Barometer

“ High-class instruments, such as that here described, yield exact readings; but, in order to note them accurately, it is important that the eye, the zero edge of the vernier, the top of the mercurial column, and the back of the vernier should be in the same horizontal plane; conditions which may be obtained after some practice.
The accompanying illustration shows a form of barometer which, though not much used in this country, is deservedly popular on the Continent as a standard station barometer. ”

“ To the great majority of observers any barometer scale is more or less empiric. By long training and habit one gradually acquires a mental value of the figures which express pressure and these become visual proportions that can be compared in the mind. ”

“ The paper being carried steadily round on a drum at a given rate per hour, the height of the column of mercury is photographed continuously on the paper. From the photograph the height of the barometer at any instant may be taken. The principle of the aneroid barometer has been applied to the construction of barographs. The lever attached to the collapsible chamber terminates in an ink-fed style which records the pressure of the atmosphere on a moving ribbon. ”

“ The action of a fly, or vane, suggests the principle of an instrument for measuring the altitude of mountains, which perhaps deserves a trial, since, if it succeed only tolerably, it will form a much more portable instrument than the barometer. It is well known that the barometer indicates the weight of a column of the atmosphere above it, whose base is equal to the bore of the tube. It is also known that the density of the air adjacent to the instrument will depend both on the weight of air above it, and on the heat of the air at that place. ”

“ In predicting the weather a great deal of use is made of an instrument called the barometer. The barometer shows how hard the air around it is pressing. If the air is pressing hard, the mercury in the barometer rises. If the air is not pressing hard the mercury sinks. Just before a storm, the air usually does not press so hard on things as at other times; so usually, just before a storm, the mercury in the barometer is lower than in clear weather. You will understand the barometer better after you make one. Here are the directions for making a barometer:
Experiment 87. ”

“ The end that the holes are nearest should be toward the south. It should be held firmly in place.
THE BAROMETER The barometer is used for measuring the pressure of the atmosphere. The principle of this instrument was first discovered by Torricelli, a pupil of Galileo, the great Italian philosopher and scientist, in 1643. Many and various types of instruments have been made, but the two most generally used, especially where accurate indications are desired, are the mercurial and aneroid barometers. ”

“ When a barometer is sent by messenger it should be sent either in a leather case or a box designed for the purpose, with the handle so placed that it must be carried cistern uppermost. It should not be allowed to rest with the end on the floor of a moving vehicle. ”

“ As our barometer had been broken in March 1838, the register of a sympiesometer will be given in describing the gale of June in that year; but as this instrument had been found (by comparison with the barometer) to act exceedingly well, it will be sufficient for our purpose; the GENERAL use of a marine barometer being merely that of a weather glass, for which purpose a sympiesometer is equally good, and more sensitive. ”

“ Taking the state of the barometer in connexion with the appearance of the weather one could make a satisfactory conclusion as to the quarter from which any violent wind would come. And the barometer, after very little practice, can be used by any man. There is no difficulty in using it sufficiently to know that danger is coming on; and if danger is coming on, a man refrains, of course, from exposing himself to it; the quarter from which the wind comes being of minor consequence. ”

“ These are simply the highest and lowest readings of the barometer, usually designated maxima and minima, and should for the object in view receive particular attention. Whenever there is reason to believe that the barometer is approaching either a maximum or minimum, additional observations should be resorted to, so as to secure as nearly as possible the precise time as reckoned at the ship, with her position, of its occurrence, as well as the altitude of the mercurial column at that time and place. ”

“ BAROMETER WITH ELECTRICAL ADJUSTMENT.
This barometer is useful to persons whose eyesight may be defective; and is capable of being read off to greater accuracy than ordinary barometers, as will be seen by the following description:—The barometer consists of an upright tube dipping into a cistern, so contrived, that an up-and-down movement, by means of a screw, can be imparted to it. In the top of the tube a piece of platina wire is hermetically sealed. The cistern also has a metallic connection, so that by means of [Pg 9] covered copper wires (in the back of the frame) a circuit is established ”

“ The conditions of satisfactory work with barometers are certainly peculiar, and to field workers familiar only with the level and transit, may seem unique. But when the conditions are fully understood, the engineer may easily take precautions which will avoid too large errors, and conduct surveys in hilly regions with a celerity not heretofore attained. ”

“ The close observer gets a much better view-point of daily variations than does the casual observer.
Specific Uses of the Aneroid Barometer.—A custom that is wellnigh universal makes the mercurial barometer the standard instrument for the measurement of pressure at Weather Bureau stations. There is no doubt of the wisdom of this practise. For a substantial instrument not easily getting out of order, and susceptible of close reading, no other form of barometer approaches it. Such instruments as the Marvin normal barograph represent the highest skill in precision instruments. ”

“ These figures, with our own tables of wind and flight pressures under all revolution speeds, give us approximately the exact rate of advance. He must watch and keep in operation the barograph or self-recording altitude gauge which takes the place of the usual barometer; as a check he also notes and records periodical readings of the regular barometer; he keeps constant watch on the car equilibrium by means of the statoscope; he records the compass course, sets down the latitude and longitude by following the compass bearing and the advance in miles ”

“ Instead of confining the case to the impossibly small area of two square miles, let us suppose a gradual fall of the barometer from one part of the ocean to the other—such a fall, in fact, over such an area as often comes within the experience of every naval officer—and it stands to reason that waves of the ocean, like those of the air, only smaller and more sluggish, are consequent upon every change of the barometer. ”

“ It is a well-known fact that the barometer is as much, or even more affected by a change of wind as it is by rain; and the objection raised against a simple barometer reading, as leaving the observer in doubt whether to expect wind or rain, is removed by the addition of the Hygrometer, an instrument indicating the comparative degree of dryness or dampness of the air;—a most important item in the determination of the coming weather. ”

“ Not much, I don't!" laughed Brackett. "I mean carry up a barometer, and measure with it."
"How?" asked Glen, to whom this was a novel idea.
"Easy enough. We know that, roughly speaking, a barometer varies a little less than one tenth of an inch with every hundred feet of elevation. For instance, if it reads 21.22 where we now are, it will read 21.14 a hundred feet higher, or 20.40 at an elevation of a thousand feet above this. There are carefully prepared tables showing the exact figures." ”

“ A young and rapidly growing tree is not a desirable object for a station mark; but a mark made on a full-grown tree is not subject to material change. A stone post set firmly in the ground, or a piece of painted scantling attached firmly to the corner of a building will answer the purpose. The mark should be of such a character that it will resist ordinary weathering. The final point in the determination is the station barometer, that is, the chain of determinations which begin at an established plane of reference and end with the ivory point within the cistern of the barometer. ”

“ Although a barometer is an instrument artificially constructed by man, it should not be forgotten that when once made the column of mercury is placed in a passive or quiescent state in direct relation with the great forces of nature, so that its indications become to some extent natural phenomena. ”

“ The vernier of a barometer reading to thousandths of an inch, is on a similar principle, though differently divided. In this application of it, generally, twenty-five vernier spaces equal twenty-four of the scale spaces, which are each half a tenth, or five hundredths of an inch ”

“ So it is with the barometer and the atmosphere: when it is carried up in the air through several strata of 87 feet, the observer does not find that it falls a tenth of an inch for every successive 87 feet upward through which he may carry it. To get it to fall a tenth of an inch, he must carry it higher and higher for every successive layer. ”

“ A self-recording barometer (usually an aneroid) is called a barograph. In its ordinary form, this instrument carries a pen, which traces a continuous record of the barometric pressure on a strip of paper wound around a cylinder turned by clockwork. Generally the instrument runs for a week before the paper has to be changed. The barograph is a very instructive instrument, because it shows, not only the pressure, but also the changes of pressure—i. e., just how fast the barometer is rising or falling, or, as meteorologists say, the “barometric72 tendency.” ”