MOUNTAIN BAROMETERS.
=37. The Syphon Tube Mountain Barometer, on Gay Lussac's principle=,
constructed as described at page 31, and fixed in a metallic tubular
frame, forms a simple and light travelling instrument. The graduations are
made upon the frame, and it is suspended for reading by a ring at the top,
from beneath an iron tripod stand, which is usually supplied with it.
Considerable care is requisite in adjusting the verniers, so as to keep
the instrument steady and vertical. A drawback to the convenience of this
barometer is the movement of the mercury in the short limb, which is
generally not confined, and hence has every facility for becoming quickly
oxidised in travelling. To remedy this, Messrs. Negretti and Zambra so
construct the Mountain Syphon Barometer that by a simple half turn of a
screw the mercury can be confined for portability, while the lower limb
can be taken out for cleaning whenever found requisite.
=38. Mountain Barometer on Fortin's principle.=--This barometer, with
Fortin's cistern, as arranged by Messrs. Negretti and Zambra, is an
elegant, manageable, and very accurate instrument for travelling purposes,
and well adapted for careful measurement of heights. The cistern is made
large enough to receive all the mercury that will fall from the tube at
the highest attainable elevation. The screw at the bottom confines the
mercury securely for carriage, and serves to adjust the surface of the
mercury to the zero of the scale when making an observation. The vernier
reads to .002 of an inch, and slides easily on the brass frame, which is
made as small in diameter as is compatible with the size of the tube. The
tube in this barometer should be altogether without contractions, so that
the mercury will readily fall when it is set up for observation. It must
be carefully calibrated, and its internal diameter ascertained, in order
that correction may be made for capillarity. This correction, however,
should be combined with the error of graduation, and form a permanent
index error, ascertainable at any time by comparison with an acknowledged
standard barometer.
The barometer is supported in the tripod stand (furnished as part of the
instrument) when used for observation. It is suspended by placing two
studs, in the ring on the frame, in slots formed on the top of the stand,
so that it hangs freely and vertically in gimbals. To the metal top of
the stand, mahogany legs are hinged. To make the barometer portable, it
must be lifted out of the stand, sloped gently until the mercury reaches
the top, turning the screw at the bottom meanwhile; then invert and screw
until the mercury is made tight. The inverted instrument packs in the
stand, the legs being formed to fit round the frame; and receptacles are
scooped out for the cistern, thermometer, gimbals, and vernier; so that
the instrument is firmly surrounded by the wooden legs, which are held
fast together by brass rings passed over them.
[Illustration: Fig. 29.]
=39. Newman's Mountain Barometer.=--Fig. 29 is an illustration of the
mountain barometer known as Newman's. The cistern consists of two separate
compartments;--the top of the lower and the bottom of the upper, being
perfectly flat, are pivoted closely together at the centres, so that the
lower can move through a small arc, when turned by the hand. This movement
is limited by two stops. The top of the lower compartment and the bottom
of the upper have each a circular hole, through which the mercury
communicates. When the instrument is required for observation, the cistern
is turned close up to the stop marked "_open_" or "_not portable_." When
it is necessary to pack it for travelling, the mercurial column must be
allowed to fill the tube by sloping the barometer gently; then invert it,
and move the cistern to the stop marked "_shut_" or "_portable_." In this
condition, the upper compartment is completely filled with mercury, and
consequently that in the tube cannot move about, so as to admit air or
endanger the tube. Nor can the mercury pass back to the lower compartment,
as the holes are not now coincident, and the contact is made too perfect
to allow the mercury to creep between the surfaces. The tube does not
enter the lower compartment, which is completely full of mercury when the
instrument is arranged for observation. The spare capacity of the upper
cistern is sufficient to receive the mercury which descends from the tube
to the limit of the engraved scale, which in these barometers generally
extends only to about 20 inches. A lower limit could of course be given by
increasing the size of the cisterns, which it is not advisable to do
unless for a special purpose. This barometer may be had mounted in wood,
or in brass frame. If in wood, it has a brass shield, which slides round
the scale part of the frame, so as to be easily brought in front of the
tube and scale as a protection in travelling; the vernier screw, in this
case, being placed at the top of the instrument. When the scale is
graduated with true inches, the neutral point, the capacity and
capillarity corrections should be marked on the frame. The graduated
scales, however, placed on these barometers in brass frames, are usually
artificial inches, like the Kew plan of graduation; the advantage being
that one simple correction only is required, viz. one for index error and
capillarity combined, which can always be readily determined by comparison
with a standard barometer; moreover, as no adjustment of cistern is
required in reading, the instrument can be verified by artificial pressure
throughout the scale, by the plan practised at Kew, Liverpool, &c., and
already described (see p. 18).
40. NEGRETTI & ZAMBRA'S PATENT MOUNTAIN AND OTHER BAROMETERS.
This invention is intended to make mountain and other barometers of
standard accuracy stronger, more portable, and less liable to derangement,
when being carried about, than heretofore, by dispensing with the ordinary
flexible cistern containing the mercury at the bottom of the instrument,
and adapting in lieu thereof a rigid cistern constructed of glass and
iron. The cistern is composed of a glass cylinder, which is secured in a
metallic tube or frame. In order to render the cistern mercury-tight at
top and bottom, metal caps are screwed into the tube or frame, and bear
against leather washers placed between them and the edges of the glass
cylinder. The upper cap of the cistern is tapped with a fine threaded
screw to receive the iron plug or socket, into which the barometer tube is
securely fixed. The whole length of this plug has a fine screw cut upon it
by which the cistern can be screwed up or down. At the side of this plug
or socket, extending from the lower end to within half an inch of the top,
is cut a groove for admitting the air to the surface of the mercury within
the cistern when the barometer is in use. An ivory point is screwed into
the under surface of the plug, carrying the barometer tube. This ivory
point is very carefully adjusted by measurement to be the zero point of
the instrument, from which the barometer scale of inches is divided. The
surface of the mercury in the cistern is adjusted to the zero point by
screwing the cistern up or down until the ivory point and its reflected
image are in contact.
[Illustration: Fig. 30.]
The instrument (fig. 30) is shown in a state of adjustment, ready to take
an observation; but _when it is desired to render it portable, it must be
inclined, until mercury from the cistern fills the tube; the cistern must
then be screwed up on the socket_, so as to bring the face of the upper
cap against the under side of the shoulder of the cover immediately above
it; the instrument may then be carried without being liable to
derangement.
_Precautions necessary in using the Mountain Barometer._--On removing the
barometer from its case after a journey, allow it to remain with its scale
end downward, whilst the cistern is unscrewed to the extent of _one turn
of the screw_, after which slightly shake the cistern; the mercury in it
will then completely fill the end of the barometer tube, should any
portion of it have escaped therefrom.
The barometer is then inverted, and if it be desired to make an
observation, suspend it vertically from its stand by the ring at top. The
cistern must then be unscrewed, until the surface of the mercury is
brought just level with the extreme end of the ivory or zero point fixed
to the iron plug on which the glass cistern moves up and down.
Should the elevation of the place where the barometer is to be used be
considerably above the sea level, it will be well--after suspending it
from the stand--to unscrew the cistern several turns, _holding the
barometer in an oblique position_, as at great heights the mercury will
fall considerably quicker than the cistern can be unscrewed, thereby
filling it to overflowing; but by partly unscrewing the cistern first,
room is given for the reception of a fall of mercury to the extent of
several inches.
The cistern must not be unscrewed when the _Instrument is_ INVERTED _more
than_ two turns of the screw, otherwise the mercury will flow out through
the groove.
It is found safer when travelling to carry the barometer in a horizontal
position, or with its cistern end uppermost.
_To clean the Barometer._--Should at any time the mercury in the cistern
become oxidised, and reading from its surface be difficult, it can be
readily cleaned by removing the cistern and its contained mercury from the
barometer frame by unscrewing it _when in a horizontal position_; this
precaution is necessary that the mercury in the tube may not escape, and
thereby allow air to enter. The cistern must then be emptied, and with a
dry clean leather, or silk handkerchief, well cleaned.
The operation of cleaning being performed, return the cistern to the
frame, and screw it until the face is brought up against the under side of
the shoulder, still keeping the instrument _horizontal_. The cistern is
now ready for re-filling, to do which stand the barometer on end _head
downwards_, and remove the small screw at bottom; through the aperture
thus opened, pour in mercury, passing it through a paper funnel with a
very small aperture. It is well to pass the mercury through a very small
funnel two or three times before returning it to the barometer cistern, as
by this process all particles of dust or oxide adhere to the paper, and
are effectually removed.
Should any small quantity of the mercury be lost during the operation of
cleaning, it is of no importance so long as sufficient remains to allow of
adjustment to the zero point. This latter constitutes one of the great
advantages of this new instrument over the ordinary barometer; for, in the
majority of cases, after an instrument has been compared carefully with a
standard, should mercury be lost, there is no means of correcting the
error unless a standard barometer be at hand; the new barometer is, in
this respect, independent, a little mercury more or less being
unimportant.
=41. Short Tube Barometer.=--This is simply a tube shorter, as may be
required, than that necessary to show the atmospheric pressure at the sea
level. It is convenient for balloon purposes, and for use at mountain
stations, being of course a special construction.
=42. Method of Calculating Heights by the Barometer.=--The pressure of the
atmosphere being measured by the barometer, it is evident that as the
instrument is carried up a high mountain or elevated in a balloon, the
length of the column must decrease as the atmospheric pressure decreases,
in consequence of a stratum of air being left below. The pressure of air
arises from its weight, or the attraction of gravitation upon it, and
therefore the quantity of air below the barometer cistern cannot influence
the height of the column. Hence it follows that a certain relation must
exist between the difference of the barometric pressure at the foot and at
the top of a hill or other elevation, and the difference of the absolute
heights above the sea. Theoretical investigation, abundantly confirmed by
practical results, has determined that the strata of air decrease in
density in a geometrical proportion, while the elevations increase in an
arithmetical one. Hence we have a method of determining differences of
level, by observations made on the density of the air by means of the
barometer. It is beyond our purpose to explain in detail the principles
upon which this method is founded, or to give its mathematical
investigation. We append Tables, which will be useful to practical
persons,--surveyors, engineers, travellers, tourists, &c.,--who may carry
a barometer as a travelling companion.
Table I. is calculated from the formula, height in feet = 60,200 (log.
29.922 - log. B) + 925; where 29.922 is the mean atmospheric pressure at
32 deg. F., and the mean sea-level in latitude 45 deg.; and B is any other
barometric pressure; the 925 being added to avoid minus signs in the
Table.
Table II. contains the correction necessary for the mean temperature of
the stratum of air between the stations of observation; and is computed
from Regnault's co-efficient for the expansion of air, which is .002036 of
its volume at 32 deg. for each degree above that temperature.
Table III. is the correction due to the difference of gravitation in any
other latitude, and is found from the formula, _x_ = 1 + .00265 cos. 2
lat.
Table IV. is to correct for the diminution of gravity in ascending from
the sea-level.
To use these Tables: The barometer readings at the upper and lower
stations having been corrected and reduced to temperature 32 deg. F., take out
from Table I. the numbers opposite the corrected readings, and subtract
the lower from the upper. Multiply this difference successively by the
factors found in Tables II. and III. The factor from Table III. may be
neglected unless precision is desired. Finally, add the correction taken
from Table IV.
TABLE I.
_Approximate Height due to Barometric Pressure._
+----------------------------------------------+
|Inches.| Feet.||Inches.| Feet.||Inches.| Feet.|
|-------+------++-------+------++-------+------|
| 31.0 | 0 || 28.2 | 2475 || 25.4 | 5209 |
| 30.9 | 84 || .1 | 2568 || .3 | 5312 |
| .8 | 169 || 28.0 | 2661 || .2 | 5415 |
| .7 | 254 || 27.9 | 2754 || .1 | 5519 |
| .6 | 339 || .8 | 2848 || 25.0 | 5623 |
| .5 | 425 || .7 | 2942 || 24.9 | 5728 |
| .4 | 511 || .6 | 3037 || .8 | 5833 |
| .3 | 597 || .5 | 3132 || .7 | 5939 |
| .2 | 683 || .4 | 3227 || .6 | 6045 |
| .1 | 770 || .3 | 3323 || .5 | 6152 |
| 30.0 | 857 || .2 | 3419 || .4 | 6259 |
| 29.9 | 944 || .1 | 3515 || .3 | 6366 |
| .8 | 1032 || 27.0 | 3612 || .2 | 6474 |
| .7 | 1120 || 26.9 | 3709 || .1 | 6582 |
| .6 | 1208 || .8 | 3806 || 24.0 | 6691 |
| .5 | 1296 || .7 | 3904 || 23.9 | 6800 |
| .4 | 1385 || .6 | 4002 || .8 | 6910 |
| .3 | 1474 || .5 | 4100 || .7 | 7020 |
| .2 | 1563 || .4 | 4199 || .6 | 7131 |
| .1 | 1653 || .3 | 4298 || .5 | 7242 |
| 29.0 | 1743 || .2 | 4398 || .4 | 7353 |
| 28.9 | 1833 || .1 | 4498 || .3 | 7465 |
| .8 | 1924 || 26.0 | 4598 || .2 | 7577 |
| .7 | 2015 || 25.9 | 4699 || .1 | 7690 |
| .6 | 2106 || .8 | 4800 || 23.0 | 7803 |
| .5 | 2198 || .7 | 4902 || 22.9 | 7917 |
| .4 | 2290 || .6 | 5004 || .8 | 8032 |
| .3 | 2382 || .5 | 5106 || .7 | 8147 |
+----------------------------------------------+
TABLE I.--_continued_.
_Approximate Height due to Barometric Pressure._
+-------------------------------------------------+
|Inches.| Feet. ||Inches.| Feet. ||Inches.| Feet. |
|-------+-------++-------+-------++-------+-------|
| 22.6 | 8262 || 18.9 | 12937 || 15.2 | 18632 |
| .5 | 8378 || .8 | 13076 || .1 | 18805 |
| .4 | 8495 || .7 | 13215 || 15.0 | 18979 |
| .3 | 8612 || .6 | 13355 || 14.9 | 19154 |
| .2 | 8729 || .5 | 13496 || .8 | 19330 |
| .1 | 8847 || .4 | 13638 || .7 | 19507 |
| 22.0 | 8966 || .3 | 13780 || .6 | 19685 |
| 21.9 | 9085 || .2 | 13923 || .5 | 19865 |
| .8 | 9205 || .1 | 14067 || .4 | 20046 |
| .7 | 9325 || 18.0 | 14212 || .3 | 20228 |
| .6 | 9446 || 17.9 | 14358 || .2 | 20412 |
| .5 | 9567 || .8 | 14505 || .1 | 20597 |
| .4 | 9689 || .7 | 14652 || 14.0 | 20783 |
| .3 | 9811 || .6 | 14800 || 13.9 | 20970 |
| .2 | 9934 || .5 | 14949 || .8 | 21159 |
| .1 | 10058 || .4 | 15099 || .7 | 21349 |
| 21.0 | 10182 || .3 | 15250 || .6 | 21541 |
| 20.9 | 10307 || .2 | 15402 || .5 | 21734 |
| .8 | 10432 || .1 | 15554 || .4 | 21928 |
| .7 | 10558 || 17.0 | 15707 || .3 | 22124 |
| .6 | 10684 || 16.9 | 15861 || .2 | 22321 |
| .5 | 10812 || .8 | 16016 || .1 | 22520 |
| .4 | 10940 || .7 | 16172 || 13.0 | 22720 |
| .3 | 11069 || .6 | 16329 || 12.9 | 22922 |
| .2 | 11198 || .5 | 16487 || .8 | 23126 |
| .1 | 11328 || .4 | 16646 || .7 | 23331 |
| 20.0 | 11458 || .3 | 16806 || .6 | 23538 |
| 19.9 | 11589 || .2 | 16967 || .5 | 23746 |
| .8 | 11721 || .1 | 17129 || .4 | 23956 |
| .7 | 11853 || 16.0 | 17292 || .3 | 24168 |
| .6 | 11986 || 15.9 | 17456 || .2 | 24381 |
| .5 | 12120 || .8 | 17621 || .1 | 24596 |
| .4 | 12254 || .7 | 17787 || 12.0 | 24813 |
| .3 | 12389 || .6 | 17954 || 11.9 | 25032 |
| .2 | 12525 || .5 | 18122 || .8 | 25253 |
| .1 | 12662 || .4 | 18291 || .7 | 25476 |
| 19.0 | 12799 || .3 | 18461 || .6 | 25700 |
+-------------------------------------------------+
TABLE II.
_Correction due to Mean Temperature of the Air._
+-------------------------------------------+
|Mean |Factor.||Mean |Factor.||Mean |Factor.|
|Temp.| ||Temp.| ||Temp.| |
|-----+-------++-----+-------++-----+-------|
| 10 deg. | 0.955 || 35 deg. | 1.006 || 60 deg. | 1.057 |
| 11 | .957 || 36 | 1.008 || 61 | 1.059 |
| 12 | .959 || 37 | 1.010 || 62 | 1.061 |
| 13 | .961 || 38 | 1.012 || 63 | 1.063 |
| 14 | .963 || 39 | 1.014 || 64 | 1.065 |
| 15 | .965 || 40 | 1.016 || 65 | 1.067 |
| 16 | .967 || 41 | 1.018 || 66 | 1.069 |
| 17 | .969 || 42 | 1.020 || 67 | 1.071 |
| 18 | .971 || 43 | 1.022 || 68 | 1.073 |
| 19 | .974 || 44 | 1.024 || 69 | 1.075 |
| 20 | .976 || 45 | 1.026 || 70 | 1.077 |
| 21 | .978 || 46 | 1.029 || 71 | 1.079 |
| 22 | .980 || 47 | 1.031 || 72 | 1.081 |
| 23 | .982 || 48 | 1.033 || 73 | 1.083 |
| 24 | .984 || 49 | 1.035 || 74 | 1.086 |
| 25 | .986 || 50 | 1.037 || 75 | 1.088 |
| 26 | .988 || 51 | 1.039 || 76 | 1.090 |
| 27 | .990 || 52 | 1.041 || 77 | 1.092 |
| 28 | .992 || 53 | 1.043 || 78 | 1.094 |
| 29 | .994 || 54 | 1.045 || 79 | 1.096 |
| 30 | .996 || 55 | 1.047 || 80 | 1.098 |
| 31 | 0.998 || 56 | 1.049 || 81 | 1.100 |
| 32 | 1.000 || 57 | 1.051 || 82 | 1.102 |
| 33 | 1.002 || 58 | 1.053 || 83 | 1.104 |
| 34 | 1.004 || 59 | 1.055 || 84 | 1.106 |
+-------------------------------------------+
TABLE III.
+-------------------------------------------------------+
|Latitude.|Factor.||Latitude.|Factor.||Latitude.|Factor.|
|---------+-------++---------+-------++---------+-------|
| 80 deg. |0.99751|| 50 |0.99954|| 20 |1.00203|
| 75 |0.99770|| 45 |1.00000|| 15 |1.00230|
| 70 |0.99797|| 40 |1.00046|| 10 |1.00249|
| 65 |0.99830|| 35 |1.00090|| 5 |1.00261|
| 60 |0.99868|| 30 |1.00132|| 0 |1.00265|
| 55 |0.99910|| 25 |1.00170|| | |
+-------------------------------------------------------+
TABLE IV.
+----------------------------------------------------+
| Height in |Correction|| Height in |Correction|
|Thousand Feet.| Additive.||Thousand Feet.| Additive.|
|--------------+----------++--------------+----------|
| 1 | 3 || 14 | 44 |
| 2 | 5 || 15 | 48 |
| 3 | 8 || 16 | 52 |
| 4 | 11 || 17 | 56 |
| 5 | 14 || 18 | 60 |
| 6 | 17 || 19 | 65 |
| 7 | 20 || 20 | 69 |
| 8 | 23 || 21 | 74 |
| 9 | 26 || 22 | 78 |
| 10 | 30 || 23 | 83 |
| 11 | 33 || 24 | 88 |
| 12 | 37 || 25 | 93 |
| 13 | 41 || 26 | 98 |
+----------------------------------------------------+
EXAMPLE 1. On October 21st, 1852, when Mr. Welsh ascended in a balloon, at
3h. 30m. p.m., the barometer, corrected and reduced, was 18.85, the air
temperature 27 deg., while at Greenwich, 159 feet above the sea, the barometer
at the same time was 29.97 inches, air temperature 49 deg., the balloon not
being more than 5 miles S.W. from over Greenwich; required its elevation.
Feet.
Barometer in Balloon 18.85, Table I. = 13007
" at Greenwich 29.97 " 883
-----
12124
Mean Temperature, 38 deg., Table II. Factor 1.012
-----
12269.
-----
Latitude 51-1/2 deg., Factor from Table III. .99941
-----
12262
Correction from Table IV. 38
-----
12300
Elevation of Greenwich 159
-----
" Balloon 12459 feet.
=====
The following examples, from the balloon ascents of J. Glashier, Esq.,
F.R.S., will serve for practice.[4]
2. Ascended from Wolverhampton, 18th August, 1862, at 2h. 38m. p.m.;
barometer (in all cases corrected and reduced to 32 deg. F) was 14.868, the
temperature of the air 26 deg.; at the same time, at Wrottesley Hall, 531 feet
above the sea, in latitude 52-1/2 deg. N, the barometer was 29.46, and the
temperature of the air 65 deg.4; find the elevation of the balloon above the
sea.
Height, 18,959 feet.
3. From the same place an ascent was made 5th September, 1862, when at 1h.
48m. p.m. barometer was 11.954, air O deg.; at Wrottesley Hall 29.38, air 56 deg.
Height, 23,923 feet.
4. From the Crystal Palace a balloon ascent was made 20th August, 1862. At
6h. 47m. p.m. barometer was 25.55, air 50 deg.5; and at the same time at
Greenwich Observatory, at 159 feet above the sea, the barometer was 29.81,
air 63 deg.
Height, 4,406 feet.
5. From the same place an ascent was made 8th September, 1862. At 5 p.m.,
the balloon being over Blackheath, barometer was 25.60, and the air 49 deg.5,
while at Greenwich, barometer was 29.92, air 66 deg.4.
Height, 4,461 feet.