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 \ 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 neat 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; besides, it will not freeze except at a very low temperature, and if a minute quantity of water be present, it never becomes solid.
A thin glass tube, 4 ft. long, can be bought for 5d. at the drug stores in cities. The glycerine and magenta will cost less than 2d. By making an ornamental case one may, with a little ingenuity, produce a beautiful instrument. It will foretell fair, changeable, and stormy weather as well as a mercurial barometer costing thirty times as much.
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.)
(14) 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. 367. The two chambers a 6 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 c 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.
(15) Hon to read the barometer. The barometer is only an eilremely sensitive balance, or a manometer showing the variations of atmospheric pressure The earl; makers of one form of the instrument had the oo-roitunate ides of marking certain points on the dial with the words "fair," "rain," "storm," 4c; 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 descendant 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
The diurnal course of the ann above the barometer, it heats the atmosphere, causing asending 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 (be day. These premises being excellent instruments, one placed, for instance, on the loner, and the other on the upper part of a house, should never agree. Proprietors of certain instruments declare thai theirs are the only barometers to be trusted; old friends will -dispute about them. With the present mode of graduation, it is rare to rind two barometers in the same bouse marking even the same division of the dial; the instrument which marks "variable" on the ground-floor will incline to "rain" on the fifth storey, for n, a house 60 ft. high the difference in the height of the column of air is about 2 mm.
Take a small aneroid wheel barometer in your hand, and walk np or down a street with a sharp ascent, and yon will find the needle derlevt towards " fine " as you descend, and fall as yon 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 mount very rapidly, the weather is not completely changed; it mounts 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.