Fig. 4.
Fig. 5.
8. The Barometer Vernier.—Thevernier, an invaluable contrivance for measuring small spaces, was invented by Peter Vernier, about the year 1630. The barometer scale is divided into inches and tenths. The vernier enables us to accurately subdivide the tenths into hundredths, and, in first-class instruments, evento thousandths of an inch. It consists of a short scale made to pass along the graduated fixed scale by a sliding motion, or preferably by a rack-and-pinion motion, the vernier being fixed on the rack, which is moved by turning the milled head of the pinion. The principle of the vernier, to whatever instrumental scale applied, is that the divisions of the moveable scale are to those in an equal length of the fixed scale in the proportion of two numbers which differ from each other by unity.
The scales of standard barometers are usually divided into half-tenths, or ·05, of an inch, as represented, in fig. 5, by AB. The vernier, CD, is made equal in length to twenty-four of these divisions, and divided into twenty-five equal parts; consequently one space on the scale is larger than one on the vernier by the twenty-fifth part of ·05, which is ·002 inch, so that such a vernier shows differences of ·002 inch. The vernier of the figure reading upwards, the lower edge, D, will denote the top of the barometer column; and is the zero of the vernier scale. In fig. 4, the zero being in line exactly with 29 inches and five-tenths of the fixed scale, the barometer reading would be 29·500 inches. It will be seen that the vernier line,a, falls short of a division of the scale by, as we have explained, ·002 inch;b, by ·004;c, by ·006;d, by ·008; and the next line by one hundredth. If, then, the vernier be moved so as to makeacoincide withz, on the scale, it will have moved through ·002 inch; and if 1 on the vernier be moved into line withyon the scale, the space measured will be ·010. Hence, the figures 1, 2, 3, 4, 5 on the vernier measure hundredths, and the intermediate lines even thousandths of an inch. In fig. 5, the zero of the vernier is intermediate 29·65 and 29·70 on the scale. Passing the eye up the vernier and scale, the second line above 3 is perceived to lie evenly with a line of the scale. This gives ·03 and ·004 to add to 29·65, so that the actual reading is 29·684 inches. It may happen that no line on the vernieraccuratelylies in the same straight line with one on the scale; in such a case a doubt will arise as to the selection of one from two equally coincident, and the intermediate thousandth of an inch should be taken.
For the ordinary purposes of the barometer as a “weather-glass,” such minute measurement is not required. Hence, in household and marine barometers the scale need only be divided to tenths, and the vernier constructed to measurehundredths of an inch. This is done by making the vernier either 9 or 11-10ths of an inch long, and dividing it into ten equal parts. The lines above the zero line are then numbered from 1 to 10; sometimes the alternate divisions only are numbered, the intermediate numbers being very readily inferred. Hence, if the first line of the vernier agrees with one on the scale, the next must be out one-tenth of a tenth, or ·01 of an inch from agreement with the nextscaleline; the following vernier line must be ·02 out, and so on. Consequently, when the vernier is set to the mercurial column, the difference shown by the vernier from the tenth on the scale is the hundredths to be added to the inches and tenths of the scale.
A little practice will accustom a person to set and read any barometer quickly; an important matter where accuracy is required, as the heat of the body, or the hand, is very rapidly communicated to the instrument, and may vitiate, to some extent, the observation.
Fig. 6.
9. SELF-COMPENSATING STANDARD BAROMETER.
This barometer has been suggested to Messrs. Negretti and Zambra by Wentworth Erk, Esq. It consists of a regular barometer; but attached to the vernier is a double rack worked with one pinion, so that in setting or adjusting the vernier in one position, the second rack moves in directly the opposite direction, carrying along with it a plug or plunger the exact size of the internal diameter of the tube dipping in the cistern, so that whatever the displacement that has taken place in the cistern, owing to the rise or fall of the mercury, it is exactly compensated by the plug being more or less immersed in the mercury, so that no capacity correction is required.
A barometer on this principle is, however, no novelty, for at the Royal Society’s room a very old instrument may be seen reading somewhat after the same manner.
Fig. 6 is an illustration of the appearance of this instrument. The cistern is so constructed that the greatest amount of light is admitted to the surface of the mercury.
10. BAROMETER WITH ELECTRICAL ADJUSTMENT.
This barometer is useful to persons whose eyesight may be defective; and is capable of being read off to greater accuracy than ordinary barometers, as will be seen by the following description:—The barometer consists of an upright tube dipping into a cistern, so contrived, that an up-and-down movement, by means of a screw, can be imparted to it. In the top of the tube a piece of platina wire is hermetically sealed. The cistern also has a metallic connection, so that by means ofcovered copper wires (in the back of the frame) a circuit is established; another connection also exists by means of a metallic point dipping into the cistern. The circuit, however, can be cut off from this by means of a switch placed about midway up the frame; on one side of the tube is placed a scale of inches; a small circular vernier, divided into 100 parts, is connected with the dipping point, and works at right angles with this scale.
To set the instrument in action for taking an observation, a small battery is connected by means of two small binding screws at the bottom of the frame. The switch is turned upwards, thereby disconnecting the dipping point; the cistern is then screwed up, so that the mercury in the tube is brought into contact with the platina wire at the top; the instant this is effected the magnetic needle seen on the barometer will be deflected. The switch is now turned down; by so doing the connection with the upper wire or platina is cut off, and established instead only between the dipping point carrying the circular vernier and the bottom of the cistern; the point is now screwed by means of the milled head until the needle is again deflected. We may now be sure that the line on the circular vernier that cuts the division on the scale is the exact height of the barometer. Although the description here given may seem somewhat lengthy, the operation itself is performed in less time than would be taken in reading off an ordinary instrument.
11. PEDIMENT BAROMETERS.
These Barometers, generally for household purposes, are illustrated by figs. 7 to 11.They are intended chiefly for “weather glasses,” and are manufactured to serve not only a useful, but an ornamental purpose as well. They are usually framed in wood, such as mahogany, rosewood, ebony, oak or walnut, and can be obtained either plain or handsomely and elaborately carved and embellished, in a variety of designs, so as to be suitable for private rooms, large halls, or public buildings. The scales to the barometer and its attached thermometer may be ivory, porcelain, or silvered metal. It is not desirable that the vernier should read nearer than one-hundredth of an inch. Two verniers and scales may be fitted one on either side of the mercurial column, so that one can denote the last reading, and thus show at a glance the extent of rise or fall in the interval. The scale and thermometer should be covered with plate glass. A cheap instrument has an open face and plain frame, with sliding vernier instead of rack-and-pinion motion. The barometer may or may not have a moveable bottom to the cistern, with screw for the purpose of securing the mercury for portability. The cistern should not, however, require adjustment to a zero or fiducial point. It should be large enough to contain the mercury, which falls from 31 to 27 inches, without any appreciable error on the height read off on the scale.
12. The Words on the Scale.—The following words are usually engraved on the scales of these barometers, although they are not now considered of so much importance as formerly:—
The French place upon their barometers a similar formula:—
Manufacturers of barometers have uniformly adopted these indications for all countries, without regard to the elevation above the sea, or the different geographical conditions; and as it can readily be shown that the height and variations of the barometer are dependent on these, it follows that barometers have furnished indications which, under many circumstances, have been completely false. Even in this country, and near the sea-level, storms are frequent with the barometer not below29; rain is not uncommon with the glass at 30; even fine weather sometimes occurs with a low pressure; while it is evident that at an elevation of a few thousand feet the mercury would never rise to 30 inches; hence, according to the scale, there should never be fair weather there. If tempests happened as seldom in our latitude as the barometer gets down to 28 inches, the maritime portion of the community at least would be happy indeed. These words have long been ridiculed by persons acquainted with the causes of the barometric fluctuations; nevertheless opticians continue to place them on the scales, evidently only because they appear to add to the importance of the instrument in the eyes of those who have not learned their general inutility. In different regions of the world, the indications of the barometer are modified by the conditions peculiar to the geographical position and elevation above the sea, and it is necessary to take account of these in any attempt to found rules of general utility in connection with the barometer as a weather guide. All that can be said in favour of these words is, that within a few hundred feet of the sea-level, when the column rises or falls gradually during two or three days towards “Fair” or “Rain,” the indications they afford of the coming weather are generally extremely probable; but when the variations are quick, upward or downward, they presage unsettled or stormy weather.
Admiral FitzRoy writes:—“The words on the scales of barometers should not be so much regarded, for weather indications, as the rising or falling of the mercury; for if it stands atChangeable, and then rises a little towardsFair, it presages a change of wind or weather, though not so great as if the mercury had risen higher; and, on the contrary, if the mercury stands aboveFairand falls, it presages a change, though not to so great a degree as if it had stood lower; besides which, the direction and force of wind are not in any way noticed. It is not from the point at which the mercury stands that we are alone to form a judgment of the state of the weather, but from itsrisingorfalling; and from the movements of immediately preceding days as well as hours, keeping in mind effects of change ofdirectionand dryness, or moisture, as well as alteration of force or strength of wind.”[1]
13. Correction due to Capacity of Cistern.—These barometers, having no adjustment for the zero of the scale, require a correction for the varying level of the mercury in the cistern, when the observations are required for strict comparison with other barometric observations, or when they are registered for scientific purposes; but for the common purpose of predicting the weather, this correction is unnecessary. The neutral point, and the ratio of the bore of the tube to the diameter of the cistern, must be known (seep. 3). Then the capacity correction, as it is termed, is found as follows:—Take the fractional part, expressed by the capacity ratio, of the difference between the observed reading and the height of the neutral point; then, if the mercury standbelowthe neutral point,subtractthis result from the reading; if it standabove,addit to the reading.
For example, suppose the neutral point to be 29·95 inches, and the capacity ratio1⁄50, required the correction when the barometer reads 30·78.
Of course the correction could as easily be found to three decimal places, if desirable. It is evident that the correction is more important the greater the distance of the top of the mercury from the neutral point.
14. PUBLIC BAROMETERS.
Since the increased attention paid to the signs of forthcoming weather of late years, and the good which has resulted therefrom to farmers, gardeners, civil engineers, miners, fishermen, and mariners generally, by forewarning of impending wet or stormy weather, the desirability of having good barometers exposed in public localities has become evident.
Barometers may now be seen attached to drinking fountains, properly protected, and are frequently consulted by the passers-by. But it is among those whose lives are endangered by sudden changes in the weather, fishermen especially, that the warning monitor is most urgently required. Many poor fishing villages and towns have therefore been provided by the Board of Trade, at the public expense, and through the humane effort of Admiral FitzRoy, with first-class barometers, each fixed in a conspicuous position, so as to be easily accessible to all who desire to consult it. Following this example, the Royal National Life Boat Institution has supplied each of its stations with a similar storm warner; the Duke of Northumberland and the British Meteorological Society have erected several on the coast of Northumberland; and many other individuals have presented barometers to maritime places with which they are connected.
These barometers have all been manufactured by Messrs. Negretti and Zambra. The form given to the instrument seems well adapted for public purposes.
Fig. 12.
15. Fishery or Sea-coast Barometers.—Fig. 12 gives a representation of these coast and fishery barometers. The frame is ofsolid oak, firmly screwed together. The scales are very legibly engraved on porcelain by Negretti and Zambra’s patent process. The thermometer is large, and easily read; and as this instrument is exposed, it will indicate the actual temperature sufficiently for practical purposes. The barometer tube is three-tenths of an inch in diameter of bore, exhibiting a good column of mercury; and the cistern is of such capacity, in relation to the tube, that the change of height in the surface of the mercury in the cistern corresponding to a change of height of three inches of mercury in the tube, is less than one-hundredth of an inch, and therefore, as the readings are only to be made to this degree of accuracy, this small error is of no importance. The cistern is made of boxwood, which is sufficiently porous to allow the atmosphere to influence the mercurial column; but the top is plugged with porous cane, to admit of free and certain play.
16. Admiral FitzRoy’s Scale Words.—The directions given on the scales of these barometers were drawn up by Admiral FitzRoy, F.R.S. They appear to be founded on the following considerations:—
Supposing a compass diagram, with the principal points laid down, the N.E. is the wind for which the barometer stands highest; for the S.W. wind it is lowest. This is found to be so in the great majority of cases; but there are exceptions to this, as to all rules. The N.E. and S.W. may therefore be regarded as the poles of the winds, being opposite each other. When the wind veers from the S.W. through W. and N. to N.E., the barometer gradually rises; on the contrary, when the wind veers from N.E. and E. to S.E., S. and S.W., the mercury falls. A similar curious law exists in relation to the veering of the wind, and the action of the thermometer. As the wind veers from the S.W. to W. and N., the thermometer falls; as it veers from N.E. to E. and S., it rises, because the wind gets from a colder to a warmer quarter. The polar winds are cold, dry, and heavy. Those from the equatorial regions are warm, moist, and comparatively light.
These laws have been clearly developed and expressed by Professor Dové in his work on the “Law of Storms.” The warm winds of Europe are those which bring the greatest quantity of rain, as they blow from the ocean, and come heavily laden with moisture. The cold winds, besides containing less moisture, blow more from the land. The weight of the vapour of the warm winds tends to raise the barometric column; but, at the same time, the increased dilatation of the air tends to lower it. This latter influence being the stronger, the barometer always falls for these winds; and in regions where they traverse a large extent of land, retain their heat, and become necessarily very dry, the fall in the barometer will be greater. Admiral FitzRoy’s words for the scales of barometers for use in northern latitudes, then, are as follows:—
It will be perceived that the exception in each case applies to N.E. winds. The barometer may fall with north-easterly winds, but they will be violent and accompanied with rain, hail, or snow; again, it will rise with these winds accompanied with rain, when they are light, and bring only little rain. It rises, however, highest with the dry and light N.E. winds.
These directions are very practically useful; they provide for geographical position—also for elevation above the sea—since they are not appended to any particular height of the column. They are suited to the northern hemisphere generally, as well as around the British Isles. The same directions are adapted to the southern hemisphere, by simply substituting for the letter N the letter S, reading south for north, andvice versa. South of the equator the cold winds come from the south; the warm, from the north. The S.E. wind in the southern hemisphere corresponds to the N.E. in the northern. The laws there are, while the wind veers from S.E. through E. to N. and N.W., the barometer falls and the thermometer rises. As the wind veers from N.W. through W. and S. to S.E., the barometer rises and the thermometer falls.
17. Instructions for the Sea-coast Barometer.—The directions for fixing the barometer, and making it portable when it has to be removed, should be attended to carefully. The barometer should be suspended against a frame or piece of wood, so that light may be seenthroughthe tube. Otherwise a piece of paper, or awhite place, should be behind the upper orscale partof thetube.
When suspended on a hook, or stout nail, apply the milled-head key (which will be found just below the scales) to the square brass pin at the lower end of the instrument, and turngentlytoward the left hand till the screw stops; then take off the key and replace it for use, near the scale, as it was before. The cistern bottom being thuslet down, the mercury will sink to its proper level quickly.
In removing this barometer it is necessary toslope it gradually, till the mercuryis at the top of the tube, and then, with the instrument reversed, to screw up the cistern bottom, or bag, by the key, usedgently, till it stops. It will then be portable, and may be carried with thecisternendupwards, or lying flat; but it must not be jarred, or receive a concussion.
18. French Sea-coast Barometer.—The French have imitated this form of barometer for coast service, and have translated Admiral FitzRoy’s indications for the scale as follows:—
Fig. 13.
Fig. 14.
MARINE BAROMETERS.
19. The Common Form.—The barometer is of great use to the mariner, who, by using it as a “weather glass,” is enabled to foresee and prepare for sudden changes in the weather. For marine purposes, the lower portion of the glass tube of the barometer must be contracted to a fine bore, to prevent oscillation in the mercurial column, which would otherwise be occasioned by the movements of the ship. This tube is cemented to the cistern, which is made of boxwood, and has a moveable leathern bottom, for the purpose of rendering the instrument portable,by screwing up the mercury compactly in the tube. The tube is enclosed in a mahogany frame, which admits of a variety of style in shape, finish, and display, to meet the different fancies and means of purchasers. The frame is generally enlarged at the upper part to receive the scales and the attached thermometer, which are covered by plate glass. The cistern is encased in brass for protection, the bottom portion unscrewing to give access to the portable screw beneath the cistern. Figs. 13 and 14 illustrate this form of barometer. Marine barometers require to be suspended, so that they may remain in a vertical position under the changeable positions of a vessel at sea. To effect this they are suspended in gimbals by a brass arm. The gimbals consist of a loose ring fastened by thumb-screws to the middle part of the frame of the barometer, in front and back. The forked end of the arm supports this ring at the sides, also by the aid of thumb-screws. Hence the superior weight of the cistern end is always sufficient to cause the instrument to move on its bearing screws, so as always to maintain a perpendicular position; in fact, it is so delicately held that it yields to the slightest disturbance in any direction. The other end of the arm is attached to a stout plate, having holes for screws, or fitted to slip into a staple or bracket, by which it may be fixed to any part of the cabin of a ship; the arm is hinged to the plate, for the purpose of turning the arm and barometer up whenever it is desirable.
Other forms of barometer (to be immediately described) have superseded this in the British Marine, but the French still give the preference to the wooden frames. They think the barometer can be more securely mounted in wood, is more portable, and less liable to be broken by a sudden concussion than if fitted in a metal frame. The English deem the ordinary wooden barometers not sufficiently accurate, owing to the irregular expansion of wood, arising from its hygrometric properties. Some of the English opticians have shown that very portable, and really accurate barometers can be made in brass frames, and therefore the preference is now given to this latter material.
20. The Kew Marine Barometer.—The form of barometer so-called, is that recommended by the Congress of Brussels, held in 1853, for the purpose of devising a systematic plan of promoting meteorological observations at sea.
The materials employed in its construction are mercury, glass, iron, and brass. The upper part of the tube is carefully calibrated to ensure uniformity of bore, as this is a point upon which the accuracy of the instrument to some extent depends. At sea, the barometer has never been known to stand above 31 inches, nor below 27. These extremes have been attained with instruments of undoubted accuracy, but they are quite exceptional. It is not necessary, therefore, to carry the scales of marine barometers beyond these limits, but they should not be made shorter. If the vernier is adjusted to read upward, the scale should extend to 32 inches, to allow room for the vernier to be set to 31 inches at least. Cases have occurred in which this could not be done, and rare, but valuable observations have been lost in consequence. If the scale part of the tube be not uniform in bore, the index errorwill be irregular throughout the scale. Whether the bore of the rest of the tube varies in diameter, is of no moment. From two to three inches below the measured part, the bore is contracted very much, to prevent the pulsations in the mercurial column—called “pumping”—which, otherwise, would occur at sea from the motion of the ship. In ordinary marine barometers, this contraction extends to the end of the tube. Below the contracted part is inserted a pipette—or Gay Lussac air-trap—which is a little elongated funnel with the point downwards. Its object is to arrest any air that may work in between the glass and the mercury. The bubble of air lodges at the shoulder, and can go up no farther. It is one of those simple contrivances which turn out remarkably useful. If any air gets into the tube, it does not get to the top, and therefore does not vitiate the performance of the barometer; for the mercury itself works up and down through the funnel. Below this, the tube should not be unnecessarily contracted.
Fig. 15.
The open end of the tube is fixed into an iron cylinder, which forms the cistern of the barometer. Iron has no action upon mercury, and is therefore used instead of any other metal. One or two holes are made in the top of the cistern, which are covered on the inside with strong sheep-skin leather, so as to be impervious to mercury, but sufficiently porous for the outer air to act upon the column. The cistern is of capacity sufficient to receive the mercury which falls out of the tube until the column stands lower than the scale reads; and when the tube is completely full, there is enough mercury to cover the extremity so as to prevent access of air. There is no screw required for screwing up the mercury.
The glass tube thus secured to the cistern is protected by a brass tubular frame, into which the iron cistern fits and screws compactly. Cork is used to form bearings for the tube. A few inches above the cistern is placed the attached thermometer. Its bulb is enclosed in the frame, so as to be equally affected by heat with the barometric column. The upper end of the frame is fitted with a cap which screws on, and embraces a glass shield which rests in a gallery formed on the frame below the scale, and serves to protect the silvered scale, as well as the inner tube, from dust and damp. A ring, moveable in a collar fixed on the frame above the centre of gravity of the instrument, is attached to gimbals, and the whole is supported by a brass arm in the usual manner; so that the instrument can be moved round its axis to bring any source of light upon it, and will remain vertical in all positions of the ship. The vernier reads to five-hundredths of an inch. No words are placed upon the scale, as the old formulary was deemed misleading. The vernier can be set with great exactness, as light is admitted to the top of themercury by a front and a back slit in the frame. The lower edge of the vernier should be brought to the top of the mercury, so as just to shut out the light.
It is evident that this form of barometer must be more reliable in its indications than those in wooden frames. The graduations can be accurately made, and they will be affected only by well-known alterations due to temperature. Some think the tube is too firmly held, and therefore liable to be broken by concussion more readily than that of an inferior instrument. This, however, appears a necessary consequence of greater exactness. It is an exceedingly good portable instrument, and can be put up and taken down very readily. These barometers are preferred to marine barometers in wood, wherever they have been used. In merchant ships, and under careful treatment, they have been found very durable. They may be sent with safety by railway, packed carefully in a wooden box.
Directions for Packing.—In removing this barometer it is necessary to slope it gradually till the mercury reaches the top of the tube. It is then portable, if carried cistern end upwards or lying flat. If carried otherwise, it will very probably be broken by the jerking motion of the heavy mercury in the glass tube. Of course it must not be jarred, or receive concussion.
Position for Marine Barometer.—Admiral FitzRoy, to whose valuable papers we are much indebted, writes in his “Barometer Manual”:—“It is desirable to place the barometer in such a position as not to be in danger of a side blow, and also sufficiently far from the deck above to allow for the spring of the metal arm in cases of sudden movements of the ship.
“If there is risk of the instrument striking anywhere when the vessel is much heeled, it will be desirable to put some soft padding on that place, or to check movement in that direction by a light elastic cord; in fixing which, attention must be paid to have it acting only where risk of a blow begins, not interfering otherwise with the free swing of the instrument: a very light cord attached above, when possible, will be least likely to interfere injuriously.”
21. Method of verifying Marine and other Barometers.—“In nearly all the barometers which had been employed at sea till recently the index correction varied through the range of scale readings, in proportion to the difference of capacity between the cistern and the tube. To find the index correction for a land barometer, comparison with a standard, at any part of the scale at which the mercury may happen to be, is generally considered sufficient. To test the marine barometer is a work of much more time, since it is necessary to find the correction for scale readings at about each half inch throughout the range of atmospheric pressure to which it may be exposed; and it becomes necessary to have recourse to artificial means of changing the pressure of the atmosphere on the surface of the mercury in the cistern.
“The barometers to be thus tested are placed, together with a standard, in an air-tight chamber, to which an air-pump is applied, so that, by partially exhausting the air, the standard can be made to read much lower than the lowest pressure towhich marine barometers are likely to be exposed; and by compressing the air it can be made to read higher than the mercury ever stands at the level of the sea. The tube of the standard is contracted similarly to that of the marine barometer, but a provision is made for adjusting the mercury in its cistern to the zero point. Glass windows are inserted in the upper part of the iron air-chamber, through which the scales of the barometers may be seen; but as the verniers cannot be moved in the usual way from outside the chamber, a provision is made for reading the height of the mercury independent of the verniers attached to the scales of the respective barometers. At a distance of some five or six feet from the air-tight chamber a vertical scale is fixed. The divisions on this scale correspond exactly with those on the tube of the standard barometer. A vernier and telescope are made to slide on the scale by means of a rack and pinion. The telescope has two horizontal wires, one fixed and the other moveable by a micrometer screw, so that the difference between the height of the column of mercury and the nearest division on the scale of the standard, and also of all the other barometers placed by the side of it for comparison, can be measured either with the vertical scale and vernier or the micrometer wire. The means are thus possessed of testing barometers for index error in any part of the scale, through the whole range of atmospheric pressure to which they are likely to be exposed; and the usual practice is to test them at every half inch from 27·5 to 31 inches.
“In this way barometers of various other descriptions have been tested, and some errors found to be so large that a few barometers read half an inch and upwards too high, while others read as much too low. In some cases those which were correct in one part of the scale were found to be from half an inch to an inch wrong in other parts. These barometers were of an old and ordinary, not to say inferior, construction. In some the mercury would not descend lower than about 29 inches, owing to a fault very general in the construction of many common barometers till lately in frequent use:—thecistern was not large enoughto hold the mercury which descended from the tube in alow atmospheric pressure.
“When used on shore, this contraction of the tube causes the marine barometer to besometimesa little behind an ordinary land barometer, the tube of which is not contracted. The amount varies according to the rate at which the mercury is rising or falling, and ranges from 0·00 to 0·02 of an inch. As the motion of the ship at sea causes the mercury to pass more rapidly through the contracted tube, the readings are almost the same there as they would be if the tube were not contracted, and in no case do they differ enough to be of importance in maritime use.”
The cistern of this marine barometer is generally made an inch and a quarter in diameter, and the scale part of the tube a quarter of an inch in bore. The inches on the scale, instead of being true, are shortened by ·04 of an inch, in order to avoid the necessity of applying a correction due to the difference of capacity of the tube and cistern. This is done with much perfection, and the errors of the instruments, when compared with a standard by the apparatus used at Kew andLiverpool Observatories, are determined to the thousandth of an inch, and are invariably very uniform and small. The error so determined includes the correction due to capillarity, capacity, and error of graduation, and forms a constant correction, so that only one variable correction, that due to temperature, need be applied, when the barometer is suspended near the water line of the ship, to make the observations comparable with others. With all the advantages of this barometer, however, it has recently been superseded, to some extent, because it was found to require more care than could ordinarily be expected to be given to it by the commander of a ship. Seamen do not exactly understand the value of such nice accuracy as the thousandth part of an inch, but prefer an instrument that reads only to a hundredth part.
22. THE FITZROY MARINE BAROMETER.
Admiral FitzRoy deemed it desirable to construct a form of barometer as practically useful as possible for marine purposes. One that should be less delicate in structure than the Kew barometer, and not so finely graduated. One that could be set at a glance and read easily; that would be more likely to bear the common shocks unavoidable in a ship of war. Accordingly, the Admiral has devised a barometer, which he has thus described:—
“This marine barometer, for Her Majesty’s service, is adapted togeneralpurposes.
“It differs from barometers hitherto made in points of detail, rather than principle:—1. The glass tube is packed with vulcanised india-rubber, which checks vibration from concussion; but does not hold it rigidly, or prevent expansion. 2. It does not oscillate (or pump), though extremely sensitive. 3. The scale is porcelain,very legible, and not liable to change. 4. There is no iron anywhere (to rust). 5. Every part can be unscrewed, examined, or cleaned, by any careful person. 6. There is asparetube, fixed in a cistern, filled with boiled mercury, andmarkedfor adjustment in this, orany similarinstrument.
“These barometers are graduated to hundredths, and they will be found accurate tothatdegree, namely the second decimal of an inch.
“They are packed with vulcanised caoutchouc, in order that (by this, and by a peculiar strength of glass tube) guns may be fired near these instruments without causing injury to them by ordinary concussion.
“It is hoped that all such instruments, for the public service at sea, will be quite similar, so that any spare tube will fitanybarometer.
Fig. 16.
“To Shift a Tube.—Incline the barometer slowly, and then take it down, after allowing the mercury to fill the upper part. Lay the instrument on a table, unscrew the outer cap at the joining just below the cistern swell, then unscrew the tubeandcistern, by turning the cistern gently, against the sun, or tothe left, and draw out the tube very carefullywithout bending it in the least,turningit a little, if required, as moved. Then insert the new tube very cautiously, screw in, and adjust to thediamond-cut mark for 27 inches. Attach the cap, and suspend the barometer for use.
“If the mercury does not immediately quit the top of the tube, tap the cistern end rather sharply. In a well-boiled tube, with a good vacuum, the mercury hangs, at times, so adhesively as to deceive, by causing a supposition of some defect.
“In about ten minutes the mercurial column should be nearly right; but as local temperature affects the brass, as well as the mercury, slowly and unequally, it may be well to defer anyexact comparisons with other instrumentsfor some few hours.”
Messrs. Negretti and Zambra are the makers of these barometers for the Royal Navy. Fig. 16 is an illustration.
The tube is fixed to a boxwood cistern, which is plugged with very porous cane at the top, to allow of the ready influence of a variation in atmospheric pressure upon the mercury. Round the neck of the cistern is formed a brass ring, with a screw thread on its circumference. This screws into the frame, and a mark on the tube is to be adjusted to 27 inches on the scale, the cistern covering screwed on, and the instrument is ready to suspend. The frame and all the fittings are brass, without any iron whatever; because the contact of the two metals produces a galvanic action, which is objectionable. The spare tube is fitted with india-rubber, and ready at any time to replace the one in the frame. The ease with which a tube can be replaced when broken is an excellent feature of the instrument. The spare tube is carefully stowed in a box, which can also receive the complete instrument when not in use. All the parts are made to a definite gauge; the frames are, therefore, all as nearly as possible similar to each other, and the tubes—like rifle bullets—are adjustible to any frame. If, then, the tube in use gets broken, the captain can replace it by the other; but, as it is securely packed with india-rubber, there is very little liability of its being broken by fair usage. Every person who knows the importance of the barometer on board ship, will acknowledge that the supplementary tube is a decided improvement. Many instruments of this description are afloat in the Royal Navy, and in a short time it may be expected that all the frames and tubes of barometers in the public service at sea will be similar in size and character; so that should a captain have the misfortune to get both his tubes broken, he would be able to borrow another from any ship he fell in with that had one to spare, which would be perfectly accurate, because it would have been verified before it was sent out.
23. Admiral FitzRoy’s Words for the Scale.—The graduation of inches and decimals are placed in this barometer on the right-hand side of the tube; and on asimilar piece of porcelain, on the left-hand, are engraved, as legibly as they are expressed succinctly, the following words, of universal application in the interpretation of the barometer movements:—
Reverting to the explanation of the words on the “Coast” barometers (atpage 14), and comparing and considering them as given for northern latitudes, and as they must be altered for southern latitudes, it will be perceived, that for allcoldwinds the barometer rises; and falls forwarmwinds. The mercury also falls forincreasedstrength of wind; and rises as the windlulls. Likewise before or with rain the column of mercury falls; but it rises with fine dry weather. Putting these facts together, and substituting for the points of the compass the terms “cold” and “warm,” the appropriateness of the words on the scale of this barometer is readily perceived. These concise and practical indications of the movements in the barometer are applicable for instruments intended for use in any region of the world, and are in perfect accordance with the laws of winds and weather deduced by Dové and other meteorologists. There is nothing objectionable in them, and being founded upon experience and the deductions made from numerous recorded observations of the weather in all parts of the world, as well as confirmed by the theories of science, they may consequently be considered as generally reliable. They involve no conjecture, but express succinctly scientific principles.
24. Trials of the FitzRoy Marine Barometer under Fire of Guns.—Some of the first barometers made by Messrs. Negretti and Zambra on Admiral FitzRoy’s principle were severely tried under the heaviest naval gun firing, on board H.M.S.Excellent; and under all the circumstances, they withstood the concussion. The purpose of the trials was “to ascertain whether thevulcanized india-rubber packinground the glass tube of anew marine barometerdid check the vibration caused by firing, and whether guns might be fired close to these instruments without causing injury to them.” In the first and second series of experiments, a marine barometer on Admiral FitzRoy’s plan was tried against a marine barometer on the Kew principle, both instruments being new, and treatedin all respects similarly. They were “hung over the gun, under the gun, and by the side of the gun, the latter both inside and outside a bulkhead,—in fact, in all ways that they would be tried in action with the bulkheads cleared away.” The result was that the Kew barometer was broken and rendered useless, while the new pattern barometer was not injured in the least. In a third series of experiments, Mr. Negretti being present, five of the new pattern barometers were subjected to the concussion produced by firing a 68-pounder gun with shot, and 16 lbs. charge of powder. They were suspended from a beam immediately under the gun, then from a beam immediately over the gun, and finally they were suspended by the arm to a bulkhead, at a distance of only 3 ft. 6 in. from the axis of the gun; and the result was, according to the official report, “that all these barometers, however suspended, would stand, without the slightest injury, the most severe concussion that they would ever be likely to experience in any sea-going man-of-war.” These trials were conducted under the superintendence of Captain Hewlett, C.B., and the guns were fired in the course of hisusualinstructions. His reports to Admiral FitzRoy, giving all the particulars of the trials, are published in the “Ninth Number of Meteorological Papers,” issued by the Board of Trade.[2]
25. NEGRETTI AND ZAMBRA’S FARMER’S BAROMETER AND DOMESTIC WEATHER-GLASS.
It is a well-known fact that the barometer is as much, or even more affected by a change of wind as it is by rain; and the objection raised against a simple barometer reading, as leaving the observer in doubt whether to expect wind or rain, is removed by the addition of the Hygrometer, an instrument indicating the comparative degree of dryness or dampness of the air;—a most important item in the determination of the coming weather.
The farmer should not be content to let his crops lie at the mercy, so to speak, of the weather, when he has within his command instruments which may be the means of preventing damage to, and in cases total loss of, his crops.
The farmer hitherto has had to depend for his prognostication of the weather on his own unassisted “Weather Wisdom;” and it is perfectly marvellous how expert he has become in its use. Science now steps in, not to ignore this experience, but on the contrary, to give it most valuable assistance by extending it, and enabling it to predict, with an accuracy hitherto unknown, the various changes that take place in this most variable of climates.
To the invalid, the importance of predicting with tolerable accuracy the changesthat are likely to occur in the weather, cannot be over-rated. Many colds would be prevented, if we could know that the morning so balmy and bright, would subside into a cold and cheerless afternoon. Even to the robust, much inconvenience may be prevented by a due respect to the indications of the hygrometer and the barometer, and the delicate in health will do well to regard its warnings.