A Treatise on Meteorological Instruments Part 2

Admiral FitzRoy's words for the scales of barometers for use in northern lat.i.tudes, then, are as follows:--

_RISE._ _FALL._ FOR FOR N. ELY. S. WLY.

NW.--N.--E. SE.--S.--W.

DRY WET OR OR LESS MORE WIND. WIND.

------- ------- EXCEPT EXCEPT WET FROM WET FROM N. ED. N. ED.

------- ------- Long foretold, long last; First rise after low, Short notice, soon past. Foretells stronger blow.

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 subst.i.tuting for the letter N the letter S, reading south for north, and _vice 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 seen _through_ the tube. Otherwise a piece of paper, or a _white place_, should be behind the upper or _scale part_ of the _tube_.

When suspended on a hook, or stout nail, apply the milled-head key (which will be found just below the scales) to the square bra.s.s pin at the lower end of the instrument, and turn _gently_ toward 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 thus _let down_, the mercury will sink to its proper level quickly.

In removing this barometer it is necessary to _slope it gradually_, till the mercury is at the top of the tube, and then, with the instrument reversed, to screw up the cistern bottom, or bag, by the key, used _gently_, till it stops. It will then be portable, and may be carried with the _cistern_ end _upwards_, 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:--

LA LA HAUSSE BAISSE INDIQUE. INDIQUE.

--------- --------- DES VENTS DE LA DES VENTS DE LA PARTIE DU PARTIE DU N.E. S.O.

(DU N.O. a l'E) (DU S.E. a l'O.) (PAR LE NORD. ) (PAR LE SUD. ) DE LA DE SeCHERESSE. L'HUMIDITe.

--------- --------- UN VENT UN VENT PLUS FAIBLE PLUS FORT EXCEPTe S'IL PLEUT EXCEPTe S'IL PLEUT AVEC DE FORTES BRISES AVEC DE PEt.i.tES BRISES DU N.E. DU N.E.

--------- --------- Mouvements lents, Le commencement Temps durable. de la hausse, --------- apres une grande Mouvements rapides, baisse presage Temps variable. un Vent violent.

MARINE BAROMETERS.

=19. The Common Form.=--The barometer is of great use to the mariner, who, by using it as a "weather gla.s.s," is enabled to foresee and prepare for sudden changes in the weather. For marine purposes, the lower portion of the gla.s.s 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 s.c.r.e.w.i.n.g 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 gla.s.s. The cistern is encased in bra.s.s for protection, the bottom portion uns.c.r.e.w.i.n.g to give access to the portable screw beneath the cistern. Figs. 13 and 14 ill.u.s.trate 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 bra.s.s 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.

[Ill.u.s.tration: Fig. 13.]

[Ill.u.s.tration: Fig. 14.]

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 bra.s.s 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, gla.s.s, iron, and bra.s.s. 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 error will 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 gla.s.s 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.

[Ill.u.s.tration: 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 s.c.r.e.w.i.n.g up the mercury.

The gla.s.s tube thus secured to the cistern is protected by a bra.s.s 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 gla.s.s 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 bra.s.s 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 the mercury 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 gla.s.s 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 to which 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. Gla.s.s 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 275 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:--the _cistern was not large enough_ to hold the mercury which descended from the tube in a _low atmospheric pressure_.

"When used on sh.o.r.e, this contraction of the tube causes the marine barometer to be _sometimes_ a 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 000 to 002 of an inch. As the motion of the ship at sea causes the mercury to pa.s.s 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 and Liverpool 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 to _general_ purposes.

"It differs from barometers. .h.i.therto made in points of detail, rather than principle:--1. The gla.s.s 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 a _spare_ tube, fixed in a cistern, filled with boiled mercury, and _marked_ for adjustment in this, or _any similar_ instrument.

"These barometers are graduated to hundredths, and they will be found accurate to _that_ degree, namely the second decimal of an inch.

"They are packed with vulcanised caoutchouc, in order that (by this, and by a peculiar strength of gla.s.s 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 fit _any_ barometer.

"_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 tube _and_ cistern, by turning the cistern gently, against the sun, or to _the left_, and draw out the tube very carefully _without bending it in the least_, _turning_ it a little, if required, as moved. Then insert the new tube very cautiously, screw in, and adjust to the diamond-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 bra.s.s, as well as the mercury, slowly and unequally, it may be well to defer any _exact comparisons with other instruments_ for some few hours."

Messrs. Negretti and Zambra are the makers of these barometers for the Royal Navy. Fig. 16 is an ill.u.s.tration.

[Ill.u.s.tration: Fig. 16.]