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

l st 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

-----------+-----------++-----------+-----------++-----------+-----------