Part 7

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 || | || | -----------+-----------++-----------+-----------++-----------+-----------

MARINE BAROMETER,

ADOPTED BY

HER MAJESTY'S GOVERNMENT,

On the recommendation of the Kew Observatory Committee of the British Association for the Advancement of Science.

This instrument should be suspended in a good light for reading, but out of the reach of sunshine or the heat of a fire or lamp. It should be as nearly amidships, and exposed as little to sudden changes of temperature, gusts of wind, or injuries, as possible. In a ship of war it should be below the lowest battery or gun-deck. Light should have access to the back of the tube, to admit of setting the index so as to have its lower edge a tangent to the surface of the mercury--the eye being on the same level, which is known by the back and front edges of the index being in one line with the mercury surface. White paper or card will reflect light for setting the vernier correctly. The height of the cistern above or below the ship's water-line should be ascertained, and entered on the register.

It is desirable to place the barometer in such a position as not to be in danger of a side blow, and also sufficiently far from the deck above to allow for the spring of the metal arm in cases of sudden movements of the ship.

If there is risk of the instrument striking anywhere when the vessel is much inclined, it will be desirable either to put some soft padding on that place, or to check movement in that direction by a light elastic cord; in fixing which, attention must be paid to have it acting only where risk of a blow begins, not interfering otherwise with the free swing of the instrument: a very light cord attached above, when possible, will be least likely to interfere injuriously.

The vernier, as usual in standard barometers, reads to the two thousandth (.002) part of an inch. Every long line cut on the vernier corresponds to .01 part; each small division on the scale is .05; the hundredth parts on the vernier being added to the five when its lower edge is next above one of the short lines; or written down as shown by the figures on the vernier only, when next above one of the divisions marking tenths.

In placing this barometer, it is only necessary to fix the instrument carefully, as indicated in the above directions, and give a few gentle taps with the fingers on the bottom, to move the mercury. Without further operation it will usually be ready for observation in less than an hour.

When moving the barometer, or replacing it in its case, the mercury should be allowed to run gently up to the top of the tube, by holding the instrument for a few minutes inclined at an angle. The vernier should be brought down to the bottom of the scale. No other adjustment for portability is required. During carriage, it ought to be kept with the cistern end uppermost, or lying flat, the former position being preferable.

If the mercury should not descend at first by a few gentle taps, use sharper (but of course without violence), by which, and two or three taps, with the finger ends, on the tube--between the scale and the tangent screw--the mercury will be made to begin to descend.