CHAPTER IV.THE USE OF ANEROID BAROMETERS.
The Aneroid, like the Mercurial barometer may be used either as a weather indicator or in the measurement of altitudes. When used in the former capacity, the Aneroid, especially at sea, possesses some obvious advantages. Aside from its superior compactness of form and its portability, it responds more readily to the changes in atmospheric pressure than the Mercury column, and thereby serves more efficiently to warn the mariner of sudden tempests.
The words Rain—Change—Fair seen stamped or engraved on the dial of many barometers have, of course, no special significance, and are now rarely seen on first-class instruments of either kind. The probable changes of weather indicated by changes of the barometer are briefly set forth in the following:
RULES FOR FORETELLING THE WEATHER.A Rising Barometer.A “rapid” rise indicates unsettled weather.A “gradual” rise indicates settled weather.A “rise” with dry air, and cold increasing in summer, indicates wind from Northward; and if rain has fallen better weather is to be expected.A “rise” with moist air, and a low temperature, indicates wind and rain from Northward.A “rise” with southerly wind indicates fine weather.A Steady Barometer.With dry air and seasonable temperature, indicates a continuance of very fine weather.A Falling Barometer.A “rapid” fall indicates stormy weather.A “rapid” fall, with westerly wind, indicates stormy weather from Northward.A “fall,” with a northerly wind, indicates storm, with rain and hail in summer, and snow in winter.A “fall,” with increased moisture in the air, and the heat increasing, indicates wind and rain from Southward.A “fall” with dry air and cold increasing (in winter) indicates snow.A “fall” after very calm and warm weather indicates rain with squally weather.
RULES FOR FORETELLING THE WEATHER.
A Rising Barometer.
A “rapid” rise indicates unsettled weather.
A “gradual” rise indicates settled weather.
A “rise” with dry air, and cold increasing in summer, indicates wind from Northward; and if rain has fallen better weather is to be expected.
A “rise” with moist air, and a low temperature, indicates wind and rain from Northward.
A “rise” with southerly wind indicates fine weather.
A Steady Barometer.
With dry air and seasonable temperature, indicates a continuance of very fine weather.
A Falling Barometer.
A “rapid” fall indicates stormy weather.
A “rapid” fall, with westerly wind, indicates stormy weather from Northward.
A “fall,” with a northerly wind, indicates storm, with rain and hail in summer, and snow in winter.
A “fall,” with increased moisture in the air, and the heat increasing, indicates wind and rain from Southward.
A “fall” with dry air and cold increasing (in winter) indicates snow.
A “fall” after very calm and warm weather indicates rain with squally weather.
It does not require the highest quality in the mechanism of an Aneroid to serve the purpose indicated in the above rules.
For the accurate measurement of differences of altitude, however, the best skill in construction and the most careful adjustment of the parts is indispensably necessary. 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 principlesinvolved, will often lead to erroneous conclusions. This is owing in many cases to a method adopted by some makers of adding a circle markedfeetoutside of the common graduation to inches of mercury.
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 thezeromark 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. And the further condition is also assumed, that the temperature of the air is of no account in estimating heights; an assumption equally at variance with fact.
It is only an inferior class of Aneroids that bear a fixed graduated circle of feet, with the zero of altitude corresponding to 30 inches of pressure.
Prof. Airy, the former Astronomer Royal of Great Britain, prepared atablefor the use of barometer makers—a scale from which is now engraved on many English Aneroids. It places the zero of altitude at 31 inches of pressure. This affords such large numbers for slight elevations that the proper use of the rule is suggested to the observer. He is led to subtract the two readings of feet to get difference in height. But this again assumes that the average temperature is 50° F.
Table Iexhibits Prof. Airy’s series of heights.
Some makers, designing to improve upon the simple construction just described, have engraved the outer circle of feet on a movable ring encircling the dial, so that when an observer is at any locality whose height is known, he may bring the proper mark of the altitude scale against the index pointer. Then if the observer travels about over a section of country, the pointer will indicate with fair approximation for some hours the altitude of the new positions.
FIELD’S ENGINEERING ANEROID.For description,see page 57.
FIELD’S ENGINEERING ANEROID.
For description,see page 57.
This device is convenient to a skilled observer who only requires rapid and approximate results, but to the novice it is misleading in two ways; first, because the temperature is left out of the calculation, and furthermore, such a use of the movable scale will, at times, involve a large error, as it is not a scale of equal parts.
Mr. Rogers Field, C. E., in 1873, applied the movable scale to the Aneroid, so as to convert it from a source of inaccuracy into an aid towards accuracy. He employs the altitude scale proposed by Sir G. Airy for temperature 50°, but he makes it movable so as to adjust it for any othertemperature. The shifting of the scale into certain fixed positions, is made to answer the same purpose as if the original scale were altered to suit various temperatures of the air. In theJournal of the Meteorological Societyfor 1874, January, Mr. Field says:
“The object aimed at in designing this improved form of Aneroid was, tosimplify the correct determination of altitudes in cases such as ordinarily occur in England, and the instrument is therefore arranged to suit moderate elevations, say of 2000 feet and under, and is not intended for more considerable heights.
“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. The exact position of the scale for different temperatures has been determined partly by calculation and partly by trial, and marked by figures engraved on the outside of the Aneroid. In order to insure the altitude scale not being shifted, after it has once been set in itsproper position there is a simple contrivance for locking it in the various positions. This consists of a pin, which fits into a series of notches on the outside of the ring carrying the glass. By slightly raising the glass it is freed from this locking pin, and can be turned until the figures corresponding to the air temperature are opposite to the pin, when the glass should be depressed so as to relock it, and the scale becomes correct for that temperature. The altitudes are in all cases determined by taking two readings, one at each station, and then subtracting the reading at the lower station from that at the upper.
“It will be seen from the foregoing description that the movable scale of the instrument requires to be set for temperatures before taking any observations, and must not be shifted during the progress of the observations.
“This may appear at first sight as a defect, inasmuch as the temperature of the air may alter during the progress of the observations; but practically it will not be found to be any drawbackin the case of moderate altitudes, as small variations of temperature will not appreciably affect the result. A variation of 5° of temperature gives only about 1 per cent. variation in the altitude, an amount that would under ordinary circumstances be inappreciable, so that as long as the temperature does not vary during the course of the observations more than 5° from that at which the instrument is set, the results may be accepted as correct, and, generally speaking, even a greater variation than this, say 6° or 8°, would be practically of no importance. Of course, if it should be found at any time that the temperature has varied considerably, during the course of the observations, from that at which the instrument was set, this variation can be allowed for by calculation in the usual way.”
The principle of allowing for variation of temperatures of the air by shifting the altitude scale is not theoretically accurate, but sufficiently so for practical purposes. For altitudes within the rangeof the instrument (say 3000 feet and under) and temperatures between 30° and 70°, the maximum error from using the shifted scale, instead of the calculation, is only 2 feet, which is inappreciable on the scale. The same principle might even be applied to altitudes up to 6000 feet, as the maximum error would be only 10 feet. For considerable elevations, however, the variations of the temperature between the base and the summit would interfere with the application of the principle.
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 isbelow 26 inches or the temperature above 70°, use 10 for a multiplier.
A higher degree of accuracy is obtained by using the multiplier obtained from the following table prepared by Mr. G. J. Symons:
To find the difference in height between two stations:Find the mean pressure; also the mean temperature. The number in the table corresponding to these two means, if multiplied by the difference of the barometric pressures in hundredths of an inch, will give the difference in altitude very nearly.
In the absence of a table to aid in computation, but having an Aneroid with the scale of feet, use the formula,
addingof the estimated altitude for every degree, theaveragetemperature is above 55°, and subtracting a like amount when it is below. D, is the difference of altitude in feet; H andhare the readingsin feetfrom the Aneroid scale. This gives fair approximations up to 3000 feet.
For accurate results use one of the following methods: Having Airy’s table (Table 1) and an Aneroid carefully graduated to inches;Take the reading in inches of the barometric scale at both lower and upper stations; also the temperature at both stations. Find from the table the heights in feet corresponding to the barometer readings. Subtract them and multiply the remainder by
The complete formula is
T andtare the observed temperatures; H andhare the heights in feet taken from the table.
In the absence of this table, but with a table of logarithms at hand, the barometric heights in inches are to be taken, and the following formula used:
B andbare the barometric readings in inches; D, T andtas in the other formulas. (See Table II.)
To avoid error from the constant changes in barometric pressure, the observations should be simultaneous. This is accomplished in the best manner by using two instruments, and requires, when the distance between the stations is considerable, two observers. With one instrument only, large errors are avoided by repeating the observation at the first station after taking that at the 2d station, and assuming that any change in barometric pressure that has occurred has been gradual during the absence.
When it is impracticable to repeat the observation at the firststation, the error which, in case of a changing pressure, might be a large one, may be reduced if the observation at the 2d station be continued for an hour or two, or until the rate of change can be estimated and a proportionate correction applied.
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. While making such comparisons, it is well to remember that the mercurial column and the scale by which it is measured both require correcting, and that during times of rapid changes, in atmosphericpressure, the Aneroid shows such changes more readily than the mercurial barometer. (See Table IV.)
In measuring heights with the Aneroid care should be taken that the instrument is not influenced by the heat of the hand nor by the direct rays from the sun.
The instrument should always be tapped gently with the finger at the moment of taking an observation. It should also be held in the same position for both observations; preferably with the face horizontal.
Considerable care is also required to determine exactly where the index points. It is best accomplished by sighting along the pointer, using one eye only for the purpose.
The following example will illustrate the use of the tables.
FromTable Iwe find height corresponding to reading at A is 857 feet. The height for B is 2120 feet.
The approximate height is 2120-857 = 1263 feet; but the sum of the temperatures is 143°. An additional correction ofis, therefore, to be applied to the above difference; this is 54 feet. The total estimated difference of altitude is then 1263 + 54 = 1317 feet.
The formula directly applied is
Applying the logarithmic formula we have:
As before remarked, the Goldschmid Aneroid requires that both the temperature of the air and of the instrument be carefully taken. Two examples of altitudes taken with the instrument previously referred to (No. 3187) will serve to show the kind of correction necessary, and asboth examples apply to the same mountain (Kiarsarge of Conway, N. H.,) they will together indicate the character of the instrument
Ex. I.—July 9th, 1881.
Ex. II.—August 9th, 1881.
In both these examples another reading would have been taken at Fryeburgon the return, if the better alternative of securing hourly readings of a stationary barometer at Fryeburg had not been followed. On July 9th there was no change in the Fryeburg barometer. On August 9th the following readings were taken at Fryeburg:
As this set of observations indicates a fall of .07 in the interval between the base and summit readings, it becomes necessary to make another correction to the last column.
Correcting the first reading to accord with the fall indicated by the stationary barometer, we get after all corrections:
The logarithmic formula for estimating heights from barometric observations is
in which
Applying this formula to our first example we have:
The second example gives:
As the station at Fryeburg is 434 feet above the sea, the estimated total height of Kiarsarge would be, in one case, 3321 feet, and in the other 3315 feet.
Prof. Airy’s tablegives 3319 and 3314 from the same data.
The instrument employed in the above measurements has been used in manyother cases of altitudes from 3000 to 4000 feet. An error of about 2 per cent. in excess has been detected in those cases where the altitude has been measured by more accurate means. It seems likely that the special correction needs some slight revision.
The following measurement was made with an aneroid only 1½ inches diameter, made by Casella.
Neversink, Sullivan Co., N. Y.,and Slide Mountain, Ulster Co.
As Neversink had been satisfactorily determined to be 1350 feet above the sea, the total height of Slide Mountain is estimated from this observation to have an altitude of 4206 feet.
(Note).—Return observations were made only at the camp of the Fly Club. Between 11a.m.and 9p.m., no change occurred in the barometer.
TheTribunereport, however, indicates a rise on this date of .07 between 7 and 11a.m.If such a change was felt in this region, then the calculated height of the mountain is too low by at least 60 feet. On the other hand, a height given by railway survey in this vicinity, (Johnson’s Mill) near the camp, seems to confirm the figures given here.
Also, the height of Helsinger Notch, taken incidentally on this excursion, was estimated at 2660 feet. Guyot makes the Notch 2677 and the summit of Slide Mountain 4205 feet.
The height of the base at Neversink was established by four observations, between New York Bay and this base, and was confirmed by comparison with the height of the railway track at Liberty, six miles south-west.
Neversink to Blue Mountain,August 18, 1880.
The corrected reading would be 28.875, if the second reading had been midway in point of time between the first and last.
This mountain is in Ulster Co., N. Y. Long. 74° 35 W, and Lat. 41° 52 N.
Neversink and Denman Mountains (Casella Aneroid).September 11, 1880.
This mountain is S. S. W. of Slide Mountain, and near Claraville. Long. 74° 28'; Lat. 41° 53' N.
Fryeburg, Me., and Kiarsarge Mountain, N. H.—Fryeburg base 434 feet above the sea, July 9th, 1881.
(Casella Aneroid)
(Note)—Barometer at Fryeburg remained stationary.
Fryeburg, Me., and Mt. Kiarsarge, N. H. (second survey) August 17th, 1881.
(Casella Aneroid.)
(Note)—Barometer at Fryeburg stationary till 4p.m.
Diff. = 2,789 × 1.012 = 2822 feet.Total ht. = 3256 feet above the sea.Height of this summit according to the Geological Survey is 3251 feet.
Liberty Hill, N. H. (near Laconia), and Mt. Belknap.—The base station was at Mr. Rowe’s 1130 feet above the sea, July 9th, 1878.
(Casella Barometer.)
As the interval between the observations was very short, and the general pressure sensibly stationary, no record was made of the time nor the return reading.
(Note)—An average of three measurements of this mountain gave 2392 feet. The other observations yielding 2369 and 2425 ft. respectively.
The height given in the Guide Books quoted from the Geological Survey is 2394 feet.