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METALLIC barometers (in _outer_ shape and size like aneroids) have not yet been tested adequately in very moist, hot, or cold air for a sufficient time. They, as well as sympiesometers, are likewise dependent or secondary instruments, and liable to deterioration. For limited employment, when sufficiently compared, they may be very useful, especially in a few cases of electrical changes not foretold or shown by mercury.

The SYMPIESOMETER is considered to be more sensitive than the marine barometer, falling sooner, and rising earlier: but this is partly in consequence of the marine barometer tube being contracted, to prevent oscillation or "pumping." In the sympiesometer a gas is used, which presses on the confined surface of the liquid with an uniform pressure at an equal state of temperature. The liquid is raised or depressed by an increase or diminution in the density of the atmosphere, and change of temperature is allowed for, by the sliding scale of the instrument being always set to agree with the height of the mercury in the attached thermometer, bringing the _pointer_ on the sliding scale of the sympiesometer to the same degree on the inverted scale (over which it slides) as is indicated by the thermometer. The height of the fluid, as then shown by the sliding scale, indicates the pressure of the atmosphere.

As the instrument is delicate, great care should be taken, in carrying or handling, to keep the top always upwards, and to exclude casual rays of the sun, or a fire, or lamp.

Oil sympiesometers seem to be affected more than mercurial, or others, and much more than the barometer, by lightning or electricity. That they, and the hermetically sealed "STORM GLASSES," are influenced by causes besides pressure and temperature, appears now to be certain.

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_[36] 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.

-----------+-----------++-----------+-----------++-----------+----------- 310 0 268 3829 227 8201 309 85 267 3927 226 8317 308 170 266 4025 225 8434 307 255 265 4124 224 8551 306 341 264 4223 223 8669 305 427 263 4323 222 8787 304 513 262 4423 221 8906 303 600 261 4524 220 9025 302 687 260 4625 219 9145 301 774 259 4726 218 9266 300 862 258 4828 217 9388 299 950 257 4930 216 9510 298 1038 256 5033 215 9632 297 1126 255 5136 214 9755 296 1215 254 5240 213 9878 295 1304 253 5344 212 10002 294 1393 252 5448 211 10127 293 1482 251 5553 210 10253 292 1572 250 5658 209 10379 291 1662 249 5763 208 10506 290 1753 248 5869 207 10633 289 1844 247 5976 206 10760 288 1935 246 6083 205 10889 287 2027 245 6190 204 11018 286 2119 244 6297 203 11148 285 2211 243 6405 202 11278 284 2303 242 6514 201 11409 283 2396 241 6623 200 11541 282 2489 240 6733 199 11673 281 2582 239 6843 198 11805 280 2675 238 6953 197 11939 279 2769 237 7064 196 12074 278 2864 236 7175 195 12210 277 2959 235 7287 194 12346 276 3054 234 7399 193 12483 275 3149 233 7512 192 12620 274 3245 232 7625 191 12757 273 3341 231 7729 190 12894 272 3438 230 7854 189 12942 271 3535 229 7969 188 13080 270 3633 228 8085 187 13219 269 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.

In reading off from a barometer, it should hang freely, not inclined by holding, or even by touch.

Sometimes, though rarely, at sea the mercury seems _stopped_. If so, take down the instrument (after _sloping_), reverse it, tap the tube gently while the cistern end is upwards, and then replace as before.

TESTING BAROMETERS, HYDROMETERS, AND THERMOMETERS.

In the year 1853 a conference of maritime nations was held at Brussels, on the subject of meteorology at sea. The report of this conference was laid before Parliament, and the result was a vote of money for the purchase of instruments and the discussion of observations, under the superintendence of the Board of Trade. Arrangements were then made, in accordance with the views of the Royal Society and the British Association for the Advancement of Science, for the supply of instruments properly tested.

In the barometers now in general use by meteorologists on land, the diameters of the tubes are nearly equal throughout their whole length, and a provision is made for adjusting the mercury in the cistern to the zero point, previous to reading the height of the top of the column. The object of the latter arrangement, it is well known, is to avoid the necessity of applying a correction to the readings for the difference of capacity between the cistern and the tube. At sea, barometers of this construction cannot be used. Part of the tube of the marine barometer must be very much contracted to prevent "pumping," and the motion of the ship would render it impracticable to adjust the mercury in the cistern to the zero point. In the barometer usually employed on shore, the index error is the same throughout the whole range of scale readings, if the instrument be properly made; but in nearly all the barometers which have till recently been employed at sea, the index correction varies through the range of scale readings, in proportion to the difference of capacity between the cistern and the tube. To find the index correction for a land barometer, comparison with a Standard at any part of the scale at which the mercury may happen to be, is generally considered sufficient.

To test the marine barometer is a work of much more time, since it is necessary to find the correction for scale readings at about each half inch throughout the range of atmospheric pressure to which it may be exposed; and it becomes necessary to have recourse to artificial means of changing the pressure of the atmosphere on the surface of the mercury in the cistern.

The barometers intended to be tested are placed, together with a Standard, in an air-tight chamber, to which an air pump is applied, so that, by partially exhausting the air, the Standard can be made to read much lower than the lowest pressure to which marine barometers are likely to be exposed; and by compressing the air it can be made to read higher than the mercury ever stands at the level of the sea. The tube of the Standard is contracted similarly to that of the marine barometer, but a provision is made for adjusting the mercury in its cistern to the zero point. Glass windows are inserted in the upper part of the iron air-chamber, through which the scales of the barometers may be seen; but as the verniers cannot be moved in the usual way from outside the chamber, a provision is made for reading the height of the mercury independent of the verniers attached to the scales of the respective barometers. At a distance of some five or six feet from the air-tight chamber a vertical scale is fixed. The divisions on this scale correspond exactly with those on the tube of the Standard barometer. A vernier and telescope are made to slide on the scale by means of a rack and pinion. The telescope has two horizontal wires, one fixed, and the other moveable by a micrometer, screw so that the difference between the height of the column of mercury and the nearest division on the scale of the Standard, and also of all the other barometers placed by the side of it for comparison, can be measured either with the vertical scale and vernier or the micrometer wire. The means are thus possessed of testing barometers for index error in any part of the scale, through the whole range of atmospheric pressure to which they are likely to be exposed, and the usual practice is to test them at every half inch from 275 to 31 inches.

In this way barometers of various other descriptions have been tested, and their errors found to be so large that some barometers read half an inch and upwards too high, while others read as much too low. In some cases those which were correct in one part of the scale were found to be from half an inch to an inch wrong in other parts. These barometers were of the old and ordinary construction. In some the mercury would not descend lower than about 29 inches, owing to a fault very common in the construction of the marine barometer till lately in general use, that the cistern was not large enough to hold the mercury which descended from the tube in a low atmospheric pressure.

The practice which has long prevailed of mounting the marine barometer in wood is objectionable. The instrument recently introduced agreeably to the recommendation of the Kew Committee, is greatly superior to any other description of marine barometer which has yet been tested, as regards the accuracy with which it indicates the pressure of the atmosphere. The diameter of the cistern is about an inch and a quarter, and that of the tube about a quarter of an inch. The scale, instead of being divided into inches in the usual way, is shortened in the proportion of about 004 of an inch for every inch. The object of shortening the scale is to avoid the necessity of applying a correction for difference of capacity between the cistern and the tube. The perfection with which this is done may be judged of from the fact, that of the first twelve barometers tested at the Liverpool Observatory with an apparatus exactly similar to that used at Kew (whence these instruments were sent by railway, after being tested and certified), the index corrections in the two pressures of 28 and 31 inches in three of them were the same; two differed 0001 of an inch; and for the remainder the differences ranged from 0002 to 0006 of an inch. The corrections for capacity were therefore considered perfect, and, with one unimportant exception, agreed with those given at Kew.

In order to check the pumping of the mercury at sea, the tubes of these barometers are so contracted, through a few inches, that, when first suspended, the mercury is perhaps twenty minutes in falling from the top of the tube to its proper level. When used on shore, this contraction of the tube causes the marine barometer to be always a little behind an ordinary barometer, the tube of which is not contracted. The amount varies according to the rate at which the mercury is rising or falling, and ranges from 000 to 002 of an inch. As the motion of the ship at sea causes the mercury to pass more rapidly through the contracted tube, the readings are almost the same there as they would be if the tube were not contracted, and in no case do they differ enough to be of importance in maritime use.

The method of testing thermometers is so simple as scarcely to require explanation. For the freezing point, the bulbs and a considerable portion of the tubes of the thermometers, are immersed in pounded ice.

For the higher temperatures, the thermometers are placed in a cylindrical glass vessel containing water of the required heat; and the scales of the thermometers intended to be tested, together with the Standard with which they are to be compared, are read through the glass.

In this way the scale readings maybe tested at any required degree of temperature, and the usual practice is to test them at every ten degrees from 32 to 92 of Fahrenheit. For this range of 60 the makers who supply Government are limited to 06 of a degree as a maximum error of scale reading; but so accurately are these thermometers made, that it has not been found necessary to reject more than a very few of them.

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