Aneroid barometer

“ Although a carefully constructed aneroid is a very useful instrument indeed, it is not to be trusted like a mercurial barometer kept in a proper place. But a good aneroid is likely to be much more serviceable to the ordinary traveller on the march than a standard mercurial barometer, every packing and unpacking of which exposes it to the risk of breakage, or to the equally fatal risk of air obtaining access to the vacuous space at the top of the tube. ”

“ The Aneroid Barometer indicates variations in atmospheric pressure by the elevation and depression of the sides of an elastic metallic box from which the air is exhausted and which is kept from complete collapse by a powerful spring. In cases where extreme accuracy is not indispensable, the portability and sensibility of this instrument recommend it for use by tourists and fishermen. It is “quick in showing the variations of atmospheric pressure.” [8] “The Aneroid readings may be safely depended upon.” [9] “Its movements are always consistent.” ”

“ In the use of either form of Aneroid, whether it has been furnished with a correction or not, the observer should take early means to become acquainted with its limits of error under various conditions of temperature or pressure. Repeated measurements of a known altitude afford good data for such information, but direct comparisons, for a long time, with a standard cistern barometer will yield, with a minimum of labor, the greatest number of comparisons. ”

“ The use of an Aneroid of even medium quality will frequently lead to considerable errors in estimating heights. It may also be added here that instruments of the best manufacture in the hands of observers unacquainted with the principles [Pg 55] involved, will often lead to erroneous conclusions. This is owing in many cases to a method adopted by some makers of adding a circle marked feet outside of the common graduation to inches of mercury. ”

“ Many Aneroids marked “compensated” exhibit a sensible change when the temperature is varied; such instruments may be serviceable and quite accurate if allowance be made for the error of the instrument. This correction the owner had better determine by experiment. It is easy to subject the Aneroid to such variation of temperature as shall embrace the range at which it is likely to be used, and the movement of the index for each 10° or 20° of temperature recorded.
Aneroids require to be compared from time to time with a good mercurial barometer. ”

“ The movement of the mercury over a certain minute distance within the tube served as a switch to energize an electrical recording system. Hipp, who was perhaps the latest of this group, first applied the aneroid barometer (fig. 8) to self-registration. The idea of the aneroid—an air-tight bellows against which the atmospheric pressure would act—had been advanced by Leibniz in the 17th century and had been the subject of a few abortive experiments in the 18th century. Not until 1848 was an instrument produced that was acceptable to users of the barometer. ”

“ No! purity and virtue are filthy rags; salvation is to be purchased only through vicarious merits and 'imputed' holiness," etc.
The Aneroid Barometer: Its Construction and Use. New York: D. Van Nostrand. Pp. 106. Price, 50 cents.
It is generally understood that the aneroid barometer is a little instrument, the size of a watch, which depends for its action upon the changes in form of a thin metallic box, partially exhausted of air. ”

“ They have no significance whatever. The rising or falling of the mercury in the tube is caused by the beginning of those atmospheric changes which precede a storm but are not discernible by our senses. The barometer discerns them for us, and gives warning of weather changes. Of course there are many different conditions which affect the instrument, and the weather observers are instructed in these matters. The aneroid barometer is round, like one of the cheap nickel-plated clocks that are so numerous, and the changes are indicated by a hand moving across a scale on the dial. ”

“ The gas in the box, changing it bulk on a change of temperature, was intended to compensate for the effect on the metals of which the aneroid is made. Besides which, a further and more, reliable compensation has lately been effected by a combination of brass and steel bars. [35]
[23]
Metallic barometers (in outer shape and size like aneroids) have not yet been tested adequately in very moist, hot, or cold air for a sufficient time. They, as well as sympiesometers, are likewise dependent or secondary instruments, and liable to deterioration. ”

“ A thermometer should be placed beside the aneroid during the trial. A range of temperature from 15° F. to 175° F. may easily be produced, and a coefficient of correction if the instrument is not compensated, may be determined.
The graduations of a good instrument are neatly engraved on the dial.
The divisions corresponding to the inches and fractions of a mercurial barometer are the only essential ones. The circle of feet, whether movable or fixed, is a convenience of secondary importance. ”

“ The difference is clearly owing to the direct weight of the mechanism exerted on the vacuum box. There is no objection to allowing this weight to be always added, but the practice of the observer should be uniform, and to read from the horizontal dial is probably the most convenient practice.
A tap with the finger just before taking the reading is required to bring the springs to their proper bearing. Also, in case of rapid ascents, as some aneroids will not, at the moment of attaining an altitude, indicate the entire fall of pressure, a few minutes’ delay is necessary. ”

“ The layman also may forewarn himself with a reasonable degree, if not of certainty, at the least, of probability. An aneroid barometer, if watched closely, may be serviceable; unless intelligently used it is of doubtful service to any but a trained observer. ”

“ For ascertaining the altitude of high lands, ranges of hills or mountains, a thoroughly good “compensated” aneroid barometer should be taken. This will not only be valuable for measurements, but will be of considerable service in the observation of weather signs. We have one now in use made expressly for us by Mr. Cary, of 181, Strand. It is protected by a smooth wooden cover or case, enveloped in tightly-stretched leather. A sling is fitted to it by a swivel loop, which admits of its being carried over the shoulder or in the jacket pocket. ”

“ Figure 16.—Richard's registering aneroid barometer, an instrument used at the U.S. Weather Bureau about 1888. The Richard registering thermometer is similar, the aneroid being replaced by an alcohol-filled Bourdon tube. (USNM 252981; Smithsonian photo 46740-C.)
To attribute the success of self-registering instruments in the late 19th century to the unquestionable improvements in the techniques of the instrument-maker is to beg the question, for it is by no means clear that the techniques of the 17th-century instrument-maker were unequal to the task. ”

“ It was also clear that England is a most unfit country for the pursuit of such investigations, as, from whatever place the balloon started, it was never safe to be more than an hour above the clouds for fear of reaching the sea. It appeared from the observations that an aneroid barometer could be trusted to read as accurately as a mercurial barometer to the heights reached. The time of vibration of a horizontal magnet was taken in very many of the ascents, and the results of ten different sets of observations indicated that the time of vibration was longer than on the earth. ”

“ Many tourists carry Aneroids of the pocket size, and consult them frequently while traveling, relying upon a single observation of the index for the determination of their altitude.
If such a circle of feet be engraved on the dial plate with the zero mark made to correspond with 30 inches of the mercury column, of course every estimate of altitude made as above mentioned assumes that at the moment of observation; the barometer at the level of the sea would stand exactly at 30 inches; a condition only realized occasionally. ”

“ First-class Aneroids claim to be compensated, but a greater portion will need a correction which the purchaser can determine for himself, by subjecting the instrument to different temperatures while the pressure remains constant.
A modification of Guyot’s formula adapted to aneroid work was suggested in an excellent paper on the use of the aneroid, read before the American Society of Civil Engineers, in January, 1871. [Pg 38]
It is
D is the difference of altitude in feet. ”

“ Too much if the internal heat was to rise so high, for in this case we should be baked, not enough to measure the temperature of springs or any matter in a state of fusion.
2. An aneroid barometer, to indicate extreme pressures of the atmosphere. An ordinary barometer would not have answered the purpose, as the pressure would increase during our descent to a point which the mercurial barometer [1] would not register. ”

“ The Aneroid is graduated for inches in the usual way on the face, but the graduation only extends from 31 inches to 27 inches so as to preserve an open scale. The outer movable scale is graduated in feet for altitudes, and this graduation is laid down by fixing the movable scale with the zero opposite 31 inches. This is the normal position of the scale and it is then correct for a temperature of 50°. For temperatures below 50° the zero of the scale is moved below 31 inches; for temperatures above 50°, the zero of the scale is moved above 31 inches. ”

“ In traveling and making geographical observations, especially in hilly or mountainous regions, it is extremely convenient. Van Nostrand's little pocketbook gives all the information necessary to make the best use of the aneroid barometer, and it contains copious tables to facilitate calculations. ”

“ A smaller and ruder instrument called the Pocket Aneroid is made by the Zurich manufacturers. It is only 11⁄2 inches in diameter and 11⁄4 inches high. A bar fastened to the top of the vacuum box takes the place of the lever in the larger instrument.
A larger size is also made in which the movements of the vacuum box are directly observed with a compound microscope. [Pg 50]
Fig. ”

“ Mr. Ball carried an aneroid barometer, by Secrétan of Paris, which, during many mountain journeys before and since, has performed very satisfactorily; and Mr. Maw had a small pocket aneroid of ordinary construction, not deserving of much confidence.
At Sir J. Hooker’s request, Mr. Carstensen, then British Vice-consul, and M. Beaumier, French Consul at Mogador, both recorded observations of the barometer and thermometer twice daily (at 10 A.M. and 4 P.M. ) during the period of our stay in and near to the range of the Great Atlas. Mr. ”

“ An increase of temperature produces greater, a diminution less elasticity in this air. The compensation for effects of temperature is adjusted by the process of “trial and error,” and only a few makers do it well. It is very often a mere sham. Admiral FitzRoy writes, in his Barometer Manual, “The known expansion and contraction of metals under varying temperatures, caused doubts as to the accuracy of the aneroid under such changes; but they were partly removed by introducing into the vacuum box a small portion of gas, as a compensation for the effects of heat or cold. ”

“ When the air falls below the temperature of the dew-point, part of its moisture is deposited in dew, and since some time must elapse before the vapor of the upper strata can diffuse itself downwards to supply the deficiency, the barometer falls—most markedly at 10 p.m., when the deposition of dew is greatest. ”

“ In India we have seen that the atmospheric pressure oscillates at each station even when these are quite near to each other, independently of the known laws of equilibrium of gases. When we turn to the semi-diurnal oscillation of the barometer we are only amused at the attempts made to explain it by shuffling the atmospheric strata. Nothing can be more certain than that the theories of expansion, or resistance to expansion and overflow, are the vain efforts to make the [Pg 86] laws of nature agree with a theory. ”

“ Nevertheless, the best plan is to dispense with altitude scales, whether fixed or movable, and to calculate the heights. Simple rules, giving more reliable results than the attached scales, are at the service of those who need easy processes. Among these are the following:
Note the rise or fall of the barometer in hundredths of an inch, in passing from one station to the other; multiply by 9. The product is the difference of altitude in feet.
This is for ordinary temperatures and pressures. If the pressure is [Pg 62] below 26 inches or the temperature above 70°, use 10 for a multiplier. ”

“ It stands lower, about a tenth of an inch for each hundred feet of height directly upwards, or vertically, above the sea; where its average height, in England, is 29·94 inches (at 32°) .
[12] In an Aneroid, a metallic, or a wheel barometer, the hand's motion should correspond to that of mercury in an independent instrument. ”

“ The weight W0 refers to the height h1 the lower of the two [Pg 33] stations. At the surface of the earth, this weight would be greater in the ratio of . But as h is always small compared with R this correction may be neglected.
But there is another of more importance which should be taken into account. On account of the spheroidal form of the globe weight varies with the latitude. If G represent the weight of a body at latitude 45°, then at any other latitude l, its weight, is found by multiplying G by
This factor is to be applied to W0 in the formula. ”