CHAPTER XXII.

ON BAROMETRICAL HEIGHTS.

To determine barometrically the difference of height between two places, the implements required are two Barometers with their attached Thermometers and two detached common air Thermometers. Before giving the formula and rules however for this process, it is necessary to say a few words on the construction and method of using the Mercurial Barometer, a description of which instrument has been inadvertently omitted in Part 2. We have therefore given in the foot notes, some useful remarks lately published in a small Manual by Mr. Belville of the Royal Observatory, Greenwich.*

The Barometers which are commonly made use of in the measurement of a height are Mountain Barometers, so called from their extreme portability, being constructed so that the tripod stand, when closed, serves as a safe and convenient packing case. They can be opened, observed, and packed up again in the space of about ten minutes. The Mountain Barometer is capable of being used extensively by one individual, and the observations, if performed with care, will give results very near the truth. The instrument is not liable to injury in travelling, if proper precautions are taken, the most essential of

*On the Construction and Method of Using the Mercurial Barometer. There are various forms of the Barometer, but the one best suited for meteorological observations consists of a tube about 33 inches in length, the extremity of which is inserted into a small reservoir or cistern; and in order

which is always to carry the cistern inverted, and, when in this position, to turn the screws at the bottom of the cistern until the mercury almost touches the top of the tube and thereby prevent the oscillations from breaking it. Newman's instruments differ in their construction from the Englefield Barometer in the adoption of a double iron cistern with a solid bottom in lieu of the wooden cistern and leather bag. In the old instrument the screw at the bottom compresses the

to maintain the mercury in the cistern always at the same level, the cistern is constructed partly of leather, that by means of a screw at the bottom, the surface of the mercury in it may be so adjusted, as to have it always at the place from which the scale commences. Some Barometers are furnished with

a gauge or float, that in great elevations and depressions the observer may perceive when the mercury in the cistern sinks too low or rises too high. Let a b, fig. 1, be the glass tube plunged into the mercury

in the cistern C, and D the surface-line of the fluid in the cistern level with the commencement of the scale, and adjusted to the particular height of the mercury in the tube, which has been actually measured from the surface of the cistern, in the construction of the instrument (which height is called its neutral point) : when the mercury rises in the tube, a portion, equal to that rise, leaves the cistern, and the surface-line falls towards the dotted line e; and being lower than the surface from which its neutral point was measured, the actual variation in the atmosphere is indicated too little turn the screw f until the lines on the float h coincide, and the mercury then records the exact change : when depressions occur, the mercury sinking from the tube into the cistern raises the surface-line towards g: in this case the screw f must be unscrewed until the leather at the bottom of the cistern be sufficiently loosened to allow the mercury to assume its proper level at the surface D.

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When there is not a gauge to the Barometer, the relative capacities of the cistern and tube are ascertained by experiment, in the construction of the instrument, and marked thereon; as is also its neutral point. In this case, when the mercury in the g tube is above the neutral point, the difference between it and the neutral point is to be divided by the capacity, and the quotient added to the observed height will give the correct height; if the mercury be below the neutral point, the difference is to be divided as before, and the quotient subtracted from the observed height will give the correct height.

Fig. 1.

whole of the mercury in the cistern as well as in the tube, frequently forcing it through the pores of the wood, thereby rendering the Barometer useless; this defect has now been remedied, and the mercury secured for travelling, or set at

Let capacity for every inch of elevation of the mercury in the tube be equal to which, reduced to a decimal, will be

=0.025 for one inch, 0·013 for inch, 0·007 for in.

The scale of the Standard Barometer used in fixed observatories is made movable, and terminates in an ivory point, which is brought down to the surface of the mercury: when this point and its reflection appear to touch one another, the height indicated is correct. This kind of Barometer requires no adjustment or correction for the cistern.

The tubes of Barometers vary in size: those of a large diameter are preferable, as the motion of the fluid is freer, and its friction against the sides of the tube is nearly inappreciable; tubes of small diameters require correction for capillarity, or the depression of the mercury caused by its adhesion to the sides of the tube.

The range of the Barometer, or the spaces passed through by the mercury in its extreme depressions and elevations, being limited to 3 inches, it is not usual to graduate the scale from the lower end of the tube: the divisions commence at 27 inches, and are continued to 31 inches. The graduations on Troughton's Mountain Barometers for measuring great elevations, commence at 15 inches and are carried on to 33 inches. Each inch is divided into ten equal parts, and these parts are subdivided into hundredths by means of a Vernier (so named from Peter Vernier, its inventor). The Vernier A, (figs. 2 & 3) is a movable plate, one inch and one-tenth of an inch (together equal to 11) in length; these eleven-tenths are divided into ten equal parts, each part being equal to one-tenth of an inch and one-tenth of a tenth, together equal to eleven hundredths. When the pointer of the Vernier coincides with a division of the Barometer scale, as in fig. 2, each division of the Vernier will exceed each division of the scale respectively by 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 parts, whose denominators are the number of parts between a, b; the excess of each division being of a tenth or do, of a tenth or 18, 1 of a tenth or of a tenth or 1, &c. The pointer in this position reads off

liberty for use, by holding the instrument with the cistern end upwards at an angle of about 45°, and moving the upper part from left to right, making the word "portable," engraved on

to inches and tenths, viz. thirty inches and one tenth, expressed in figures 30.10 inches.

When the pointer does not coincide with a division of the scale as in fig. 3, observe which division of the Vernier does coincide; and the number placed against that division of the Vernier will be the number of hundreths to be added to the inches and tenths. In fig. 3, 7 coincides with a division of the Barometer scale, and therefore 7 hundreths are to be added to the inches and tenths, and the reading is thirty inches, one tenth and seven hundredths, expressed in figures 30.17 inches. By an alteration in the divisions of the Vernier, the Mountain and Standard Barometer are read off to of an inch.

Fig. 2.

30.5

Fig. 3.

A Thermometer is attached to the Barometer to indicate the temperature of the mercury in the cistern; all bodies expand by heat and contract with cold; the expansion of mercury is easily tested by exposing a mercurial Thermometer to the heat of a fire, or by placing it in hot water as the warmth increases, the mercury will expand and ascend in the tube; as it diminishes it will contract and fall towards the bulb: if the Thermometer be plunged into a mixture of pounded ice and common salt, from the intense cold produced by the conversion of the ice into water, the mercury will sink to zero, or 32° below the freezing-point of Fahrenheit; if the tube of the Thermometer should not be long enough to admit of so low a graduation, the mercury will shrink into the bulb. The expansion of mercury is 990 of its bulk for each degree of Fahrenheit between 32° and 212.° For convenience, tables have been computed, from which may be taken out, at sight, the amount

the cistern, coincide with the stop, or by a contrary motion bringing the words "not portable" opposite the stop, when the instrument is intended for use.

to be subtracted from the height of the mercurial column, on account of the expansion of the mercury from temperature.

The words Change, Fair, and Rain, engraved on the plate of the Barometer, were placed there by the first observers of its variations: no great importance should be attached to them; for from the observations of two centuries we find, that heavy rains, and of long continuance, take place with the mercury at 29.5 inches, or Change; that rain frequently falls when it stands as high as 3000 inches, or Fair; and more particularly in winter, a fine bright day will succeed a stormy night, the mercury ranging as low as 29'00 inches, or opposite to Rain. It is not so much the absolute height as the actual rising and falling of the mercury which determines the kind of weather likely to follow. The late great elevation of 30.9 inches in February of the present year 1849, (in England) was succeeded by a minimum of 29.25 inches, which produced a storm of wind so violent that the horizontal pressure of many of the gusts amounted to 20lbs. upon the square foot; a pressure which is rarely exceeded, even when the Barometer falls as low as 28.25 inches. This may appear extraordinary if we merely take into consideration the actual height of the column, and neglect the quantity of the fall which amounted to 1.65 inch. The mean height of the greatest observed elevations for the last thirty-eight years is 30.61 inches, and the mean height of the observed depressions for the same period is 28.69 inches; therefore a fall in the mercury of 1.65 inch from the mean of the elevations would give a minimum of 28.96 inches; a depression which is contemporary with violent storms, as it is within three-tenths of the mean of the lowest depressions of the Barometer.

In fixing the Barometer great care must be taken to place it perpendicular : a situation should be selected subject to the least change of temperature, for which reason a northern aspect is preferable to a southern; the height of the cistern of the Barometer above the level of the sea, and, if possible, the difference of the height of the mercury with some standard, should be ascertained, in order that the observations made with it should be comparative with others made in different parts of the country. Before taking an observation, the instrument should be gently tapped to prevent any adhesion of the mercury to the tube, the gauge should be adjusted to the surface-line of the cistern, and the index of the Vernier brought level with the top of the mercury. If the Barometer have a Vernier which admits the light from behind, the lower part of the pointer must make a tangent with the convex part of the mercury in the tube. In reading off the observation the eye should be on a line with the mercury; as by placing it above, the reading would be too low, and by placing it below, it would be too high. This difference in the manner of reading