Part 5
CHAPTER IV.
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 bra.s.s 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 bra.s.s rings pa.s.sed over them.
[Ill.u.s.tration: Fig. 29.]
=39. Newman's Mountain Barometer.=--Fig. 29 is an ill.u.s.tration 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 pa.s.s 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 bra.s.s frame. If in wood, it has a bra.s.s 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 bra.s.s 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 gla.s.s and iron. The cistern is composed of a gla.s.s 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 gla.s.s 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 s.c.r.e.w.i.n.g the cistern up or down until the ivory point and its reflected image are in contact.
[Ill.u.s.tration: 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 gla.s.s 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 uns.c.r.e.w.i.n.g 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 uns.c.r.e.w.i.n.g 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, pa.s.sing it through a paper funnel with a very small aperture. It is well to pa.s.s 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 quant.i.ty 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 const.i.tutes 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 quant.i.ty 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.
29922 - log. B) + 925; where 29922 is the mean atmospheric pressure at 32 F., and the mean sea-level in lat.i.tude 45; 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 for each degree above that temperature.
Table III. is the correction due to the difference of gravitation in any other lat.i.tude, 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 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.
-------+------++-------+------++-------+------
310
0
282
2475
254
5209
309
84
1
2568
3
5312
8
169
280
2661
2
5415
7
254
279
2754
1
5519
6
339
8
2848
250
5623
5
425
7
2942
249
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
300
857
2
3419
4
6259
299
944
1
3515
3
6366
8
1032
270
3612
2
6474
7
1120
269
3709
1
6582
6
1208
8
3806
240
6691
5
1296
7
3904
239
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
290
1743
2
4398
4
7353
289
1833
1
4498
3
7465
8
1924
260
4598
2
7577
7
2015
259
4699
1
7690
6
2106
8
4800
230
7803
5
2198
7
4902
229
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.
-------+-------++-------+-------++-------+-------
226
8262
189
12937
152
18632
5
8378
8
13076
1
18805
4
8495
7
13215
150
18979
3
8612
6
13355
149
19154
2
8729
5
13496
8
19330
1
8847
4
13638
7
19507
220
8966
3
13780
6
19685
219
9085
2
13923
5
19865
8
9205
1
14067
4
20046
7
9325
180
14212
3
20228
6
9446
179
14358
2
20412
5
9567
8
14505
1
20597
4
9689
7
14652
140
20783
3
9811
6
14800
139
20970
2
9934
5
14949
8
21159
1
10058
4
15099
7
21349
210
10182
3
15250
6
21541
209
10307
2
15402
5
21734
8
10432
1
15554
4
21928
7
10558
170
15707
3
22124
6
10684
169
15861
2
22321
5
10812
8
16016
1
22520
4
10940
7
16172
130
22720
3
11069
6
16329
129
22922
2
11198
5
16487
8
23126
1
11328
4
16646
7
23331
200
11458
3
16806
6
23538
199
11589
2
16967
5
23746
8
11721
1
17129
4
23956
7
11853
160
17292
3
24168
6
11986
159
17456
2
24381
5
12120
8
17621
1
24596
4
12254
7
17787
120
24813
3
12389
6
17954
119
25032
2
12525
5
18122
8
25253
1
12662
4
18291
7
25476
190
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
0955
35
1006
60
1057
11
957
36
1008
61
1059
12
959
37
1010
62
1061
13
961
38
1012
63
1063
14
963
39
1014
64
1065
15
965
40
1016
65
1067
16
967
41
1018
66
1069
17
969
42
1020
67
1071
18
971
43
1022
68
1073
19
974
44
1024
69
1075
20
976
45
1026
70
1077
21
978
46
1029
71
1079
22
980
47
1031
72
1081
23
982
48
1033
73
1083
24
984
49
1035
74
1086
25
986
50
1037
75
1088
26
988
51
1039
76
1090
27
990
52
1041
77
1092
28
992
53
1043
78
1094
29
994
54
1045
79
1096
30
996
55
1047
80
1098
31
0998
56
1049
81
1100
32
1000
57
1051
82
1102
33
1002
58
1053
83
1104
34
1004
59
1055
84
1106
+-------------------------------------------+
TABLE III.
+-------------------------------------------------------+
Lat.i.tude.
Factor.
Lat.i.tude.
Factor.
Lat.i.tude.
Factor.
---------+-------++---------+-------++---------+-------
80
099751
50
099954
20
100203
75
099770
45
100000
15
100230
70
099797
40
100046
10
100249
65
099830
35
100090
5
100261
60
099868
30
100132
0
100265
55
099910
25
100170
+-------------------------------------------------------+
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 1885, the air temperature 27, while at Greenwich, 159 feet above the sea, the barometer at the same time was 2997 inches, air temperature 49, the balloon not being more than 5 miles S.W. from over Greenwich; required its elevation.
Feet.
Barometer in Balloon 1885, Table I. = 13007 " at Greenwich 2997 " 883 ----- 12124 Mean Temperature, 38, Table II. Factor 1012 ----- 12269 ----- Lat.i.tude 51-1/2, 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 F) was 14868, the temperature of the air 26; at the same time, at Wrottesley Hall, 531 feet above the sea, in lat.i.tude 52-1/2 N, the barometer was 2946, and the temperature of the air 654; find the elevation of the balloon above the sea.
