Baromacrometer

[Gr. baros, weight, makros, length, and metron, a measure. An instrument for measuring the length and weight of a new-born infant. Barometer, n. [Fr. baromètre, Gr. baros, weight, and metron, measure.] (Meteor.) A measure of weight. Specifically, an instrument for measuring the weight or pre ure of the atmosphere, and indicating 'the changes of pre ure. - The B. is one of the most important instruments of meteorology, its object being to measure the weight of the superincumbent column of air, and so to enable the inquirer to note its variations. In common estimation, this instrument is a weather-gla , prognosticating the occurrence of rain, ., . It does not, however, give any direct indication, except the one now specified: the probabilities of rain, ., are inferences only, and dependent for their degree of accuracy on the mode by which very imperfect meteorological theories have been able to connect the other pheno men a of the atmosphere with its weight. - The invention of the B. was, in some degree, owing to an acc i dent. Some workmen, employed by the Duke of Flor. ence to prepare a sucking-pump for a deep well, found, to their surprise, that, notwithstanding the utmost care in forming and fitting the valves and piston, the water would not rise higher than 18 palms, or about 32 Eng. ft. For an explanation of this unexpected difficulty, they applied to the illustrious Galileo, then pa ing the evening of his life at his villa near Arcetri; but this philosopher was not yet prepared with the true answer. In that age, the doctrine of a plenum was an axiom in philosophy; and the ascent of water in the barrel of the pump was universally ascribed to nature's horror of a vacuum. Galileo, either fearing to encounter further persecutions by propounding opinions at variance with the prejudices of the times, or pre-occupied by the prevailing metaphorical modes of expre ion, evaded the difficulty by saying that the power of nature to overcome a vacuum was limited, and did not exceed the pre ure of a column of water 32 ft. in height. That he was himself little satisfied with this explanation, is evident from the circumstance, that, previously to his death, which happened soon after, 1642, he earnestly recommended his pupil. Torr i cell i, to undertake the investigation of the subject, which the infirmities of advanced age no longer permitted him to prosecute. Torr i cell i, suspecting the true cause of the suspension of the water, namely, the weight of the atmosphere, happily conceived the idea of trying the experiment with mercury. He perceived, that, if the weight of the atmosphere forms a counterpoise to a column of water of 32 ft., it must also counterpoise a column of mercury of about 28 inches in height, the weight of mercury being about 14 times greater than that of water. Having, accordingly, procured a gla tube, of about 3 ft. in length and inch in diameter, hermetically sealed at one end, he filled it with mercury; and covering the open end with the finger, he immerged it in an open ve el containing mercury. On bringing the tube to the vertical position, and removing the finger, the mercury instantly sank, leaving a vacuum at the top of the tube, and after making several oscillations, stood in the tube at the height of about 28 inches above the surface of that in the ve el. On covering the mercury in the ve el with a portion of water, and raising the tube till the lower end came into contact with the water, the mercury all ran out, and the water rushed up to the top of the tube. This experiment, called after its author the Torr i cell i an experiment, demonstrated that the mercury was sustained in the tube, and the water in the barrel of the pump, by exactly the same counterpoise, whatever the nature of it might be. Torr i cell i died shortly after, in the flower of his age, without completing his great discovery; but the fame of his experiment was soon carried into other countries, and the subject engaged the attention of the most eminent philosophers; among others the celebrated Pascal. After a variety of ingenious experiments on the subject, all of which tended to establish the pre ure of the atmosphere, it at length occurred to Pascal, that, if the mercurial column was really supported by atmospheric pre ure, it must be affected by the weight of the superincumbent ma of air, and consequently be diminished at considerable elevations. In order to verify this conjecture, he requested his brother-in-law, Perier, to try the experiment on the Puy de Dôme, a lofty conical mountain. At the foot of the mountain, Perier filled two tubes, and observed the mercury in each to stand at precisely the same height, nearly 28 English inches. Leaving one of them under the care of a person to watch its rise and fall, he carried the other to the top of the mountain; and on repeating the experiment there, the mercury stood at the height of only 24-7 English inches. At two intermediate stations in his descent, the mercury was observed succe ively to rise, and at the foot of the mountain it stood at exactly the same height in the tube as at first. This experiment was decisive; the result of it was communicated to Pascal at Paris, who, after confirming it by similar observations made succe ively on the ground, and at the top of a gla -house, and the belfry of a church, proposed the B. as an instrument for measuring the height of mountains, or the relative altitude of places above the surface of the earth. - The B. had been but a short time invented, before it was observed that the height of the mercurial column is subject to variations connected in some way with the changes of weather. But the variations are confined within a limited range, scarcely exceeding three inches in all, and often, for many days together, do not exceed a few hundredths of an inch. It therefore was considered desirable to render these minute oscillations more apparent, by increasing their range; and, accordingly, of the numerous forms which the B. has received, or which have been suggested, the greater part have been proposed with a view to this purpose. The most remarkable or useful constructions are the following, the descriptions of which will be readily understood, with the a istance of the diagrams: Fig. 294, 4, is the Cistern Barometer, and is merely the inverted tube of Torr i cell i, already described. The tube must be about 34 inches long. When placed in the cistern, the mercury sinks till the column between the two surfaces, m and n, just counterbalances the pre ure of the air. The space above the mercury, am, is, or ought to be, a perfect vacuum, or only filled with the vapor of mercury. In this B., as the diameter of the cistern is generally much greater than that of the tube, almost the whole effect of the rise or fall is perceived in the variation of the upper surface at m. For, supposing the section of the cistern 20 times greater than that of the tube, and that the height of the column, mn, suffers • diminution of one inch, it is evident that, as all the mercury which goes out of the tube pa es into the cistern, when it falls at mit must rise at n, but le in proportion as the section of the cistern exceeds that of the tube. In the case supposed, therefore, the alteration of the level at m will be 20 times greater than at n; that is to say, there will be a fall of 20-21 of an inch at m, and IMG:content-0536.png:[merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small] phon. The variations in this are only half as great as in the cistern B .; for the tube being of the same width throughout, a diminution of the column, mn , amounting to one inch, will be marked by a fall of half an inch at m, and a rise of half an inch at n. This inconvenience may, however, be remedied by having the lower branch blown into a wide bulb; but as it is very difficult to procure the bulb to be thrown into a perfectly regular shape, this enlargement of the bulb is found to give rise to inaccuracies. Fig . 294, 3, is the Wheel Barometer , proposed by Hooke. A small weight floats on the surface of the mercury in a siphon, which is very nearly counterpoised by another weight, connected with the former by a string pa ing over a pulley, p . When the mercury rises at n, the exterior weight descends and turns the pulley. An index attached to the axle of the pulley shows on a dial, the quantity of revolution. This B. , though very commonly met with, is a mere toy, and indicates neither the absolute height of the mercurial column, nor its variations, with sufficient accuracy to be of the slightest use for any philosophical purpose whatever. Even as a weather-gla , it is the worst of all the common forms of the B. -It has been proposed to enlarge the scale, by inclining the upper part of the tube so as to form a considerable angle with the perpendicular. By this contrivance the scale is increased in the proportion of radius to the co-sine of the angle of inclination; but the friction on the sides of the tube is greatly increased, and it is very difficult to determine the exact plane of the top of the column, which requires to be read off on a vertical scale. This construction is easily conceived without a diagram.- We shall notice two other forms of the barometer, proposed with a different view from that of enlarging the scale. Fig . 294, 4, is a modification of the siphon barometer, proposed by Gay-Lu ac. It differs from the common form in this respect, that, after the tube has been filled, the short branch is hermetically closed at the top, and the communication with the atmosphere takes place through a small capillary hole, drilled laterally through the tube at o, so fine, that, though it admits the air to pa freely, it prevents the pa age of the mercury. The B. is thus rendered very convenient for carriage; but notwithstanding the promising appearance of this B. , it has been found, particularly in travelling, that a portion of air will frequently insinuate itself through the mercury. In order to prevent the po ibility of the accident, an ingenious modification has been made. It consists in causing the part of the tube to terminate in a very fine point, and to penetrate to some depth into the other part, co , to which it is joined at c, in the manner represented in Fig . 294, 5. Now if an air-bubble from the end, o, which communicates with the atmosphere, should find its way through the bent capillary tube, it will pa along the sides of the bulging part, and instead of penetrating to the vacuum at a, will be arrested at c, whence it is easily expelled by reversing the barometer. - None of the contrivances which have been described for increasing the range of the oscillations, have been found to succeed well in practice. It is found to be decidedly better to apply minute divisions than to attempt to enlarge the scale; accordingly, experimenters now adhere to one or other of the two ancient forms, the cistern barometer and the siphon barometer. The height of the column in the siphon barometer is conveniently measured by means of a movable scale attached to the frame which supports the tube; by means of a tangent screw, the scale is raised or lowered till its zero coincides exactly with the surface of the mercury in the lower branch; and with the a istance of a vernier, the height can be read off to the hundredth or two hundredth of an inch, with sufficient precision. The scale of the cistern barometer is usually fixed, and the bottom of the cistern is raised or lowered by a screw, till the surface of the mercury in it coincides with the zero of the scale; but the scale may be movable, and its zero brought to coincide with the surface of the mercury in the basin, as in the former case. In order to determine when this coincidence takes place, various expedients may be had recourse to. The most usual is to place on the surface of the mercury a float carrying a vertical needle, some point on which answers to a fixed point on the scale, and the coincidence obtains when the two points are brought into the same level. Another contrivance to effect the same purpose was employed by Fortin, a celebrated French artist. An ivory needle is attached to the scale, pointing downwards, and having its point exactly in the same level with the zero of the scale. The image of the needle is clearly reflected from the surface of the mercury in the cistern, and the cistern is raised or lowered till the point of the needle and its image precisely coincide. In order to construct a good barometer, it is indispensably nece ary that the mercury be perfectly free from impurities, and carefully purged of air; this is obtained by boiling it. The particles of air and moisture which cling obstinately to the sides of the tube, must also be expelled by heat; the mercury must then be introduced slowly and continuously in a hot state, and while the tube continues hot. It is important that the diameter of the tube be not very small; for it is found that the mercury moves with more freedom in a tube of considerable width, the oscillations following the atmospheric changes with more promptitude than in one of smaller dimensions: besides which, there is le disturbance from capillary attraction. The interior diameter should in every case exceed one-fourth of an inch. The value of the B. as a scientific instrument depends on the purity of the mercury, and the total exclusion of atmospheric air. By proper care, it is, perhaps, po ible to expel every particle of air from the mercury and the interior of the tube, when the B. is made; but it seems doubtful if, by any means whatever, it can be preserved for a considerable length of time in this state. The most carefully constructed B. are liable to a slow and gradual deterioration, by the intrusion of air, which has been supposed to insinuate itself between the metal and the tube, and not through the mercury. To obviate this inconvenience, Prof. Daniell conceived the ingenious idea of fixing to the open end of the tube of the cistern B. a substance having a greater affinity than gla to mercury. "I caused," says he, "a small, thin plate of platinum to be made, about the third of an inch in length, and of the diameter of the gla tube; this was carefully welded to its open end, so that the B. tube terminated in a ring of platinum. The tube was filled and boiled as usual, and the infiltration of air was completely prevented by the adhesion of the mercury, both to the exterior and interior surface of the platinum guard. I have no doubt that a mere ring of wire welded, or even cemented upon the exterior surface of the gla , which would be a much easier and le expensive operation, would be a sufficient protection, as the slightest line of perfect contact must effectually arrest the pa age of the air. In all barometric observations there are, in general, two e ential corrections to be made, one for the capillarity or depre ion of the mercury in the tube, and the other for temperature. Pure mercury in a gla tube always a umes a convex surface. The following are the corrections for tubes of different diameters, according to the theory of Mr. Toony. IMG:content-0537.png:[blocks in formation] These corrections, which must always be applied to cistern B. , show that wide tubes ought to be preferred; in fact, when the diameter of the tube exceeds half an inch, they may be safely omitted. In siphon B. having both branches of the same diameter, the depre ion is equal at both ends; consequently the effect is destroyed, and no correction is required. This is a considerable advantage; for notwithstanding the most elaborate calculations, some uncertainty must always remain with regard to the exact amount of the capillary repulsion. - The correction for the temperature, which is the most important, depends on the expansion of the mercury, and the expansion of the scale on which the divisions are marked. If we make a = the height of the thermometer in degrees above the freezing-point, z = the fractional part of its bulk which mercury expands for one degree of heat on Fahrenheit's scale, y = the fractional part of its length by which the scale increases, h = the observed height of the B .; then the height which would have been observed, had the thermometer stood at the freezing point, ish-ha (z-y). The expansion of mercury in part of its bulk is -0001001. The scale is generally of some mixed metal of which the expansion is not very well ascertained; supposing it to be equal to that of copper, the expansion would be 0000096; therefore it will be sufficiently accurate to neglect the temperature of the scale, and a ume that of the mercury to be -0001. Hence the following practical rule for reducing an ob served height to the corresponding height at the temperature of the freezing-point; "subtract the 10,000th part of the observed altitude for every degree of Fahr. above 32." Suppose the thermometer 54° and the B. 30 inches, the correction will be (54-32) × 30 × -001-066, to be subtracted from 30 inches. In order to find the value of this correction, a thermometer must be attached to the barometer and observed at the same time. Cause of the variations of the B.- Various theories have been proposed to account for those frequent atmospherical changes which cause the rise and fall of the B. , but none of them can be regarded as very satisfactory. Whatever tends to increase or diminish the vertical pre ure will obviously cause the B. to rise or fall; but the vertical pre ure may be increased either by an influx of winds and the accumulation of air at any place, or by a diminution of the elasticity of the atmosphere. The presence of heat or of moisture augments the elasticity, and consequently reduces the weight of the vertical column. During the prevalence of northerly and easterly winds the B. stands high, the elasticity being diminished by the cold. But the real difficulty consists in explaining why the variations of the B. should be greater in the high latitudes than between the tropics, and why they should exceed in all cases the quantities which calculation might a ign. The only mode, perhaps, of removing the difficulty is to take into consideration the comparative slowne with which any force is propagated through the vast body of the atmosphere. An inequality may continue to accumulate in one spot before the counterbalancing influence of the distant portions of the aerial influence can arrive to modify the result. In the higher latitudes, the narrow circle of air may be considered as in some measure insulated from the expanded ocean of atmosphere; and hence, perhaps, the variations of the B. are concentrated there, and swelled beyond the due proportion. - Uses of the Barometer. The B. is an instrument of great importance in astronomy, its indications forming an e ential element in determining the amount of atmospheric refraction. It is also, on account of its application to the measurement of altitudes. indispensable in all researches connected with the climate. The purpose for which it is most commonly sought after, is to prognosticate the state of the weather. On land IMG:content-0538.png:[subsumed][subsumed][ocr errors][subsumed][subsumed][subsumed][subsumed] to the axis. This flat spiral spring, which is always in a state of tension, maintains a pre ure against the force of the levers, and keeps the hand of the aneroid in obedience to the indications of the vacuum-vase. Were it not for this spring, the hand, h , would remain stationary at the point to which it had been propelled. Barometric, Barometrical, a. Pertaining or relating to the barometer.