Cooley's Cyclopaedia of Practical Receipts Volume I Part 75

9. " " " modified by M. Bunten.

_a_, Tube containing a column of mercury.

_b_, Mercurial cistern.

_c_, A column of mercury supporting another of water, _d_.

_e_, _e_, Weights, one of which floats on the surface of the mercury, and by means of the cord _f_ moves the index _g_.

_h_, Graduated dial.

_k_, Capillary hole drilled laterally to admit air.]

The construction of a barometer requires the utmost skill and care of a practised artist, and will therefore be seldom undertaken by the amateur or experimentalist--a fact which renders it unnecessary for us to enter into the details here. In the choice of his instrument the purchaser must greatly depend on the known experience and integrity of the manufacturer; as nothing but lengthened use, and frequent comparisons with other instruments, can possibly prove its excellence. An ordinary barometer, however carefully made, is found to suffer gradual deterioration, from the external air insinuating itself between the mercury and the gla.s.s tube, by which the perfection of the vacuum is destroyed. Various plans have been proposed to remedy this inconvenience and source of error. Prof. Daniell forms the bottom part of the tube, to the extent of about 1/3rd of an inch, of solid platinum, welded to the gla.s.s. This plan has proved completely satisfactory. Dr Ure proposes the use of platinum-foil for the same purpose. Before purchasing an instrument it is as well to ascertain that this has been done. In those called 'STANDARD BAROMETERS' the scale is movable and adjustable by a delicate screw, so as to enable the observer to bring the lower point or zero (0) of the scale coincident with the surface of the mercury in the cistern. Exact contact is readily effected by making the point, and its image as seen by reflection from the surface of the mercury, to coincide. In this case the cistern is made of gla.s.s. Provided the ivory scale be connected with the zero-point with a strip of bra.s.s, correction as to temperature is very nearly effected by this simple adjustment. The WHEEL-BAROMETER is chiefly serviceable as a domestic or land weather-gla.s.s.

Of the many forms of mercurial barometer, that perhaps known as Fortin's is the best. In this instrument the cistern and the lower portion of the tubes is shown in the annexed figure.

[Ill.u.s.tration]

[Ill.u.s.tration]

"The cistern is made of boxwood, with a movable leather bottom _b b_, and a gla.s.s cylinder, _b_, is inserted into it above, all except the gla.s.s being encased in bra.s.s. In the bottom of the bra.s.s box a screw, C, works on the upper end of which the leather rests, so that by elevating or depressing this screw, the bottom of the cistern, and with it the cistern level of the mercury, can also be raised or depressed at pleasure. A small ivory pin, _p_, ending in a point is fixed to the upper frame of the cistern, and when an observation is made, the surface of the mercury is made to coincide with the point of the pin as the standard level from which the barometric column is to be measured. The tube of the barometer, the upper part of which is shown in the lower figure, is enclosed in one of bra.s.s, which has two directly opposite slits in it for showing the height of the column, and on the sides of these the graduation is marked.

A bra.s.s collar, _d, d_, slides upon the tube with a vernier, _v, v_, marked on it for reading the height with the greatest exactness and in which two oblong holes are cut, a little wider than the slits in the bra.s.s tube. When a reading is taken the collar is so placed that the last streak of light is cut off by the two upper edges of the holes or until they form a tangent to the convex mercurial curve. By this means the observer is sure that his eye is on a level with the top of the column and that the reading is taken exactly for this point. Fortin's barometer is generally arranged so as to be portable, in which case the screw, _c_, is sent in until the mercury fills the whole cistern, by which the air is kept from entering the tube during transport, the leather yielding sufficiently at the same time to allow for expansion for increase of temperature. It packs in a case which serves as a tripod when the instrument is mounted for use.

On this tripod it is suspended about the middle, swinging upon two axles at right angles to each other, so that the cistern may act the part of a plummet, in keeping the tube vertical--the position essential to all measurements."[107]

[Footnote 107: Chambers's 'Encyclopaedia.']

_How to Manage a Barometer._--It is of the first importance to have the instrument hung perfectly perpendicular. This is best effected by means of a plummet line. It should be placed in a good light, but protected from direct sunlight and also from rain. If air should accidentally find its way into a common cistern barometer, it may be got rid of by first fixing the ivory piston, so as to prevent the escape of the mercury, then by means of the screw raising the mercurial column nearly to the top of the tube, then by slowly inverting the instrument and tapping the cistern gently, the air may then perhaps ascend to the cistern and thus escape. In transporting a barometer from place to place it is best to carry it by hand; and if packed it is almost needless to say that the float must be firmly fixed and the mercurial column raised by means of the screw, so as to prevent any escape of the metal.

_Reading the Barometer._--The mercury in the cistern must first be brought by means of the screw to the 'zero,' and then the vernier must be screwed up so that its horizontal edge forms a tangent to the mercurial curve. The vernier is an instrument for reading off the graduated scale of the barometer correctly to 1/100th or 1/500th of an inch.

Buchan gives the following description of the vernier and of the method of using it: "It consists (see figures _a_ and _b_) of a piece similar to the scale of the barometer along which it slides. It will be observed from figure _a_ that ten divisions of the vernier are exactly equal to eleven divisions of the scale, that is, to eleven tenths of an inch. Hence each division of the vernier is equal to a tenth of an inch, together with a tenth of a tenth, or a hundredth, or to ten hundredths, and one hundredth, that is, to eleven hundredths of an inch. Similarly two divisions of the vernier are equal to twenty-two hundredths of an inch, which expressed as a decimal fraction is 022 inch, three divisions of the vernier is 033 inch, &c. Suppose the vernier set as previously described--that is, having the zero line of the vernier a tangent to the convex curve of the mercury in the column. If the vernier and scale occupy the relative positions as in figure _a_, then the height of the barometer is 3000 inches, but if they stand as in figure _b_, we set about reading it in this way: (1) The zero of the vernier being between 29 and 30, the reading is more than 29 inches, but less than 30 inches, and we obtain the first figure 29 inches.

(2) Counting the tenths of an inch from 29 upwards we find that the vernier indicates more than seven tenths and less than eight tenths, giving the second figure seven tenths or 07 inch. (3) Casting the eye down the scale to see the point at which a division of the scale and a division of the vernier lie in one and the same straight line, we observe this to take place at line 9 of the vernier; this gives this last figure nine hundredths or 009 inch, and placing all these figures in one line we find that the height of the barometer is 2979 inches. This sort of vernier gives readings true to the hundredth of an inch. If the inch be divided into half tenths or twentieths, and twenty-five divisions of the vernier equal twenty-four divisions of the scale, it follows that the difference of these divisions is two thousandths of an inch."

[Ill.u.s.tration]

A still more divided vernier is always used with the best barometers, and though a little troublesome to read at first, yet if the method of reading the simpler one just described be understood, the difficulty will be easily overcome.

_Uses, &c._ The barometer is employed for ascertaining the amount of atmospherical refraction in astronomical calculations, for measuring alt.i.tudes, and in prognosticating the weather. For the last purpose, on land, it sometimes proves a false prophet; but at sea, its monitions are highly trustworthy. As a mere weather-gla.s.s, the indications, as read off from the scale of the instrument, are generally sufficiently accurate; but in all observations connected with meteorology, alt.i.tudes, astronomy, &c., certain corrections must be made; the height of the mercury being influenced both by the size of the tube and by the temperature of the air by which it is surrounded, as well as by variations in the weight or pressure of the atmosphere. (See _below_.)

_Barometrical Corrections_:--

1. As to CAPILLARITY:--This applies to all cistern-barometers formed of tubes of very small diameters, owing to the mercury a.s.suming a convex surface in the tube. As the tube increases in diameter, so the depression of the mercury lessens. Hence, the "interior diameter" of a barometer "should, in every case, exceed one-fourth of an inch." (Brande.) Syphon barometers that have each of their legs of equal size, require no correction, as the depression is equal at both ends.

TABLE _of Barometrical Corrections for_ CAPILLARITY, _from the_ 'Encycl. Brit.'

--------------------+------------------ Diam. of Tube. Depression.

--------------------+------------------ 10 inch. 1403 inch. + 15 " 0863 "

20 " 0581 "

25 " 0407 "

30 " 0292 "

35 " 0211 "

40 " 0153 "

45 " 0112 "

50 " 0083 "

60 " 0044 "

70 " 0023 "

80 " 0012 "

2. As to TEMPERATURE:--These depend on the expansion of the mercury, and of the scale on which the divisions are marked. The rule for reducing an observed height to the corresponding height at the freezing-point, or 32 Fahr., the usual standard temperature, is--Subtract 110000th part of the observed height of the barometer for every degree of Fahr. above 32 at the time of the observation. Or--

(obs. t. - 32) obs. h. 0001 = corr. req.

_Measurement of Heights by the Barometer._--When a barometer is at the foot of a mountain, the pressure it sustains is greater than that to which it is subjected at the top, by the weight of the column of air intervening between the top and the bottom.

The height can be obtained from the following table by calculating the number of feet which must have been ascended to cause the observed fall; and then making a correction for temperature by multiplying the number obtained from the table, which may be called A, by the following formula: _t_ is the temperature of the lower and _t'_ of the upper station:--

1 ((_t_ + _t'_ - 64) / 900) A.

To lower the barometer from 31 in. to 30 = 857 feet must be ascended.

" " " 30 " 29 = 886 " "

" " " 29 " 28 = 918 " "

" " " 28 " 27 = 951 " "

" " " 27 " 26 = 986 " "

" " " 26 " 25 = 1025 " "

" " " 25 " 24 = 1068 " "

" " " 24 " 23 = 1113 " "

" " " 23 " 22 = 1161 " "

" " " 22 " 21 = 1216 " "

" " " 21 " 20 = 1276 " "

" " " 20 " 19 = 1341 " "

" " " 19 " 18 = 1413 " "

A very complex formula is given by mathematicians for finding very nearly the true height of a mountain from barometrical and thermometrical observations made at its base and summit. The following rule by Mr Ellis will be found to give very nearly the same results:--Multiply the difference of the barometric readings by 52,400, and divide by the sum of the barometric readings. If the result be 1000, 2000, 3000, 4000, or 5000, add 0, 02, 6, 14, respectively. Subtract 2-1/3rd times the difference of the temperature of the mercury. Multiply the remainder by a number obtained by adding 836 to the sum of the temperatures of the air and dividing by 900. A correction must also be given for lat.i.tude, which can be done by the annexed table.

---------+--------++---------+--------- Lat.i.tude. Factor. Lat.i.tude. Factor.

---------+--------++--------+---------- 80 099751 35 100090 75 099770 30 100265 70 099797 25 100170 65 099830 20 100203 60 099868 15 100230 55 099910 10 100249 50 099954 5 100261 45 100000 0 100265 40 100046 ---------+--------++---------+---------

Fortin's and Gay-Lussac's barometers are employed for measuring heights.

The aneroid can be used for alt.i.tudes reaching to 5000 feet. A delicate instrument will register for as small an ascent as 4 feet.

_The Barometer as a Weather-gla.s.s._--Generally speaking when the mercurial column in the barometer falls, 'rain' is indicated, and 'fair weather'

when it rises. When it continues steady, a continuance of the weather at the time is regarded as the forecast; when low, the weather is generally broken or bad; and when high, it is fair and settled. A storm is usually preceded by a sudden fall in the mercurial column, the violence of the storm being in proportion to the suddenness of the fall. An unsteady barometer indicates an unsettled condition of weather, whilst a gradual change in it indicates the approach of some permanent condition of it. The state and direction of the wind has also to be taken into consideration when studying the changes of the barometer, and forms an important element in the calculations of the meteorologist, each different wind indicating variations of weather. The connection between changes of weather and the pressure of the atmosphere does not seem to have been satisfactorily established.

One of the reasons a.s.signed for the mercurial column in the barometer being lower in wet than in fine weather is that so long as aqueous matter remains in the air in the form of elastic vapour, its tension a.s.sists in supporting the barometric column, but that when this aqueous vapour is precipitated in the form of rain, this tension is lost or removed, and the column therefore falls.

The correspondence between wet and fine weather and an elevation and depression of the barometer seems, however, equally, if not more, dependent on the nature of the winds than on the preceding cause. "In western Europe, the south and south-western winds, which are the rain-bringing winds, are warm winds. Now, a column of warm air to be of the same weight as one of cold air must necessarily be higher, but this cannot well be the case in the atmosphere, for no sooner does the warm column rise by its lightness above the surrounding level of the upper surface of the aerial ocean, than it flows over and becomes nearly of the same height as the cold air around it. The interchange taking place less interruptedly, and consequently less slowly, in the higher strata than in those near the ground, it is some time before the equilibrium, thus disturbed, is restored; and meanwhile the barometer keeps low under the pressure of a rarer atmospheric column. On the other hand, the northerly and easterly winds, being comparatively cold and dry, are accompanied by fair weather and a high barometer. It is thus to the warmth, and not to the moisture of these winds, that the pressure is to be ascribed."[108]

[Footnote 108: Chambers's 'Encyclopaedia.']

=Barometer, An'eroid.= An instrument invented, or at least perfected, by M. Vidi, of Paris, in which the pressure of the atmosphere is measured without the employment of a fluid, as in the ordinary barometer.[109]

Externally, it somewhat resembles in appearance a carriage clock or a ship's chronometer; internally, it consists of a small air-tight cylindrical box, formed of thin corrugated copper plates, and partially exhausted of air, the sides of which yield to the pressure of the atmosphere; the effect being regulated by a spring, multiplied by a system of levers, and ultimately recorded by the index on a graduated dial.

Compensation for changes of temperature are self-effected, with almost perfect accuracy, by the elastic force of the spring being so adjusted to that of the air in the cylinder, that the loss of force in the one and the increased expansive force of the other shall, independently of changes of atmospheric pressure, preserve the lever in equilibrio.