Part 59

_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 0·22 inch, three divisions of the vernier is 0·33 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 30·00 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 0·7 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 0·09 inch, and placing all these figures in one line we find that the height of the barometer is 29·79 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."

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 altitudes, 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-glass, the indications, as read off from the scale of the instrument, are generally sufficiently accurate; but in all observations connected with meteorology, altitudes, 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 assuming 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 1·10000th 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, 0·2, 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 latitude, which can be done by the annexed table.

+--------++---------+--------- Latitude.| Factor.||Latitude.| Factor. ---------+--------++--------+---------- 80 | 0·99751|| 35 | 1·00090 75 | 0·99770|| 30 | 1·00265 70 | 0·99797|| 25 | 1·00170 65 | 0·99830|| 20 | 1·00203 60 | 0·99868|| 15 | 1·00230 55 | 0·99910|| 10 | 1·00249 50 | 0·99954|| 5 | 1·00261 45 | 1·00000|| 0 | 1·00265 40 | 1·00046|| | ---------+--------++---------+---------

Fortin's and Gay-Lussac's barometers are employed for measuring heights. The aneroid can be used for altitudes reaching to 5000 feet. A delicate instrument will register for as small an ascent as 4 feet.

_The Barometer as a Weather-glass._--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 assigned 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 assists 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 aërial 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 'Encyclopædia.']

=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.

[Footnote 109: An instrument founded on the same principle, and of nearly similar construction was described by M. Conté, in 1798, in the 'Bull. des Sci. Nat.,' t. i, No. xiii, p. 106.]

The indications of the aneroid barometer closely correspond to those of the mercurial barometer at ordinary ranges; the differences never exceeding ·01 of an inch. It is so extremely sensitive that an ascent or descent of only a few feet is distinctly indicated by it; whilst its portability adapts it for service in situations for which an ordinary barometer is unfitted. On the other hand, it is liable to move by jerks, and the elasticity of the spring, and consequently the zero-point of the scale, has been found to be sometimes affected by time and a rough journey. On this account it is necessary to compare it occasionally with some standard mercurial barometer, to determine its amount or rate of variation, if any.

=Barometer, Phi'al.= This amusing philosophic toy is made by cutting off the rim and part of the neck of a common glass-phial with a file. The phial is then nearly filled with water, either pure or tinged blue or red; and the finger being placed on its mouth, it is inverted, and suspended in a vertical position by means of a piece of twine or wire, when the finger is withdrawn. (See _engr._) In dry weather the under surface of the water remains level with the neck of the bottle, or even concave; in damp weather, on the contrary, a drop appears at the mouth and continues enlarging until it falls, and is then followed by another in the same way.

=Barometer, Fitzroy.= This, which is a very cheap instrument, is made on the syphon principle, but the cistern is formed by the lower limit, which is blown into a bulb.

=Barometer, Port'able.= The most accurate are those of Gay-Lussac and Bunten, and after them the aneroid. They should be set on universal joints, and well balanced. The common instrument made with a box and leather cistern seldom continues long correct.

=Barometer, Wheel.= The common form of the instrument having a dial-face and hands.

[For further information in connection with the above subject the reader is referred to the 'articles' ANEROID, ATMOSPHERE, GAS, HEIGHTS, STEAM, STORM-GLASS, VAPOUR, WEATHER, &c.]

=BAR'OSCOPE=[dagger] (-sk[=o]pe). [Eng., Fr.] _Syn._ BAROSCO'PIUM, L. A barometer; sometimes applied to the wheel-barometer of Hooke.

=BAR'RAS.= The concrete resinous exudation from the bark of fir-trees. That from _pi'nus marit'ima_ is called GALIPOT.

=BARSE.= [Provincial.] The common perch.

=BAR'WOOD.= A red dye-wood imported from Angola and other parts of Africa. It closely resembles cam-wood and sanders-wood in its colouring matter being of a resinous nature, and scarcely soluble in water. In _dyeing_ this difficulty is obviated by taking advantage of the strong affinity existing between it and the proto-salts of tin and iron. Thus, by strongly impregnating the goods with protochloride of tin, either with or without the addition of sumach, according to the shade of red desired, and then putting them into a boiling bath containing the rasped wood, the colour is rapidly given out and taken up, until the whole of the tin in the fibres of the cloth is saturated, and the goods become of a rich bright hue. In like manner the dark red of bandana handkerchiefs is commonly given by a mordant of acetate of iron followed by a boiling bath of this dye-stuff. See DYEING, MORDANTS, &c.

=BASALT'= (b[)a]-s[)o]lt'). [Eng., Ger.] _Syn._ BASAL'TES (-s[)a]l'-t[=e]z), L.; BASALTE, Fr. In _geology_, &c., a species of trap-rock, essentially composed of the minerals felspar and augite. It is of a fine compact texture, of a dark-green, grey, or black colour, and usually occurs in regular columns, of which the Giants' Causeway and the Island of Staffa furnish magnificent examples. It is fusible; and when rapidly cooled forms a dark brittle glass; but when slowly cooled retains its original beauty and hardness almost unimpaired. Messrs Chance, Brothers, of Birmingham, have availed themselves of this property to apply it to decorative and ornamental purposes. Their process is to melt the material[110] in a reverberatory furnace, and, when sufficiently fluid, to pour it into red-hot moulds of sand encased in iron boxes. The corresponding adj. is BASALT'IC (-s[=o]lt'-; BASAL'TICUS, -s[)a]l'-, L.; BASALTIQUE, Fr.).

[Footnote 110: Rowley-rag is used by the Messrs C.; as beside ordinary basalt, greenstone, whinstone, and other similar minerals, possess the same property.]

=BASE.= [Eng., Fr.] _Syn._ BA'SIS (pl., ba'ses), L., Gr.; GRUND, GRUNDFLÄCHE, Ger. In _chemistry_ it was formerly, and is now occasionally, applied to metallic oxides which possess the property of forming salts with acids. The alkaloids are also designated organic bases. In _pharmacy_, the characteristic or principal ingredient in any medicine or compound preparation; or that on which its qualities or efficacy depends.

=BAS'IL= (-b[)a]z'-). _Syn._ SWEET BAS'IL, CIT'RON B.; BASIL'ICUM, L.; BASILIC, Fr.; BASILIKUM, Ger. The _oc'ymum_ ([)o]s'-) _basil'icum_ (Linn.), an annual aromatic herbaceous plant, of the nat. ord. Labiatæ (DC.). It is a native of India, but is largely cultivated in every part of Europe as a pot-herb. Leaves strong-scented; popularly reputed emmenagogue; much used to flavour salads, soups, &c., especially in French cookery. Mock-turtle soup derives its peculiar flavour from this herb; as also did the original Fetter-lane sausages, once so highly esteemed by cockney gourmands. In India it is commonly employed as an anodyne in childbirth.

=Bas'il= (b[)a]z'-). _Syn._ BAS'AN; BASANE, Fr. A sheep-skin, tanned; particularly one dressed on the grain side, for book-binding.

=BASIL'ICON.= See CERATES and OINTMENTS.

=BAS'KET= (b[)a]s'-). _Syn._ COPH'INUS (k[)o]f'-), L.; PANIER, CORBEILLE, &c., Fr.; KORB, Ger. BASKETS are generally STAINED or COLOURED with the simple liquid dyes used for straw or wood; and that, for variegated work, the twigs, after being carefully peeled, washed, and wiped dry or slightly air-dried, are stained before being woven. See OSIERS, &c.

=BASS[double-dagger].= [Provin.] The linden-tree; also a hassock or mat made of its inner bark. See BAST.

=Bassia butyracea.= A tree growing in the sub-tropical Himalayas. The seeds yield by expression a concrete oil, known by the name of _Fulwa Butter_, which does not become rancid by keeping. It is held in high esteem in India as an external application in rheumatic and other painful maladies.

=BAS'SORIN= (-r[)i]n). _Syn._ BASSORI'NA, L. A substance first noticed, by Vauquelin, in _Bas'sora-gum_. See GUM, INSOLUBLE, TRAGACANTHINE, &c.

=BAST= (b[)a]st). _Syn._ BASS (which _see_). The inner bark of the linden tree or tiel tree; also matting, &c., made of it.

=BAS'TARDS= (-t[)a]rdz). _Syn._ BAS'TARD SUG'AR (sh[)o][)o]g'-), PIECES, &c. In _sugar-refining_, impure or damaged sugar resulting from the heat and chemicals used in the process of manufacture, and which will not pay for purifying.

=BA''SYL= (b[=a]se'[)i]l). In _chemistry_, any simple or compound body, acting as a basic radical.

=BATATA= [_Convolvulus batatas_, or SWEET POTATO]. This is a native of the East Indies, but is now cultivated in all tropical and sub-tropical countries for the sake of its tubers, which are highly esteemed as an article of food. They are eaten either roasted or boiled, and are sweet, wholesome, and nutritious, although somewhat laxative.

In some parts of America the Batata, next to maize, forms the principal diet of the poorer classes. The plant was introduced into England by Sir Francis Drake and Sir John Hawkins; but they do not bear the cold of our winters, and if grown here are raised in hot-houses, where they may be obtained without difficulty varying from 1 lb. to 2 lbs. in weight. They thrive better in the south of Europe. The tubers contain about 32 per cent. of solid matter, 16 of which is starch, 10 sugar, 1·5 albumen, 1·1 gum, 0·3 fat, 2·9 mineral matter. The leaves are used as a boiled vegetable.

=BATH= (bahth). _Syn._ BAL'NEUM, L.; BAIN, Fr.; BAD, Ger., Sax. A place for bathing; a vessel or receptacle, natural or artificial, containing or adapted to contain water, and used to bathe in. In _architecture_ and _hygiene_, a building fitted up for and appropriated to bathing.

_Constr., &c._ Here one of the first subjects which must engage our attention is the selection of the material of which the bath is to be formed. For FIXED BATHS polished white marble has always been in favour, owing to its cleanliness and beauty. For this purpose, slabs of sufficient thickness and free from flaws or cracks should be chosen; and they should be securely and properly bedded in good water-tight cement, in a well-seasoned wooden case. The objections to marble, independent of its costliness, are, that it is apt to get yellow or discoloured, and to lose its polish, by frequent and careless use; and that the restoration of its surface to its original purity, is a matter of considerable expense and difficulty. It is also only fitted to contain water with, at the most, soap, weak alkalies or alkaline carbonates, aromatics, or neutral organic principles; and cannot be employed with water medicated, however slightly, with acids, sulphurets, iodine, chlorine, salines (others than those just named), or calorific substances. As a cheaper material thick slabs of Welsh slate are often substituted for marble; but even this substance is attacked by chemicals, though much more slowly. A lining of large Dutch tiles is sometimes used: but here the joints are very apt to leak. For baths adapted to all the requirements of health and disease, and which are at the same time durable and comparatively inexpensive, we must, therefore, seek further. Porcelain, glass, and hard glazed stone-ware have been proposed, and are even sometimes used for baths; but they possess the disadvantages of being fragile, and very liable to crack when filled with hot water in cold weather. Wedgewood-ware is very beautiful and durable; but is expensive, and baths formed of it can only be obtained on special order. Stourbridge-ware, as produced of late years, is the only product of the potter's art that appears entirely to meet the case; but even this yields in durability to enamelled iron as a material for baths adapted to all liquids and temperatures, and to rough or careless usage. (See _engr._ 1.) The better qualities of PORT'ABLE BATHS (see _engr._ 2) are generally made of copper. Stout tinned or galvanised iron, and even stout block-tin thickly covered with waterproof paint or japan, are also employed; but though less expensive than copper, they have the disadvantage of being much less durable. All these substances are, however, readily acted on by chemicals. A durable and cheap portable bath, adapted to all purposes, must, therefore, like a fixed one, be made of one or other of the materials already noticed. For MED'ICATED BATHS large wooden troughs are frequently employed, particularly for acidulated, ioduretted, and sulphuretted baths.