Barometer (Gr. weight, and a measure), an instrument used for determining the pressure of the atmosphere. . The doctrine of a plenum in natural philosophy, and the abhorrence of nature for a vacuum, had long been too fully established in the old systems to admit the possibility of a vacuum, when Galileo, toward the close of his life, was requested to explain why water could not be raised in a suction pump more than about 32 feet. He was led to admit that nature's abhorrence of a vacuum did not exceed the pressure of a column of water 32 feet high; but subsequently, as mentioned in the last of his dialogues, he devised an experiment to ascertain the power of a vacuum. This consisted in applying weights to a piston closely fitting in a smooth tube, placed in an inverted position, to see what weight would draw it down; and previous to his death he recommended to his pupil Torricelli to continue these investigations. The decisive experiment, made by Torricelli, and called after him the Torricellian experiment, was in ascertaining the length of a column of mercury sustained by the same cause, whatever it might be, which supported the column of water.
The weight of the mercury being about 14 times greater than that of the water, the height of the two columns, he reasoned, should be proportional to their weights. Filling a glass tube three feet or more in length with mercury, and closing the open end with his finger, he introduced this by inverting the tube under the surface of mercury in a basin. On removing the finger, the mercury in the tube sank down, and after oscillating stood at about 28 inches above the surface of that in the vessel, leaving in the upper end a vacant space. (See fig. 1.) Tor-ricelli continued his experiments, and discovered the fluctuations in the height of the column of mercury caused by the changes of the weather, and in 1645 an account of his observations was published; but he soon after died, before his great discovery was fully completed. The subject was taken up with great zeal by Pascal at Rouen in France. It occurred to him that if it were the atmospheric pressure which supported the column of mercury or water, the height of the column should be lessened as the pressure is reduced by ascending to greater elevations above the surface.
He communicated his views to his brother-in-law Perier, who lived at Clermont in Auvergne, near the high conical mountain of Puy-de-Dome, with the request that he should test the theory upon this elevation. This was not accomplished, however, till Sept. 19, 1648. Perier at this time, provided with mercury and tubes, observed in the garden of a monastery in the lowest part of Clermont the height at which the mercury stood in two tubes, which was 26 French inches and 3 3/4 lines. Leaving one of the barometers to be noticed in his absence, he took the other up the mountain, and at the summit found the height of the column was only 23 inches and 2 lines. At lower points, as he descended, the mercury rose in the tube, and at the base it occupied the same space in the tube as at first. This was the first observation ever made upon the different pressures of the atmosphere at different elevations. Perier repeated the experiment upon the highest tower of Clermont; and Pascal, on learning the result, made similar observations upon the top of a high house and the belfry of a church in Paris. Satisfied with the results, he soon proposed this process for determining differences of elevation.
Attention began now to be directed to the variations in the height of the mercurial column caused by the atmospheric changes. Otto Guericke, an ingenious and wealthy burgomaster of Magdeburg, contrived a gigantic barometer for indicating the state of the weather. It was a glass tube nearly filled with water, 30 feet in length, placed within the wall of his house and rising above the roof, the lower end terminating in a cistern of water. In the upper part, which was of larger dimensions than the rest, was placed the figure of a man, large enough to be visible from the street. In fine weather this figure, floating upon the surface of the water, appeared in full size above the roof; but as the fluid subsided with the change of weather, the manikin withdrew into the building. - From the original invention of the barometer to the present time, the ingenuity of the most distinguished men of science has been exercised in improving its construction. Numerous modifications of its form have been contrived, and yet those now most approved are but slightly varied from the straight inverted tube of Torricelli, and the siphon tube also proposed by him. The liquid selected by him is still preferred to all others by reason of the required weight of it occupying so little space.
It is also not liable to be volatilized by slight elevations of temperature, and thus fill with its vapor the vacant space in the top of the tube. The simplest form of the instrument is that called the cistern barometer. The straight tube of Torricelli terminates at its foot in a cistern of mercury. By the rising and falling of the liquid in the tube, the level of that in the cistern must change. The absolute height of the mercury, therefore, is found by rendering the scale movable, and bringing its zero point always to the surface of the mercury in the cistern; or by making the scale fixed, and bringing the mercury to its zero point by means of a screw, which is made to press against a flexible bag that forms the lower part of the cylinder, as represented in fig. 2, where the details of the upper, middle, and lower part of the barometer are shown separately. The latter method is the most generally adopted in the best instruments. By means of a sliding vernier, the scale may be read to the 1/1000 of an inch. Though various contrivances have been suggested for taking the place of these minute divisions and vernier readings, no substitute has yet been found to give such good results.
By a skilful observer they can be read with great minuteness, and much within the limits of accuracy of the instrument in other respects. - The barometer adopted by the Smithsonian institution is that of Mr. James Greene of New York. A full description of this, with the drawings that are required to render it intelligible, is published in the 10th annual report of the institution. In the same article are also directions for the use of the instrument, and for making barometrical observations. The instrument is designed for service as a mountain barometer as well as for stationary uses. In fig. 3 is represented the tripod serving for its support during observations when used as a mountain or travelling barometer. This stand folds up as seen in fig. 4, and serves then as an envelope to protect the instrument. Mr. Green constructed also, at the suggestion of Prof. Henry, a sulphuric acid barometer for the Smithsonian institution. As this liquid is much heavier than water, the tube was only about 18 ft. long; but experience proved it to be behind the mercurial barometer in its indications, and its use was abandoned. - The siphon barometer of Gay-Lussac, improved by Bunten of Paris, is a very portable and convenient form for the use of the scientific traveller.
It is represented in fig. 5. The name siphon is applied to barometers of which the lower end of the tube is turned up to form a short arm, which constitutes the cistern, and may be left open for the air to press directly upon the mercury. A capillary opening in this short arm, which is otherwise tight, answers the same purpose as if the whole were open. The surface of the mercury in the lower arm corresponds to the zero point in the cistern barometer; and as this fluctuates as well as that of the longer limb, it is necessary to use a vernier at each extremity of the column, and take two readings in order to determine the height of the column. As the two limbs are made of precisely the same diameter, the reading of one and doubling this gives a correct result. In Gay-Lussac's barometer, the tube at each extremity is of the usual diameter, but in the elbow, and along the lower part of the long limb, it is drawn down to a very small bore. The instrument is thus made to occupy very little space, so that the glass is enclosed in a brass cylinder of the size of an ordinary cane.
An open slit at each end of the brass tube affords an opportunity of reading the verniers, the indexes of which traverse up and down these openings by means of toothed wheels which run in a rack made upon the edge of the brass. The improvement introduced by Bunten is in dividing the long limb into two parts, the upper one of which is drawn down at its lower end to a small opening and inserted into the lower portion, to which it is attached, making again one tube. (See fig. 6.) The object of this conical projection of the upper into the lower part is to form a chamber or trap to catch any air which may be accidentally introduced through the short branch, and thus intercept its passage to the vacuum, where by its elasticity it would counterbalance to some extent the pressure of the external air. When the barometer is inverted, the air lodged in the air trap escapes through the short branch by which it entered. - A barometer in common use is provided with an index which turns around upon a dial, and points to figures which indicate the height of the mercury, as also to words descriptive of the state of the weather, as "Cloudy," "Fair," "Rainy," etc.
The index is made to move by means of a string, which passes around its axle, and has at each end a weight attached, the larger one resting upon the surface of the mercury in the shorter limb of a siphon barometer. (See fig. 7.) This is open to the objection that the reading of one limb gives but half the actual effect; but as the length of the index is several times greater than the radius of the pulley upon its axis, this objection is really more than counterbalanced. Still, little confidence is placed in its accuracy in marking the true variations of the column, there being so much friction that slight changes do not affect it at all. The words "Fair," "Variable," "Rain," "Storm," etc, found on the barometer scales, convey an erroneous impression about this instrument to the uninstruct-ed; for the barometer does not designate by the absolute height of the mercury, but by its rising or falling, the kind of weather we may expect, and this change is not indicated by the index. - In filling a tube with mercury, particular care is required that the mercury be free from mixtures of other metals. It is introduced into the tube in small quantities at a time, and boiled as each portion is added, the heat being applied to that part of the tube containing the mercury last introduced.
By boiling the mercury in the tube in vacuo, the air and moisture are most effectually expelled. On inverting the tube when properly filled, its lower end being kept in a basin of mercury, the column sinks to the proper level to counterbalance the atmospheric pressure. When the operation has been successfully completed, the column of mercury presents a bright undimmed appearance, and emits flashes of electrical light in the vacuum above, on the column being made to oscillate up and down in the dark; and a perfect vacuum is indicated by the clicking sound of the mercury wheait is allowed to strike the top of the glass tube. Still the electrical light is supposed to be dependent on a small quantity of vapor left behind in the vacant space of the tube; but in several instances it has been observed that the mercury remains suspended in the tube when this is inverted, even if the lower end be not placed in a cistern of the metal. It is detached by a sudden jar. The adherence of the mercury to the glass tends to introduce errors in estimating the true height of the column.
Instead of forming at the top of the column a concave surface by the particles adhering to the glass and climbing up its surface, as water and other fluids do by the property called capillarity, the mercury takes a convex form, and the column is lower than it should be. The smaller the bore of the tube, the greater is this depression and the error involved; but in the siphon barometer (tig. 5) the error of one convex surface of the mercury in one limb is counteracted by the same effect from that of the other. - However well constructed and filled, all barometers are liable to vary, after years of use, by a partial oxidation of the mercury, producing a thin film, which attaches itself to and obscures the inner surface of the tube. This film can be removed only by cleaning and refilling with fresh mercury. Air is liable to creep in between the mercury and the glass, and gradually enter into the vacuum, producing in the best instruments effects that are only perceived after a series of years; instruments used. for a long period show a less height in the latter than in the former part of the period. - Prof. Daniell constructed the most perfect water barometer ever made, which is somewhat similar to that already noticed of Guericke at Magdeburg. It is fixed in the hall of the royal society at Somerset house.
The tube is of glass, 40 ft. long and an inch in diameter. The water in it stands at an average height of 400 inches above the fluid in the cistern. A layer of a solution of caoutchouc in naphtha upon the water in the cistern prevents access of any air to the tube. The column is sensitive to continual changes of pressure in the atmosphere, which do not affect other barometers. In windy weather it is in perpetual motion, vibrating up and down almost with the regularity of respiration. It indicates the horary oscillations of the pressure sooner than, does the mercurial barometer of half an inch bore. - In the use of barometers, it is often desirable to have their variations recorded without the necessity of frequently observing them. Several methods have been devised of rendering them self-registering. One method is that of Mr. Bryson of Edinburgh. Upon the mercury in the lower limb of a siphon barometer is placed an ivory float, which carries outside to the tube a knife edge. This, by proper machinery, is made to touch once every hour the surface of a vertical cylinder, which revolves with uniform motion once in 24 hours, and upon the face of which are marked spaces corresponding to the hours of the day and night. A new cylinder is used each day.
The marks are made upon a coating of fine chalk and water laid on with a camel's-hair brush. Such arrangements are, however, far inferior to the photographic method now adopted in all meteorological observatories. This consists simply in a slip of sensitive photographic paper, moving by clockwork behind the upper part of the mercurial column, which throws its shadow on it, and thus prevents the impression of the light on the lower shaded portion. The light used is a kerosene lamp, and the slips of paper, after having been exposed, are darkened upon their upper half, while the undulating line between the darkened and light portion shows the variations of the barometer during the time of exposure. Account should be taken of the temperature at the same time that the observations of the barometer are noted; for the height of the column, as in the thermometer, must vary with change of temperature, as well as by change of atmospheric pressure. It is particularly important to make allowance for this cause of variation in observations for determining elevations, and a thermometer is always attached to the barometer for this use. Between the points of boiling and freezing it is found that the space occupied by mercury amounts to 1/54 of its bulk.
For each degree of heat by the centesimal scale its volume increases 1/5412; by Fahrenheit's thermometer, 1/9742 Though little reliance can be placed upon the barometer as indicating by any single observation the condition of the weather, its fluctuations caused by changes of atmospheric pressure may, when carefully noticed, often serve to foretell the effects that must still ensue. Thus, a sudden and long-continued fall is a sure sign of an impending storm. Many instances are recorded of vessels being saved by the precautions taken, in consequence of the warning of the barometer at the immediate approach of hurricanes, of which no other notice was given. - Barometers have been constructed with particular reference to use at sea. (See fig. 8.) Their tube has a bore scarcely exceeding 1/30 of an inch. Its upper end terminates in a cylinder 4 or 5 inches high and nearly 3/10 of an inch in diameter. It is suspended by a spring and gimbals near the top. The object of the larger bore above the capillary tube is to prevent a rapid flow of the mercury, which might be caused by the motion of the ship, and break the tube by its striking against the top.
The form is liable to the objection that the rise and fall of the fluid is necessarily very slow, and several minutes may elapse before a sudden change of atmospheric pressure is indicated. - The cause of the shifting pressure of the atmosphere is to be looked for in the operations of the winds which may be blowing in distant localities. By drawing the air away from any point, the pressure is here to some extent taken off, producing a partial vacuum which must soon be filled by a rush of air from other sources. Where the winds are equable, like the trade winds of the tropics, the movements of the barometer partake of the same regularity. Humboldt, in his researches in the equatorial regions of South America, was greatly struck by the uniformity of the motion of the barometer in the different periods of the day. From 4 o'clock in the morning till 10 the mercury generally rises, and then falls until 4 in the afternoon. It then rises again till 10 at night, after which it falls till 4 in the morning. In temperate northern latitudes the barometer generally stands higher at 9 A. M. and 9 P. M. and lower at 3 A. M. and 3 P. M. than at other hours. Prof. Daniell recommends these hours as the best times for consulting the barometer as a weather glass.
Its rise between 9 A. M. and 3 P. M. indicates fine weather. A fall from this time to 9 P. M. is likely to be followed by rain. Prof. Buys Ballot of Utrecht occupied himself for many years in making with others simultaneous observations in different localities of the changes in the barometer and in wind and weather. He determined positive numerical relations between the force of the wind and the height of the barometer preceding it. He succeeded at last in finding the laws governing the forward motion of the centre of barometric depression, followed by storms, and induced the government of Holland to establish a weather bureau with public storm signals in 1860, which was followed by England in 1861, by France in 1863, and by the United States in 1870. These laws, as might be expected, differ in different localities. From this relation rules have been deduced by which the maximum force of the wind during the day may be predicted every morning, thus enabling outward-bound vessels to determine the safety of putting to sea. - The Boiling Point Barometer is an instrument whose action depends upon the variable temperature at which water boils at different elevations, or, what is the same thing, under different atmospheric pressures.
It is constructed with a small cistern for the water, arranged in a cylindrical tin tube, which contains in the lower part an alcohol lamp for heating the fluid. The temperature is best noticed by suspending the bulb of the thermometer in the partially confined steam which rises from the boiling water. The difference in the temperature observed at two different points, expressed in degrees of Fahrenheit's thermometer, being multiplied by 530, will give the approximate difference of elevation between these two points. For greater accuracy correction should be made for the difference of the temperature of the air at the two places. Although the instrument is in a very portable and convenient form, it has not proved a favorite with scientific observers, from a want of confidence in its results. - The Ane-roid Barometee (Gr. a, and , a form without fluid) is a modification of the vacuum case barometer, the earliest form of which was invented by M. Conte, professor in the aerostatical school at Meudon, near Paris, and described by him in the Bulletin des sciences, Flo-real, year 6 (1798), p. 106. M. Conte in his balloon ascents found the reading of the mercurial barometer subject to the same difficulties so much complained of on shipboard, arising from the violent oscillations of the instrument. He therefore invented a watch-like, metallic, air-tight vacuum case, the lid of which, sustained by internal springs, rose and fell under the variable pressure of the atmosphere, an index showing the motion. M. Vidi subsequently devised a case of different form, with a flat corrugated top and bottom, flanged over and soldered to a rim, first pressed together at the centre by the withdrawal of the enclosed air, and then separated a certain distance by the introduction of a compensating spring. The instrument thus improved and constructed has come into extensive use.
It is represented externally by fig. 9; fig. 10 shows the interior arrangement, while fig. 11 shows a cross section of the flexible air-tight box, which collapses when the air is withdrawn. (See fig. 12.) By means of a spring it is brought back to its original position, the spring pulling it out again, and thus counterbalancing the atmospheric pressure, which tends to make the box collapse. A change in this pressure will of course resist the spring more or less, and this slight motion, multiplied by a proper mechanical arrangement, turns the hand seen at the top of fig. 10, and also, with the scale, in fig. 9. As, however, a rise in temperature expands the spring and diminishes its resistance, it will have the same result as an increased atmospheric pressure, namely, tend to let the box collapse. Becker, a well-known balance maker of New York, corrects this by introducing into the vacuum in the box a measured but very small quantity of perfectly dry air, the expansion of which by heat counterbalances the loss of tension of the spring by the same cause.
Experience proves, however, that this kind of compensation becomes inert after a lapse of a few years; hence a correction for temperature is required, the instrument having a thermometer attached, as shown in fig. 9. Unfortunately, this correction must be found by experiment for every instrument, and changes even for the same instrument in the course of time The coast survey and the Smithsonian institution have therefore pronounced against these barometers Their objections, however, it is thought, do not apply to their use in the hands of practical surveyors, topographers, civil engineers, artists, travellers, and sailors, who all pronounce emphatically in their favor. The observer must however learn to know his instrument well, or he can do nothing with it on an extended survey. Of course the aneroid can be of no service in the high geodesy of a coast or ordnance survey. In civil engineering, on the contrary, up to the final location line, it is reasonable to expect that it will almost replace the spirit level. In geological I examinations it is invaluable.
The geologist in tracing outcrops through the woods and where the rocks are entirely concealed, across ravines, and over the shoulders of hills, in a broken country has only to discover and take the direction of the line of strike, to know by infallible rise or fall of the index hand to the level of the point of his departure precisely when he is passing up or down over the outcrop of his bed. In countries where the rocks are nearly or quite horizontal in fact over half the United States, the aneroid is to the work of a week can with its help often be done in a day. There is an external index to assist the memory of the house observer from one observation to another.