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 bra.s.s ring, with a screw thread on its circ.u.mference. 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 bra.s.s, 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 a similar 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:--

_RISE_ _FALL_ FOR FOR COLD WARM DRY WET OR OR LESS MORE WIND. WIND.

-------- -------- EXCEPT EXCEPT WET FROM WET FROM COOLER SIDE. COOLER SIDE.

Reverting to the explanation of the words on the "Coast" barometers (at page 14), and comparing and considering them as given for northern lat.i.tudes, and as they must be altered for southern lat.i.tudes, it will be perceived, that for all _cold_ winds the barometer rises; and falls for _warm_ winds. The mercury also falls for _increased_ strength of wind; and rises as the wind _lulls_. Likewise before or with rain the column of mercury falls; but it rises with fine dry weather. Putting these facts together, and subst.i.tuting for the points of the compa.s.s 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 Dove 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 circ.u.mstances, they withstood the concussion. The purpose of the trials was "to ascertain whether the _vulcanized india-rubber packing_ round the gla.s.s tube of a _new marine barometer_ did 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 treated in 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 his _usual_ instructions. 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-GLa.s.s.

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 una.s.sisted "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 a.s.sistance 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 changes that 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.

[Ill.u.s.tration: Fig. 17.]

_Description of the Instrument._--The farmer's barometer, as figured in the margin, consists of an upright tube of mercury inverted in a cistern of the same fluid; this is secured against a strong frame of wood, at the upper end of which is fixed the scale, divided into inches and tenths of an inch. On either side of the barometer, or centre tube, are two thermometers--that on the left hand has its bulb uncovered and freely exposed, and indicates the temperature of the air at the place of observation; that on the right hand has its bulb covered with a piece of muslin, from which depend a few threads of soft lamp cotton; this cotton is immersed in the small cup situated just under the thermometer, this vessel being full of water; the water rises by capillary attraction to the muslin-covered bulb, and keeps it in a constantly moist state.

These two thermometers, which we distinguish by the names "Wet Bulb" and "Dry Bulb," form the Hygrometer; and it is by the simultaneous reading of these two thermometers, and noting the difference that exists between their indications, that the humidity in the atmosphere is determined.

Admiral FitzRoy's words (see p. 22) are placed upon the scale of the barometer, as the value of a reading depends, not so much on the actual height of the mercury in the tube, as it does on whether the column is rising, steady, or falling.

The moveable screw at the bottom of the cistern is for the purpose of forcing the mercury to the top of the tube when the instrument is being carried from place to place, and it must always be unscrewed to its utmost limit when the barometer is hung in its proper place. After this it should never be touched.

The manner in which the Hygrometer acts is as follows: It is a pretty well-known fact that water or wine is often cooled by a wet cloth being tied round the bottle, and then being placed in a current of air. The evaporation that takes place in the progressive drying of the cloth causes the temperature to fall considerably below that of the surrounding atmosphere, and the contents of the bottle are thus cooled. In the same manner, then, the covered wet bulb thermometer will be found _invariably_ to read lower than the uncovered one; and the greater the dryness of the air, the greater will be the difference between the indications of the two thermometers; and the more moisture that exists in the air, the more nearly they will read alike.

The cup must be kept filled with pure water, and occasionally cleaned out, to remove any dirt. The muslin, or cotton-wick, should also be renewed every few weeks. The hygrometer may be had separate from the barometer, if the combined instruments cannot be sufficiently exposed to the external air, this being essential for the successful use of the hygrometer.

This farmer's weather-gla.s.s, then, consists of three distinct instruments: the barometer, the thermometer, and the hygrometer. He has thus at command the three instrumental data necessary for the prediction of the weather.

And now to describe--

_How to Use the Instrument._--The observations should be taken twice a day, say at 9 A.M. and 3 P.M.; and should be entered on a slip of paper, or a slate hung up by the barometer. The observer will then be able to see the different values of the readings from time to time, and to draw his conclusions therefrom.

The thermometer on the left hand should first be read, and a note made of its indication, which is the temperature of the air. The wet bulb thermometer should now be read, and also noted; and the difference should be taken of these two readings. Next read the barometer by moving the small index at the side of the tube until it is on a level with the top of the mercury. Having noted the number of inches at which the column stands, compare with the last observation, and see immediately whether the barometer is rising, steady, or falling.

Now, having taken the observations as above, we naturally ask the question, _What are we to predict from them?_

And, probably, the best way of answering this query will be by giving an example. We will suppose that our readings yesterday were as follows:--Temperature, 70; Wet Bulb, 69; Difference, 1; =very moist air. Barometer, 295, and that rain has fallen.

To-day, we read:--Temperature, 60; Wet Bulb, 55; Difference, 5; =dryer air. Barometer, 30. We may safely predict that the rain will cease, and probably we may have wind from the northward.

In spring or autumn, if the barometric height be steady any where between 295 and 30 inches, with the temperature about 60, fresh to moderate south-westerly winds, with cloudy sky, will probably characterize the weather; the indications of the hygrometer being then specially serviceable in enabling us to foretell rain; but if the mercury become steady at about 305 inches, with temperature about 40, north-easterly winds, dry air, and clear sky, may be confidently expected.

Many cases will doubtless suggest themselves to the observer where these figures do not occur, and where he might find a difficulty in interpreting the indications of his instruments. We have, therefore, drawn up some concise rules for his guidance; and although they will not prove absolutely infallible guides to this acknowledged most difficult problem, still, they will be found of much service in foretelling the weather, when added to an intelligent observation of ordinary atmospheric phenomena, as force and direction of wind, nature of any particular season, and the time of year.

26. RULES FOR FORETELLING THE WEATHER.

A RISING BAROMETER.

A "Rapid" rise indicates unsettled weather.

A "Gradual" rise indicates settled weather.

A "Rise," with dry air, and cold increasing in summer, indicates wind from northward; and if rain has fallen, better weather is to be expected.

A "Rise," with moist air and a low temperature, indicates wind and rain from northward.

A "Rise," with southerly wind, indicates fine weather.

A STEADY BAROMETER,

With dry air and a seasonable temperature, indicates a continuance of very fine weather.

A FALLING BAROMETER.

A "Rapid" fall indicates stormy weather.

A "Rapid" fall, with westerly wind, indicates stormy weather from northward.

A "Fall," with a northerly wind, indicates storm, with rain and hail in summer, and snow in winter.

A "Fall," with increased moisture in the air, and the heat increasing, indicates wind and rain from southward.

A "Fall," with dry air, and cold increasing (in winter), indicates snow.

A "Fall," after very calm and warm weather, indicates rain with squally weather.

=27. Causes which may bring about a Fall or a Rise in the Barometer.=[3]--As heat produces rarefaction, a sudden rise of temperature in a distant quarter may affect the weight of the atmosphere over our heads, by producing an aerial current outwards, to supply the place of the lighter air which has moved from its former position; in which case the barometer will fall. Now such a movement in the atmosphere is likely to bring about an intermixture of currents of air of different temperatures, and from this intermixture rain is likely to result.

On the other hand, as cold produces condensation, any sudden fall of temperature causes the column of air over the locality to contract and sink to a lower level, whilst other air rushes in from above to supply the void; and, accordingly, the barometer rises. Should this air, as often happens, proceed from the north, it will contain in general but little moisture; and hence, on reaching a warmer lat.i.tude, will take up the vapour of the air, so that dry weather will result.

It is generally observed, that wind causes a fall in the instrument; and, indeed, in those greater movements of the atmosphere which we denominate storms or hurricanes, the depression is so considerable as to forewarn the navigator of his impending danger. It is evident, that a draught of air in any direction must diminish the weight of the column overhead, and consequently cause the mercury in the barometer to sink.

The connection, therefore, of a sinking of the barometric column with rain is frequently owing to the wind causing an intermixture of the aerial currents which, by their motion, diminish the weight of the atmosphere over our heads; whilst a steady rise in the column indicates the absence of any great atmospheric changes in the neighbourhood, and a general exemption from those causes which are apt to bring about a precipitation of vapour.