barometers and weather-glasses defined in 1909 year
barometers and weather-glasses - barometers and weather-glasses; barometers and weather-glasses - A barometer is an instrument by which the weight (sometimes termed pressure) of the atmosphere is measured or indicated, and as the air we breathe has a constantly varying weight, and the weight varies to a marked degree with variations in the weather, the instrument has gained its name of weather-glass. The instrument is very simple in parts, being merely a glass tube filled with mercury, the column of mercury being held up by atmospheric pressure, this pressure being equal to, and capable of holding up, a column of water about 32 ft. high; or a column of mercury 30 in. high, these measurements varying with the altitude and with the state of the atmosphere.
An ordinary form of weather-glass has working parts more or less resembling Fig., these consisting of a tube turned up at its lower end, this lower end being open with a float in it, the upper end being closed. The float has connected a cord which, passing over a wheel, is balanced by a small weight. The tube is filled with mercury, and, as only one end of the tube is open, the mercury cannot run out owing to the pressure of the atmosphere upon this open end, yet the level of the mercury must vary with every variation in the density or weight (pressure) of the air as it occurs. With the balanced float, operating a wheel as shown, each variation is indicated by the visible pointer on the front of the dial.
To make a cheap barometer. Obtain a straight fine glass tube, about 33 in. long, and with a clean interior, sealed at one end, and having an even uniform bore of about 2½ lines diameter. The mercury to be used should be perfectly pure, and free from all air and moisture. This latter requisite may be assured by heating the mercury in a porcelain dish to nearly the boiling point, previous to using it. The tube is then held securely, with the open end uppermost, and carefully filled with the liquid metal. The imprisoned air is removed by shaking. The open end of the tube is then securely covered with the finger, the tube is inverted, and the end covered by a finger is plunged below the surface of a little mercury placed in a small vessel to receive it. The finger is then removed, when the mercury in the tube will immediately fall to a level of about 30 in. above the surface of that in the small reservoir below. The tube is again closed by pressing the fingers on to the open end, brought to a horizontal position and gently shaken for a short time. After a portion of the air embedded in the mercury has entered the vacuum, the tube is transferred to the trough, manipulated as before, and the operation is repeated two or three times until the mercury is freed from the adhering air. A simple method of ascertaining the quantity of mercury required is to fill the tube with water from a weighed measure, weigh the measure again, when the loss of weight multiplied by 13.5 will give the weight of mercury required. In order to attach the scale correctly, it will be necessary to compare the indications with those of some good instrument.
Bogen's Method. - When the section of tubing is reduced to that of a capillary tube, the filling with mercury by the foregoing method is impracticable; this difficulty is overcome by the following modification.
Having filled the barometer tube with mercury, clamp it to a stand in the position shown in Fig., and connect it by a 3-inch piece of rubber tubing (c) to a glass tube (a) of the same diameter and length, but open at both ends. Hold the open tube as shown in Fig., and fill it with mercury through a small funnel. Lightly tap the rubber tubing to free any air bubbles that may have collected there, then, while the open end of the tube (a) is partially covered by the finger, press the rubber until a small portion of the mercury has been forced out; complete the closing of the tube with the finger, and immerse the closed end in the dish of mercury (e).
On withdrawing the finger from the orifice the tubes will be in the position shown in Fig., and both quite filled with mercury.
The vacuum is formed as follows: Release the tube (b) from the clamp and raise it with one hand to a nearly vertical position, meanwhile holding the tube (a) firmly with the other hand. A vacuum (g, f) will then be formed in the tube (b) into which any air embedded in the mercury will be released; then gently lower tube (b) while tube (a) is raised, until they are in the positions shown in Fig., when the air will be in the position "hi." The operation is then reversed, and repeated two or three times, until all the air is withdrawn from the mercury, which will be shown by its giving a characteristic metallic sound, when the barometer tube is gently shaken endways.
The method is applicable to siphon and cistern barometers, and can also be used for the exhaustion of capillary tubing; the employment of an air pump and application of heat in exhausting the latter will greatly accelerate the operation. The manipulations which the method requires are simple, though great caution must be exercised in handling the glass tubes, when filled with mercury, to avoid breakage of the glass; they can be performed with good results by persons unaccustomed to experimental work, always giving an excellent vacuum.
Filling Tubes. - Application of Wright's apparatus for distilling, to the filling of barometer tubes. In Fig., a is a vessel full of impure mercury; 6 a tube about 30 in. long; c an enlargement of b; d and e tubes inclined in opposite directions; f an arm for connecting with a Sprengel pump; g a tube a little over 30 in. long; h a reservoir with an outlet to the air; h is filled with pure mercury.
The air is now exhausted through f; the mercury rises in 6 and g until c is partially filled; a Bunsen burner is placed under c, and the mercury distils over into g, and flows out through h.
If now g is cut off at x a few inches below the junction of e, with the arm f (the part h being no longer used), and a glass cock be inserted at x, then by means of a short rubber tube this cock can be connected with the open end of the barometer tube to be filled, which latter will take the general position of the whole tube g.
The rubber tube must be covered with melted sealing wax. The impure mercury in a should first be washed in acids and dried before introduction. At the beginning of operations, a is full of impure mercury, but the rest of the apparatus contains only air. The Sprengel pump is set in motion and gradually exhausts the air from 6, c, d, e, and the barometer tube, until no air bubbles can be seen in the running mercury of the Sprengel pump, and until the sharp click is heard when the drops of mercury fall. The tube f is then sealed or a stop cock in it is turned, cutting off the Sprengel pump; the Bunsen burner under c is lighted, and the mercury will distil over into the barometer tube, which will thus be filled without allowing the mercury to come into direct contact with the air.
The barometer tube should be constantly watched in order to detect any air bubbles that may be carried over; when seen they must be cooked out by heating the tube slightly by means of a Bunsen burner. When the barometer tube has become filled with the mercury, the cock at x can be closed, the sealing wax is broken and the tube is replaced by another. (F. Walde.)
See (4) and (5) as to whether the mercury be suitable. If the barometer has a Fitzroy pattern tube with its lower end turned up thus J, first fill this end with mercury then press your thumb or hand over the opening, tilt the tube over and coax the mercury round the bend into the main tube. Repeat this until the tube is quite full, then carefully invert the tube to its proper upright position. When this is done the mercury will be seen to descend for a little way at top, then stop at its right position. In doing this some mercury will overflow at the open end of the J at bottom, and, in anticipation of this, it should be held over a plate or pan. If the tube is a straight one, having its lower end open and dipping in a small cup or reservoir of mercury, then the filling can usually be done through a fine metal or paper funnel, the tube being inverted, i. e. with its open end upwards, while this is done. When it is full, a finger is pressed over the open end and kept there while the tube is inverted again and its open end put beneath the mercury in the cup or reservoir. In this case also the tube must be quite filled with mercury, and, when the finger is removed from the open end (in the cup of mercury) the mercury in the tube will fall a little way as already explained. When a tube, of either kind, is filled with mercury, and before it is turned its proper way up, it is a good plan to heat its lower closed end, as by doing this air is expelled from the mercury, but great care must be used, as the heating in unskilled hands is so liable to result in fracture, a broken tube, and possibly loss of mercury. Letting the filled tube, with closed end downwards, stand in a warm position (over a stove when the fire is just out at night) for a few hours is almost as good as heating the mercury.
Cleaning- Barometer Tubes. To clean the tube of a film, etc., get a piece of covered electric bell wire and fix on the end of it a piece of wash-leather. It must be very fine wash-leather, cut into narrow strips; wrap the wire from end to end, leaving a thicker piece at the end to tightly fit the tube.
Clean with warm water, soda, and soap-powder, afterwards with cold water, using the covered wire all the time, of course replacing the wet with dry leather to finish. If the wire is not covered, the tube will most assuredly break, if not at the time, certainly within 48 hours after using the wire. Clean the mercury with nitric acid and water, say, for 4-5 lb. of mercury, 4 teaspoonfuls of acid and 20 teaspoonfuls of water; put the whole into a soup plate, and put it in the oven or before the fire, and heat up to about 140°-150° F., stirring it at intervals until the acid forms a sort of powder or refuse on the top of the mercury. When cold, run the mercury through a fine paper cone a few times, and then it is fit for use.
Siphon Barometer. - A few words must first be said regarding the selection of the glass tube, as on its fitness for the purpose the instrument's future excellence will very much depend. Ordinary white, easily fusible glass tube should not be used, as the mercury is apt to attract its oxide of, lead, and not only become impure, but by adhesion to the inside of the bore hinder the free oscillation of the barometric column. The proper kind of tubing is that which shows a greenish tinge in the glass when looked at endways. For either of the instruments shown in Fig., it should not be less than 3/8 in. outside diameter and 1/4 in. bore; and if slightly larger may still be used with advantage.
For the siphon barometer, Fig., a piece of tube about 38 in. long is required. This is to be well cleaned by running through it plenty of warm soft water, while at the same time a little swab made from a piece of soft, fine linen, tied in the middle of a cord, is pulled through the bore from end to end. After the water has drained out, alcohol in which precipitated chalk is suspended, should be applied to the inside by means of the swab. A clean swab moistened with alcohol will remove the particles of chalk, when, the cord being withdrawn, fresh alcohol is to be poured through, after which the tube must stand in an upright position till it has drained perfectly dry, a little cap of paper meantime, being placed on its upper end to exclude dust. The inner surface of the tube must finally be polished with a small piece of soft washleather fixed on the end of a clean, smooth, covered wire.
The tube thus cleaned and dried is now to be closed at one end by drawing it apart in a gas flame about 2 in. from the extremity. The narrow pointed end, which forms when the tube is drawn asunder should be pressed and rotated in the flame till a substantial and well-rounded closing has been obtained. About 32 in. from the sealed extremity a U-shaped bend is to be made. Care must be taken to make the curve a gradual one, as failure in this respect would not only mar the appearance of the instrument, but might also tend to narrow the bore and make the bend a weak point. The arc of the curve is to be 1¼ in. The longer limb of the siphon is thus 32 in. long and a shorter one about 3 in. The short limb is not to be bent down quite parallel with the longer one, but should make a slight angle with it to render the subsequent introduction of the mercury more easy. The tube a is to have adapted to it a supporting stand b, which may be a piece of dressed walnut, 34 in. long, 3½ in. wide, and about 3/8 in. thick, rounded off at the top, and furnished with a brass screw and ring for hanging up. A shallow groove, curved to correspond with the bent tube, is made on the wood. The three small brass clasps c, provided for attaching the tube to its support, may be readily cut from sheet-brass, polished, bent to shape, and drilled with a hole in each end to receive the appropriate small brass screws. The sliding scale support d is a slip of cherry or mahogany, 1 in. wide, 3/16 in. thick, and 28 in. long, having two longitudinal cuts e, made therein, through which pass two screws which fasten it to the walnut scale and allow of its motion upward and downwards These screws may be of brass with milled heads, or a cheap and excellent substitute may be found in the brass buttons with screw-stems sold for fastening carriage aprons. These are to 'have their stems passed through the longitudinal cuts e, and screwed into appropriate holes in the walnut support till their projecting shoulders bind on the scale support and prevent it from moving, except when required. The bottom of the sliding scale support is a piece of sheet-brass cut square and attached by two small rivets or screws. Its angle or corner is used as an index, as will afterward be explained. A scale k made of a piece of ivory veneer, 4 in. long, and about 1¼ in. wide, is required for the upper end of the sliding support. This must be carefully and accurately divided into inches and tenths, the lowest inch mark being numbered "29," the next "30," and the upper one "31." It will be well to have the figures and lines done by an engraver; but, if economy be a consideration, the markings can be very well ruled with a fine pen, and after the ink has dried a coat of thin dammar varnish will protect the lines from injury by moisture. The ivory scale is now to be fixed to the sliding support with the upper end of which its top exactly corresponds. If the measures have been correctly made, its 30-in. mark will now be situated exactly 30 in. from the bottom of the brass index. An excellent cement for attaching the ivory to the wood is made of a little isinglass dissolved by heat in equal parts of alcohol and water. The walnut support 6 should receive two or three coats of copal varnish. The cherry wood slide d may either be finished with boiled linseed oil or varnish, according to fancy.
All parts of the instrument being thus fitted, it only remains to introduce the mercury. For this purpose the tube a being detached from the support, is placed upon a level table and sustained by small pieces of wire, so that the short limb is uppermost, the long limb lying flat upon the table. The mercury used should be as pure as possible: though if freshly distilled mercury cannot be had that of commerce may be used, provided it has not become contaminated by lead or kindred metals. A fair test of the goodness of mercury is made by dropping a little into a clean white plate and causing it to run about. If bright round globules are formed, which readily coalesce and leave no trails of discoloration on the china, the metal is sufficiently pure. If, however, the drops become pear-shaped and soil the plate with dull, metallic splotches, the metal must be rejected. Before being used for filling, the mercury should in any case be forced through small pinholes in a piece of thin chamois skin to remove mechanical impurities. The tube being filled, is next raised gently into a vertical position, with its closed end uppermost. The mercury will descend a few inches, showing the Torricellian vacuum in the upper part of the longer limb, while at the same time it rises and overflows from the open orifice of the short limb. From the latter, enough of it should be displaced, by inserting a small round piece of wood into the bore, to leave a couple of inches empty. After this it only remains to finish the instrument by attaching the tube a to its support with the brass clasps o and screws. A narrow strip of green surface paper, 4-5 in. long, slipped behind the upper part of the tube where the vacuum appears, is an improvement to the look of the instrument and an assistance when taking the readings. It will now be evident at a glance that by bringing the lower brass corner of the index level with the surface of the mercury in the short limb, as often as an observation is to be made, the height of the mercurial column in inches and decimals will at once be shown on the ivory scale.
A small thermometer l fixed beside the sliding scale is at once a useful and ornamental addition to the barometer, A small cap m of metal or wood must be loosely fitted over the open end of the tube to exclude dust. (A. F. Miller.)
Cistern Barometer. - The tube must be cleaned as already described, and closed at one end; but instead of being bent it is left straight, and cut off at a length of 32 in. Fig. shows a section of the cistern, which is simply a small wooden cup turned neatly out of hard wood; its outside dimensions being 1½ in. diameter and 2¼ in. high, and the inside cavity being 1 1/8 in. diameter and 2 in. deep. A cut made with a fine saw along the line a separates the underneath part of the cistern as a small wooden ring, to the bottom of which must be glued a piece of stout wash - leather 5, made loosely convex so as to bulge readily inward and outward, forming the cistern-bottom and supplying a movable surface on which the atmospheric pressure is to act. A hole c in the closed top admits the pipe d, which passes down into the cistern till its end is level with the line of division a, and is secured in place by being cemented where it goes through the wood of the top. A small hole e for adjusting the height of the mercury is made ½ in. below the closed top of the cistern, and stopped for the time with a little wooden plug.
The filling with pure mercury is to be done as already described in the case of the siphon, except that the tube may now be placed in a nearly vertical position with its closed end downward; a small straight funnel is to be used for pouring through. The cistern has also to receive as much mercury as will fill it to the edge a, after which the ring-shaped piece, bearing the wash-leather bottom b is coated with glue on its sawn surface and pressed on in place, so closing the cistern. As soon as the glued joint is firm, the tube may be turned up into proper position by placing the finger on the washleather bottom, and pressing it inward till the orifice of the tube is felt, when the whole is quickly inverted. Thus no air enters the tube during the moment of turning over; and as an instant later its opening is covered by the mercury of the cistern, the vacuum is now secured. Care should be taken however, never again to turn the cistern bottom upward. The tube being now in a vertical position, the level of the mercury is adjusted by removing the plug from the hole e, when the superfluous metal escapes and the column in the tube descends, leaving the vacuum above. The plug is then to be reinserted and glued in place.
The stand (which it is well to make and fit to the tube before the latter is filled) is shown in Fig. It may be of walnut, mahogany, or cherry, and its general style and finish must depend on the fancy of the maker. A shallow groove down the centre receives the tube f, and an oblong cavity at the bottom admits the back of the cistern, while its front may be covered with a hollow ornamental turning g as represented. The scale h, which in this case should be 5 in. long, may be ruled on ivory as already suggested, though an instrument of this description is really deserving of a well-made engraved scale, with a vernier giving readings to the hundredth part of an inch. Such a vernier i is a narrow piece of ivory 1 1/5 in. long, provided with a groove to receive the inner edge of the ivory scale along which it slides next to the tube, a hollow being cut in the wood of the stand behind the scale to admit of its motion. It is divided into 11 equal parts by 10 horizontal lines numbered downward from 1 to 10, each of the divisions measuring therefore 1/10 â€“ 1/100 in. The 30-in. line of scale is to be placed exactly 30 in. above the centre of the hole c, which marks the level of the mercury in the cistern. It is best to affix the scale to the stand by little brass screws. A small thermometer k, opposite the barometer scale, adds to the elegance and efficiency of the instrument. A slip of green surface paper should be pasted in the groove behind the tube before the latter is fixed in place. The top of the tube f should be covered by a small turned button l of bone or wood. (A. F. Miller.)
Expelling- Air Bubbles from Barometer Tubes. - First remove the tube from its board and pour out as much of the quicksilver as possible. Then invert the tube (with closed end downwards) and stand it in a, warm place for an hour or more, to get warm through. Then, holding the tube firmly near both ends, make the closed end tap on a table lightly and repeatedly, and the bubbles will work up and escape. One or two thicknesses of wash leather should be placed on the table for the tube to strike on, and, if necessary, the finger should be held over the upper open end to prevent any mercury flying out. To replace the mercury follow the directions already given, and if the mercury appears dirty filter through thin or pricked wash leather.
Glycerine Barometer. - The marked influences of the variations in the pressure of the atmosphere upon the disengagement of carburetted gases in coal mines has led engineers to devise a new barometer that will not only indicate the most minute variation of atmospheric pressure, but will indicate it so plainly that miners and others not experienced in making barometric observations can readily detect the variations.
(a) Jordan spent several years in studying the different liquids that might possibly be applicable in constructing an accurate and highly sensitive barometer, and finally found that glycerine produced the best results. The glycerine is very pure, and has a specific gravity of 1.26, and on account of its high point of ebullition the vapours have no perceptible tension at ordinary temperatures, and it will only congeal at a very low temperature. The height of a column of glycerine is 26 ft. 9 in., and a variation of 1/16 in. of mercury corresponds j to a variation of about 1 in. in the column of glycerine. As glycerine is very apt to absorb the moisture of the air, it is covered with a thin layer of prepared thickened petroleum in the cistern of the barometer. Jordan constructed barometers for the South Kensington and Jermyn Street Museums in London, and the former is still in use (1907); both gave perfect satisfaction.
The instrument, shown in Fig., was erected by him at Kew Observatory, and consists of a cylindrical cistern of tinned copper, about 6 in. high and 10 in. diameter, provided with a screw cover or cap having a small opening leading into a recess containing cotton to act as filter and keep out the dust. The large barometric tube is made of ordinary gas pipe, about ¾ in. diameter, and is rigidly attached to the cylindrical cistern or cup. The upper end of this tube fits into a piece of bronze, into which a glass tube ¾ in. diameter and about 4 ft. high is securely cemented. This tube terminates in a cup inclosing a rubber packing. Graduated scales provided with indicators are placed at each side of the glass tube, the one on the left side indicating the inches and tenths of inches, the right-hand scale shows the equivalent measure of a corresponding column of mercury. The scales are attached to an open plank which is fastened to the wall of one of the upper stories of the observatory, and the large tube passes down to a room situated 26 ft. 9 in. lower. The glycerine in the barometer is coloured with aniline red. Before putting the glycerine in the tube, it is boiled at a temperature of about 180° to expel the air and to make it purer. The air is exhausted from the barometer tube by means of an air pump.
(6) How to make a Glycerine Barometer. - A bottle about a quarter filled with glycerine, coloured red with magenta or crimson aniline, has a glass tube of about the diameter of a pencil passing airtight through the cork which is inserted airtight into the bottle. The lower end of the tube dips beneath the surface of the glycerine. The bottle is made to contain compressed air by blowing into the upper end of the tube. On removing the mouth, part of the glycerine will rise in the tube until the weight of the liquid column in the tube and the atmosphere balance the internal air pressure on the surface of the glycerine. The column in the tube will tend to rise when the pressure of the atmosphere diminishes, or the temperature of the compressed air' rises, and to fall when the atmospheric pressure increases or the temperature of the compressed air diminishes. So far as the variation in the height of the column is due to changes in atmospheric pressure, the column moves in the opposite direction from that in a mercurial barometer.
It will now be seen that it is desirable to eliminate from the reading of the barometer scale the effect due to a change in temperature. Simultaneously observe the reading of the barometer and a thermometer at hand. Next find the difference between the readings, calling that of the thermometer the minuend. The difference is regarded as the relative pressure of the atmosphere at the time of observation. The divisions on the instrument are ¼ in. apart, and the length of the tube above the bottle is 25 in. It seems better to have 100 divisions than any other number. These divisions bear no relation to those on mercurial and aneroid barometers. Each instrument is intended to be compared with itself to indicate a relative pressure of the atmosphere. In the instrument the degrees are marked and numbered with a pen on a strip of paper obtained from a ribbon roll; this is pasted upon a neat wooden case behind the tube. The case has a recess into which the bottle is let. A piece of wood, of the proper shape, secures the bottle, while leaving it almost entirely in view. Two small wire staples secure the tube to the scale. If desirable, a paper scale may be pasted upon the tube, thus dispensing with a case.
Of course, it is liable to be broken when thus constructed. The use of a thermometer is scarcely necessary if the barometer is kept in a cellar or any place where the temperature is nearly uniform.
With a tube 3-4 ft. long, the bottle may be buried in a large box of dry sawdust, or any other poor conductor of heat, in a finely divided state. The instrument will then give fair results without using either a thermometer or a cellar.
The advantage of using glycerine instead of water, is that glycerine scarcely evaporates; it is, however, highly hygroscopic and needs to be protected from the air by some method as in (a); it will not freeze except at a very low temperature, and if a minute quantity of water be present, it never becomes solid.
The upper end of the tube should be loosely filled with cotton, to keep out the dust. After having forced air into the instrument, it should not be allowed to approach a horizontal position, for the compressed air may blow the column out of the tube; if this does not happen, a large air bubble may separate the column, and render the instrument useless. No particular dimensions are requisite for either the bottle or the tube. The magenta is used merely to render the column more readily visible. Other colours may be used, but this is the most beautiful. (J. Asher.)
Portable Mercurial Barometer. - The chief faults in the ordinary barometer are imperfect vacuum and a degree of capillarity in the mercury which makes it difficult to read the true level. With portable barometers it is rare to get accurate results. Kralvitch overcomes the obstacles by the instrument shown in Fig. The two chambers a b, as well as the tubes joining them, are filled with pure dry mercury. The chamber a communicates with the chamber d by a capillary tube. On tipping the barometer, the mercury in b flows into a, displacing the air, which escapes by the tube o into chamber d, and cannot return to the barometric chamber a. The reservoir h assists in filling the tube, which is done cold. On reversing the tube, the air collects and escapes at the hole o. At m is a rubber tube uniting the two portions of the instrument and capable of being closed by a pinch-cock. By repeatedly reversing the tube, all air is at last forced out of the barometric chamber a. The instrument is rendered portable by reversing it, and putting the pinch-cock on the rubber tube.
How to Read the Barometer. - The barometer is only an extremely sensitive balance, or a manometer, showing the variations of atmospheric pressure. The early makers of one form of the instrument had the unfortunate idea of marking certain points on the dial with the words "fair," "rain," "storm," etc.; their example has been followed blindly and hence the bad reputation of the barometer. The passage of dry winds over our heads naturally causes the barometer to rise, while damp winds have the reverse effect; but it must not be forgotten that rainy winds in Europe come from the south-west, and are ascendant in latitude - they raise the air, and in the same degree lighten the barometer; on the contrary, dry winds come from the north and east, are cold, and descendent in latitude - they drive the air towards the surface of the earth, and cause the barometer to fall. The barometer shows very well the great atmospheric perturbations - the only condition being that we should learn how to use it. The diurnal course of the sun above the horizon exercises its influence on the barometer, it heats the atmosphere, causing ascending currents of air, which create a fall in the level of the mercury in the afternoon, and a return towards the former level in the evening. It is evident that the barometer may vary from three distinct causes; by change of altitude, under the influence of dry and moist winds, and under the action of the solar rays dependent on the hour of the day. These premises being stated, it is not astonishing that two excellent instruments, one placed, for instance, at the lower, and the other at the upper part of a house, should never agree. Proprietors of certain instruments declare that theirs are the only barometers to be trusted; old friends will dispute about them. With the present mode of graduation, it is rare to find two barometers in the same house marking even the same division of the dial; the instrument which marks " variable " on the ground-floor will incline to "rain" at the fifth storey, for in a house 60 ft. high the difference in the height of the column of mercury is about 2 mm. Take a small aneroid wheel-barometer in your hand, and walk up or down a street with a sharp ascent, and you will find the needle deflect towards "fine " as you descend, and fall as you rise - every 30 ft. representing about 1 mm. in the barometric variation.
French barometers are generally graduated for Paris, and cannot possibly be correct in places of different altitudes. The position of the index is altered. The barometer is affected much by latitude, and a little by longitude; the oscillation is altered, and no change in the index will correct the error.
Suppress the deceptive indications on the dial, and the barometer may be consulted anywhere with profit. When the mercury is rising or falling, the indication of the same foretells faithfully the probable weather to be expected. The only exception occurs when two opposing currents are struggling against each other. In such a case the barometer will be scarcely affected, yet the rain may fall suddenly. Generally, rapid variations of the instrument indicate change; when the fall is rapid, rain may be expected; when very rapid, storms. The importance of the atmospheric perturbation is in proportion to the rapidity of the fall of the mercury, but the duration of bad weather is in general long in proportion as the fall has been gradual and continuous. If the mercury I mounts very rapidly, the weather is not completely changed; it mounts j more rapidly than it falls, but still there are differences to be observed. In testing the condition of the mercury by tapping gently with the finger, it is not safe to accept the rising of the index as a sign of fine weather; it must be remembered that the barometer, unless acted upon by a tolerably energetic current, has a marked tendency to rise between 5 o'clock in the afternoon and midnight, to fall between midnight and 5 o'clock in the morning, and to rise again between 5 a.m. and mid-day.
Baroscope. - Take any bottle; pour coloured water into it, about ¼ of the quantity the bottle will hold; insert in it a glass tube, from 3 to 4 ft. long, and passing air-tight through the stopper, which must also be air-tight. Let a paper index, divided according to any scale of division, say into inches and fractions of an inch, be glued to the glass tube. Blow into the glass tube so as to cause the water to ascend the tube a few inches, say 10 in., and the j instrument is constructed. The bottle must be placed in another vessel, and protected by sawdust, or some other material, from the influence of changes in the temperature of the atmosphere. This very sensitive instrument records faithfully any change in the density of the external air, and the approach of a storm will infallibly be indicated by a sudden rise of the water in the glass tube.
The Aneroid Barometer is so named from its indications being obtained without the use of mercury or any other fluid. Its action depends on atmospheric pressure on a metallic box, which has been hermetically sealed after exhaustion of air. An index, traversing a dial, records the changes in the weight or pressure of the air on a given surface. A flat circular box, about J in. in depth, is made of some white metal, the upper and under surfaces of which are corrugated in concentric circles to give it greater elasticity. This box being exhausted of air through a short tube, and then made air-tight by soldering, forms a spring which is affected by every variation of pressure in the external atmosphere. It is attached to the bottom of a metallic case, which encloses the mechanism of the instrument. At the centre of the upper surface of the elastic box is a solid projection, about half an inch high, to the top of which the principal lever is attached. This lever rests partly on a spiral spring, and is also supported by two vertical pins, with perfect freedom of motion. The end of the principal lever is attached to a second or small lever, from which a chain extends to the centre, where it works on a drum attached to the arbour of the hand. A hair spring, the attachments of which are made to the metallic plate, regulates the motion of the hand.
As the weight or pressure of the atmosphere is increased or diminished, the surface of the elastic box is depressed or elevated, and this motion is communicated through the levers to the arbour of the hand. The spiral spring on which the lever rests is intended to compensate for the effects of alterations of temperature on the minute portion of air which the box must contain, however perfect the exhaustion. The actual movement at the centre of the elastic box, from which the indications emanate, is very slight, but this is increased 657 times at the point of the hand; so that a movement to the extent of one-220th part of an inch in the box, carries the point of the hand through 3 in. on the dial. The tension of the box in its construction is equal to 44 lb. At the back of the outer case is a screw, to adjust the hand to the height of a standard mercurial barometer.