Foretelling Weather Important Observations Final Part

The daily movement of the barometer may be noted (in a form or table of
double entry) at the time of each observation, by a dot at the place
corresponding to its altitude, and the time of observing; which dot
should be connected with the previous one by a line. The resulting free
curve (or zig-zag) will show at a glance what have been the movements
during the days immediately previous, by which, and not merely by the
last observation, a judgment may be formed of the weather to be
expected.

Such a diagram may be filled up by uncorrected observations, its
object being to serve as a weather guide for immediate use, rather than
for future investigation. If closely kept up, it will prove to be of
utility, and will in some degree reward the trouble of keeping a regular
record. For purely scientific objects much more nicety and detail are
required.

* * * * *

HESITATION is sometimes felt by young seamen, at first using the vernier
of a barometer, for want of some such familiar explanation as the
following:--

The general principle of this moveable dividing scale is, that the total
number of the smallest spaces or subdivisions of the vernier are made
equal, taken altogether, to one less than that number of the smallest
spaces in an equal length of the fixed scale.

For example: ten spaces on the vernier being made equal to nine on the
scale, each vernier space is one tenth less than a scale space; and if
the first line or division of the vernier agree exactly with any line of
the scale, the next line of the vernier must be one tenth of a tenth (or
one hundredth) of an inch from agreement with the next scale division;
the following vernier line must be two hundredths out, and so on:
therefore, the number of such differences (from the next tenth on the
scale) at which a vernier line agrees with a scale line, when set, is
the number of hundredths to be added to the said tenth; (in a common
barometer, reading only to hundredths of an inch).

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;
therefore, the difference between one of the vernier and one of the
scale is two-tenths of a hundredth, or two thousandths of an inch
[25).050(.002].

This is the usual graduation of scientific barometers; but for ordinary
purposes, as weather-glasses, a division, or reading, to the hundredth
of an inch is sufficient.

When set properly, the vernier straight edge, the top of the mercury,
and the observer's eye, should be on the same level; the edge (or
pointer) just touching the middle and uppermost point of the
column.

Great care should be taken to look thus square, or at right angles to
the scale.

Light, or something white, at the back of the tube, assists in
accurately setting the vernier, and may be shifted about to aid in
reading off.

* * *

THE ANEROID has been recommended, in these pages, as a weather-glass;
but it may increase its usefulness to append a table for measuring
heights (approximately) by this, or any barometer, which can be compared
with another, or itself, at a higher or lower station.

If the measure of a height rather greater than the aneroid will commonly
show, be required, it may be re-set thus--When at the upper station
(within its range), and having noted the reading carefully, touch the
screw behind so as to bring back the hand a few inches (if the
instrument will admit), then read off and start again. Reverse the
operation when descending. This may add some inches of measure
approximately.

* * * * *

In the following Table, the difference between the number of feet
opposite the height of a barometer, at one station, and that at another
station, is their approximate difference of height.

TABLE.

-----------+-----------++-----------+-----------++-----------+-----------
Barometer Height in Barometer Height in Barometer Height in
Inches. feet. Inches. feet. Inches. feet.
-----------+-----------++-----------+-----------++-----------+-----------
31.0 0 26.8 3829 22.7 8201
30.9 85 26.7 3927 22.6 8317
30.8 170 26.6 4025 22.5 8434
30.7 255 26.5 4124 22.4 8551
30.6 341 26.4 4223 22.3 8669
30.5 427 26.3 4323 22.2 8787
30.4 513 26.2 4423 22.1 8906
30.3 600 26.1 4524 22.0 9025
30.2 687 26.0 4625 21.9 9145
30.1 774 25.9 4726 21.8 9266
30.0 862 25.8 4828 21.7 9388
29.9 950 25.7 4930 21.6 9510
29.8 1038 25.6 5033 21.5 9632
29.7 1126 25.5 5136 21.4 9755
29.6 1215 25.4 5240 21.3 9878
29.5 1304 25.3 5344 21.2 10002
29.4 1393 25.2 5448 21.1 10127
29.3 1482 25.1 5553 21.0 10253
29.2 1572 25.0 5658 20.9 10379
29.1 1662 24.9 5763 20.8 10506
29.0 1753 24.8 5869 20.7 10633
28.9 1844 24.7 5976 20.6 10760
28.8 1935 24.6 6083 20.5 10889
28.7 2027 24.5 6190 20.4 11018
28.6 2119 24.4 6297 20.3 11148
28.5 2211 24.3 6405 20.2 11278
28.4 2303 24.2 6514 20.1 11409
28.3 2396 24.1 6623 20.0 11541
28.2 2489 24.0 6733 19.9 11673
28.1 2582 23.9 6843 19.8 11805
28.0 2675 23.8 6953 19.7 11939
27.9 2769 23.7 7064 19.6 12074
27.8 2864 23.6 7175 19.5 12210
27.7 2959 23.5 7287 19.4 12346
27.6 3054 23.4 7399 19.3 12483
27.5 3149 23.3 7512 19.2 12620
27.4 3245 23.2 7625 19.1 12757
27.3 3341 23.1 7729 19.0 12894
27.2 3438 23.0 7854 18.9 12942
27.1 3535 22.9 7969 18.8 13080
27.0 3633 22.8 8085 18.7 13219
26.9 3731
-----------+-----------++-----------+-----------++-----------+-----------

Next: Marine Barometer Adopted By Her Majesty's Government
Previous: Other Types Of Barmeters Fortelling Weather Part 4

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