. The scale of elevation, which is for the temperature of 50 deg., was computed by Professor Airy, the Astronomer Royal, who kindly presented it to Messrs. Negretti and Zambra, at the same time suggesting its application. Moderate-sized aneroids, fitted in leathern sling cases, are also good travelling instruments, and will be found serviceable to pilots, fishermen, and for use in coasting and small vessels, where a mercurial barometer cannot be employed, because requiring too much space.
Admiral FitzRoy, in a communication to the _Mercantile Marine Magazine_, December, 1860, says:--"Aneroids are now made more portable, so that a pilot or chief boatman may carry one in his pocket, as a railway guard carries his timekeeper; and, thus provided, pilots cruising for expected ships would be able to caution strangers arriving, if bad weather were impending, or give warning to coasters or fishing boats. Harbours of Refuge, however excellent and important, are not always accessible, even when most wanted, as in snow, rain, or darkness, when neither land, nor buoy, nor even a lighthouse-light can be seen."
=51. Measurement of Heights by the Aneroid.=--For measuring heights not exceeding many hundred feet above the sea-level by means of the aneroid, the following simple method will suffice:--
Divide the difference between the aneroid readings at the lower and upper stations by .0011; the quotient will give the approximate height in feet.
Thus, supposing the aneroid to read at the
Lower Station 30.385 inches. Upper Station 30.025 ------ Difference .360 ======
Divided gives .360/.0011 = 327 feet.
As an illustration of the mode in which the aneroid should be used in measuring heights, the following example is given:--
A gentleman who ascended Helvellyn, August 12th, 1862, recorded the following observations with a pocket aneroid by Negretti and Zambra:--
Near 10 a.m., at the first milestone from Ambleside, found by survey to be 188 feet above the sea, the aneroid read 29.89 inches; about 1 p.m., at the summit of Helvellyn, 26.81; and at 5 p.m., at the milestone again, 29.76. The temperature of the lower air was 57 deg., of the upper, 54 deg. Hence the height of the mountain is deduced as follows:--
Inches.
Reading at 10 a.m. 29.89 " 5 p.m. 29.76 ------ Mean 29.825 Table I.[5] 1010 Upper Reading 26.81 " 3796 ----- Difference 2786 Mean Temperature 55 deg.5, gives in Table II. 1.048 ----- 2920 Lat. 55 deg. N., gives in Table III. .9991 ----- 2917 Table IV. 5 ----- Difference of height 2922 Height of lower station 188 ----- " Helvellyn 3110
In Sir J. Herschell's _Physical Geography_ it is given as 3115 ft.
So near an agreement is attributable to the excellence of the aneroid, and the careful accuracy of the observer.
52. METALLIC BAROMETER.
This instrument, the invention of M. Bourdon, has a great resemblance to the aneroid, but is much simpler in arrangement. The inventor has applied the same principle to the construction of metallic steam-pressure gauges. We are here, however, only concerned with it as constructed to indicate atmospheric pressure. It consists of a long slender flattened metallic tube, partially exhausted of air, and hermetically closed at each end, then fixed upon its centre, and bent round so as to make the ends face each other. The transverse section of this tube is an elongated ellipse. The principle of action is this: interior pressure tends to straighten the tube, external pressure causes it to coil more. Hence as the atmospheric pressure decreases, the ends of the tube become more apart.
This movement is augmented and transferred by a mechanical arrangement of small metallic levers to a radius bar, which carries a rack formed on the arc of its circle. This moves a pinion, upon the arbour of which a light pointer, or "hand," is poised, which indicates the pressure upon a dial. When the pressure increases, the ends of the tube approach each other, and the pointer moves from left to right over the dial. The whole mechanism is fixed in a brass case, having a hole at the back for adjusting the instrument to the mercurial barometer by means of a key, which sets the pointer without affecting the levers. The dial is generally open to show the mechanism, and is protected by a glass, to which is fitted a moveable index.
This barometer is very sensitive, and has the advantage of occupying little space, although it has not yet been made so small as the aneroid. Both these instruments admit of a great variety of mounts to render them ornamental. The metallic barometer can be constructed with a small clock in its centre, so as to form a novel and beautiful drawing-room ornament.
Admiral FitzRoy writes, "Metallic barometers, by Bourdon, have not yet been tested in very moist, hot, or cold air for a sufficient time. They are dependent, or secondary instruments, and liable to deterioration. For limited employment, when sufficiently compared, they may be very useful, especially in a few cases of electrical changes, _not foretold or shown by mercury_, which these seem to indicate remarkably."
They are not so well adapted for travellers, nor for measurements of considerable elevations, as aneroids.