Familiar as the practical use of weather-glasses is, at sea as well as on land, only those who have long watched their indications, and compared them carefully, are really able to conclude more than that the rising glass[3] USUALLY foretells less wind or rain, a falling barometer more rain or wind, or both; a high one fine weather, and a low, the contrary. But useful as these general conclusions are in most cases, they are sometimes erroneous, and then remarks may be rather hastily made, tending to discourage the inexperienced.

By attention to the following observations (the results of many years' practice and many persons' experience) any one not accustomed to use a barometer may do so without difficulty.

The barometer shows whether the air[4] is getting lighter or heavier, or is remaining in the same state. The quicksilver falls as the air becomes lighter, rises as it becomes heavier, and remains at rest in the glass tube while the air is unchanged in weight. Air presses on everything within about forty miles of the world's surface, like a much lighter ocean, at the bottom of which we live—not feeling its weight, because our bodies are full of air, but feeling its currents, the winds. Towards any place from which the air has been drawn by suction,[5] air presses with a force or weight of nearly fifteen pounds on a square inch of surface. Such a pressure holds the limpet to the rock when, by contracting itself, the fish has made a place without air[6] under its shell. Another familiar instance is that of the fly which walks on the ceiling with feet that stick. The barometer tube, emptied of air, and filled with pure mercury, is turned down into a cup or cistern containing the same fluid, which, feeling the weight of air, is so pressed by it as to balance a column of about thirty inches (more or less) in the tube, where no air presses on the top of the column.

If a long pipe, closed at one end only, were emptied of air, filled with water, the open end kept in water and the pipe held upright, the water would rise in it more than thirty feet. In this way water barometers have been made. A proof of this effect is shown by any well with a sucking pump—up which, as is commonly known, the water will rise nearly thirty feet, by what is called suction, which is, in fact, the pressure of air towards an empty place.

The words on scales of barometers should not be so much regarded for weather indications, as the rising or falling of the mercury; for, if it stand at Changeable, and then rise towards Fair, it presages a change of wind or weather, though not so great, as if the mercury had risen higher; and, on the contrary, if the mercury stand above fair and then fall, it presages a change, though not to so great a degree as if it had stood lower: besides which, the direction, and force of wind, are not in any way noticed. It is not from the point at which the mercury may stand that we are alone to form a judgment of the state of the weather, but from its rising or falling; and from the movements of immediately preceding days as well as hours, keeping in mind effects of change of direction, and dryness, or moisture, as well as alteration of force or strength of wind.

In this part of the world, towards the higher latitudes, the quicksilver ranges, or rises and falls, nearly three inches—namely, between about thirty inches and nine-tenths (30·9), and less than twenty-eight inches (28·0) on extraordinary occasions; but the usual range is from about thirty inches and a half (30·5), to about twenty-nine inches. Near the Line, or in equatorial places, the range is but a few tenths, except in storms, when it sometimes falls to twenty-seven inches.

The sliding-scale (vernier) divides the tenths into ten parts each, or hundredths of an inch. The number of divisions on the vernier exceeds that in an equal space of the fixed scale by one.[7]

By a thermometer the weight of air is not shown. No air is within the tube. None can get in. But the bulb of the tube is full of mercury, which contracts by cold, and swells by heat—according to which effect the thread of metal in the small tube is drawn down or pushed up so many degrees: and thus shows the temperature.[8]

If a thermometer have a piece of linen tied round the bulb, wetted enough to keep it damp by a thread or wick dipping into a cup of water, it will show less heat than a dry one, in proportion to the dryness of the air, and quickness of drying.[9] In very damp weather, with or before rain, fog, or dew, two such thermometers will be nearly alike.

For ascertaining the dryness or moisture of air, the readiest, and surest method is the comparison of two thermometers; one dry, the other just moistened, and kept so. Cooled by evaporation as much as the state of the air admits—the moist (or wet) bulb thermometer shows a temperature nearly equal to that of the other one, when the atmosphere is extremely damp, or moist; but lower at other times,—in proportion to the dryness of air, and consequent evaporation,—as far as twelve or fifteen degrees in this climate; twenty or even more elsewhere. From four to eight degrees of difference is usual in England; and about seven is considered healthy for living rooms.

The thermometer fixed to a barometer intended to be used only as a weather-glass shows the temperature of air about it nearly—but does not show the temperature of mercury within exactly. It does so however near enough for ordinary practical purposes—provided that no sun, nor fire, nor lamp heat is allowed to act on the instrument partially.

The mercury in the cistern and tube being affected by cold or heat, makes it advisable to consider this when endeavouring to foretell coming weather by the length of the column.

Briefly, the barometer shows weight or pressure of the air; the thermometer—heat and cold, or temperature; and the wet thermometer, compared with a dry one, the degree of moisture or dampness.[10]

It should be remembered that the state of the air foretells, rather than shows present weather (an invaluable fact too often overlooked); that the longer the time between the signs and the change foretold by them, the longer such altered weather will last; and, on the contrary, the less the time between a warning and a change, the shorter will be the continuance of such foretold weather.

If the barometer has been about its ordinary height, say near thirty inches, at the sea level,[11] and is steady, or rising—while the thermometer falls, and dampness becomes less—North-westerly, Northerly, or North-easterly wind—or less wind—may be expected.

On the contrary—if a fall takes place, with a rising thermometer and increased dampness, wind and rain may be expected from the South-eastward, Southward, or South-westward.

A fall, with a low thermometer, foretells snow.

Exceptions to these rules occur when a North-easterly wind, with wet (rain or snow) is impending, before which the barometer often rises (on account of the direction of the coming wind alone), and deceives persons who, from that sign only, expect fair weather.

When the barometer is rather below its ordinary height, say, below twenty-nine inches and nine-tenths (at the sea level only), a rise foretells less wind, or a change in its direction towards the Northward,—or less wet; but when the mercury[12] has been low, say near 29 inches—the first rising usually precedes, and foretells, strong wind—(at times heavy squalls)—from the North-westward—Northward—or North-eastward—after which violence a rising glass foretells improving weather—if the thermometer falls. But, if the warmth continue, probably the wind will back (shift against the sun's course), and more Southerly, or South-westerly wind will follow. "Backing" is a bad sign, with any wind.

The most dangerous shifts of wind, and the heaviest Northerly[13] gales happen after the mercury first rises from a very low point.

Indications of approaching changes of weather, and the direction and force of winds are shown less by the height of mercury in the tube, than by its falling or rising. Nevertheless, a height of about 30 inches (at the level of the sea) with a continuance of it, is indicative of fine weather and moderate winds.

The barometer is said to be falling when the mercury in the tube is sinking, at which time its upper surface is sometimes concave or hollow. The barometer is rising when the mercurial column is lengthening; its upper surface being then, as in general, convex or rounded.[14]

A rapid rise of the barometer indicates unsettled weather. A slow rise, or steadiness, with dryness, shows fair weather.

A considerable and rapid fall is a sign of stormy weather and rain. Alternate rising and sinking show very unsettled weather.

The greatest depressions of the barometer are with gales from the S.E., Southward, or S.W.; the greatest elevations, with winds from the N.W., Northward, or N.E., or when calm.

Although the barometer generally falls with a Southerly, and rises with a Northerly wind, the contrary sometimes occurs; in which cases the Southerly wind is dry and the weather fine; or the Northerly wind is wet and violent.[15]

When the barometer sinks considerably, high wind, rain, or snow will follow: the wind will be from the Northward if the thermometer is low (for the season)—from the Southward if the thermometer is high.

Sudden falls of the barometer, with a Westerly wind, are sometimes followed by violent storms from N.W. or North.

If a gale sets in from the Eastward or S.E., and the wind veers by the South, the barometer will continue falling until the wind becomes S.W., when a comparative lull may occur; after which the gale will be renewed; and the shifting of the wind towards the N.W. will be indicated by a fall of the thermometer as well as a rise of the barometer.

Three things appear to affect the mercury in a barometer:—

1. The direction of the wind—the North-east wind tending to raise it most—the South-west to lower it the most, and wind from points of the compass between them proportionally as they are nearer one or the other extreme point.

N.E. and S.W. may therefore be called the wind's extreme bearings (rather than poles?)

The range, or difference of height, of the mercury, due to change of direction only, from one of these bearings to the other (supposing strength or force, and moisture, to remain the same) amounts in these latitudes to about half an inch (shown by the barometer as read off).

2. The amount, taken by itself, of vapour, moisture, wet, rain, hail, or snow, in the wind or current of air (direction and strength remaining the same) seems to cause a change amounting, in an extreme case, to about half an inch.

3. The strength or force alone of wind from any quarter (moisture and direction being unchanged) is preceded, or foretold, by a fall or rise, according as the strength will be greater or less, ranging, in an extreme case, to more than two inches.

Hence, supposing the three causes to act together—in extreme cases—the mercury might range from about 31 (30·9) inches to near 27 inches, which has happened occasionally.

Generally, however, as the three act much less strongly, and are less in accord—ordinary varieties of weather (the wind varying as usual—with more or less cloudiness, or rain) occur much more frequently than extreme changes.

Another general rule requires attention; which is, that the wind usually veers, shifts, or goes round, with the sun, (right-handed in northern places, left-handed in the southern parts of the world,) and that, when it does not do so, or backs, more wind or bad weather may be expected instead of improvement.

In a barometer the mercury begins to rise occasionally before the conclusion of gale, sometimes even at its commencement, as the equilibrium of the atmosphere begins to be restored. Although the mercury falls lowest before high winds, it frequently sinks considerably before heavy rain only. The barometer falls, but not always, on the approach of thunder and lightning, or when the atmosphere is highly charged with electricity.[16] Before and during the earlier part of serene and settled weather, the mercury commonly stands high, and is stationary.[17]

Instances of fine weather, with a low glass, occur exceptionally, but they are always preludes to a duration of wind or rain, if not both.

After very warm and calm weather, rain or a storm is likely to occur; or at any time when the atmosphere has been heated much above the usual temperature of the season.

Allowance should invariably be made for the previous state of the instrument during some days as well as hours, because its indications may be affected by remote causes, or by changes close at hand. Some of these changes may occur at a greater or less distance, influencing neighbouring regions, but not visible to each observer whose barometer, nevertheless, feels their effect.

There may be heavy rains or violent winds beyond the horizon, out of view of an observer, by which his instruments may be affected considerably, though no particular change of weather occurs in his immediate locality.

It may be repeated, that the longer a change of wind or weather is foretold by the barometer before it takes place, the longer the presaged weather will last; and, conversely, the shorter the warning, the less time whatever causes the warning; whether wind or a fall of rain, hail, or snow, will continue.

Sometimes severe weather from an equatorial[18] direction, not lasting long, may cause no great fall of the barometer, because followed by a duration of wind from polar regions:—and at times it may fall considerably with polar winds and fine weather, apparently against these rules, because a continuance of equatorial wind is about to follow. By such changes as these one may be misled, and calamity may be the consequence if not thus forewarned.

The veering of the winds is a direct consequence of the earth's rotation, while currents of air from the polar regions are alternating or contending with others from the equator.

The polar currents are cold, dry, and heavy. Those from the equatorial parts of the world are warm, moist, and comparatively light. Their alternate or combined action, with the agencies of solar heat and electricity, cause the varieties of weather that we experience.

It is not intended to discourage attention to what is usually called "weather wisdom." On the contrary, every prudent person will combine observation of the elements with such indications as he may obtain from instruments.

The more carefully and accurately these two sources of foreknowledge are compared and combined, the more satisfactory will the results prove.

A few of the more marked signs of weather—useful alike to seaman, farmer, and gardener, are the following:

Whether clear or cloudy, a rosy sky at sunset presages fine weather; a red sky in the morning, bad weather, or much wind (if not rain):—a grey sky in the morning fine weather; a high dawn, wind; a low dawn; fair weather.[19]

Soft-looking or delicate clouds foretell fine weather, with moderate or light breezes;—hard edged oily-looking clouds, wind. A dark, gloomy, blue sky is windy;—but a light, bright blue sky indicates fine weather. Generally, the softer clouds look, the less wind (but perhaps more rain) may be expected;—and the harder, more "greasy," rolled, tufted, or ragged, the stronger the coming wind will prove. Also, a bright yellow sky at sunset presages wind; a pale yellow, wet:—and thus by the prevalence of red, yellow, or grey tints, the coming weather may be foretold very nearly: indeed, if aided by instruments, almost exactly.[20]

Small inky-looking clouds foretell rain; a light scud, driving across heavy clouds, wind and rain; but if alone, wind only.

High upper clouds crossing the sun, moon, or stars, in a direction different from that of the lower clouds, or wind then blowing, foretell a change of wind (beyond tropical latitudes).[21]

After fine clear weather the first signs (in the sky) of change are usually small, curled, streaked, or spotty clouds, followed by an overcasting of vapour, that grows into cloudiness. This murky appearance, more or less oily or watery, as wind or rain will prevail, is a sure sign. The higher and more distant the clouds seem to be, the more gradual, but extensive, the coming change of weather will prove.

Generally speaking, natural, quiet, delicate tints or colours, with soft undefined forms of clouds, foretell fine weather: but gaudy or unusual hues, with hard, definite outlines, presage rain and wind.

Misty clouds forming, or hanging on heights, show wind and rain coming—if they remain, or descend. If they rise, or disperse, the weather will improve, or become fine.

When sea birds fly out early, and far to seaward, moderate wind and fair weather may be expected. When they hang about the land, or over it, sometimes flying inland, expect a strong wind, with stormy weather. As many creatures, besides birds, are affected by the approach of rain or wind, such indications should not be slighted by the observer of weather.

There are other signs of a coming change in the weather known less generally than may be desirable; and, therefore worth notice here.

When birds of long flight, such as swallows and others, hang about home and fly low—rain or wind may be expected. Also when animals seek sheltered places, instead of spreading over their usual range: when pigs carry straw to their sties; and when smoke from chimneys does not ascend readily, (straight upwards during a calm,) an unfavourable change may be looked for.

Dew is an indication of fine weather. So is fog. Neither of of these two formations occurs under an overcast sky, or when there is much wind. One sees the fog occasionally rolled away, as it were, by wind—but not formed while it is blowing.

Remarkable clearness of atmosphere, near the horizon; distant objects, such as hills, unusually visible; or raised (by refraction); and what is called "a good hearing day" may be mentioned among signs of wet, if not wind, to be expected.[22]

More than usual twinkling of the stars; indistinctness or apparent multiplication of the moon's horns; haloes; "wind-dogs;" and the rainbow; are more or less significant of increasing wind, if not approaching rain.[23]

Near land, in sheltered harbours, in valleys, or over low ground, there is usually a marked diminution of wind during part of the night—and a dispersion of clouds. At such times an eye on an overlooking height may see an extended body of vapour below; which the cooling of night has rendered visible.

Although the preceding remarks are probably sufficient for their principal purpose—these pages may fall into the hands of persons familiar with the subject, to whom the following observations may be addressed, as some of the reasons for what has been so briefly, if not too positively outlined.

As the mercurial column rises with increase of pressure by the atmosphere, and descends when the pressure diminishes, it indicates a greater or less accumulation of air, which, like other fluid, such as water (when heaped above its average level or reduced below it, from whatever cause),—will have a tendency to fall or rise till the general equilibrium is restored. An observer may be under the centre of such accumulation or depression, he may be more or less distant from it, though within the influence of whatever horizontal movement of air may be caused by such temporary increase or diminution of pressure. Hence the barometer shows, and generally foretells, changes of wind; but as complications always occur, and as changes are of greater or less extent, affecting or extending through a wider or more limited area, accompanied by hygrometric and electrical alterations, it is extremely difficult at times to say beforehand what particular change of weather is to be expected, and at what interval of time; although after the event the correspondence of barometric changes with those of the weather can be readily traced. However, notwithstanding occasional perplexity, the general character of weather during the next few days may be predicted by an observer who understands the nature and use of this instrument and the thermometer, and has watched them in the few immediately preceding days.

In endeavouring to foretell weather, the general peculiarity should always be remembered, that the barometric column usually stands higher with easterly than it does with westerly winds; and with winds from the polar regions higher than with those from the direction of the equator. Hence the highest columns are observed with north-east winds in northern latitudes, and with south-east in the southern hemisphere.

In middle latitudes there is an average difference (unreduced or observed height as read off) of about half an inch, other things being similar, between the heights of the mercury with North-easterly, and with South-westerly winds.

The steadier the column, or the more gradually it moves, the more settled in character will the weather be, and conversely: because it shows a quiet settled state of the atmosphere; or, if otherwise, the reverse. In the tropics, when the barometric column moves contrary to its usual daily motion, inferior weather may be expected (temporarily), because the usual air currents are disturbed.

This regular movement, whether tidal, or otherwise connected with the sun's influence—sensible in tropical latitudes, but more or less masked elsewhere—amounts to nearly two-tenths of an inch near the equator, the highest being at about nine, and the lowest near three o'clock.

Some movements of the atmosphere may be illustrated by reference to the motion of water drawn off from a reservoir by a small opening below; or by similar upward draught through a syphon; or by a gradual pouring in at the upper surface.

From a slight motion at the commencement, affecting only that portion of the fluid adjoining either of those places of diminution or repletion, gradually all the water becomes influenced and acquires more or less rapid movement. But suppose a long reservoir or canal of fluid which has two such points of exhaustion or two of such repletion (as imagined above), and that one of either is near each end of the vessel. If each aperture be opened at the same moment, equal effects will be caused in each half of the fluid towards either end of the vessel, but in the middle there must be a neutral point at which the water falls, yet has no horizontal motion. The converse takes place in raising the level. And in the case of fluid drawn off or diminished in weight at one end while increased by repletion at the other, the whole body of water will move similarly to that in the former vessel, but unequally. Hence it is evident, that before horizontal motion occurs, an augmentation or a diminution of pressure must take place somewhere more or less remote; and so it is with the lighter fluid atmosphere,—which has centres, lines, or areas of depression towards which currents flow.

Such considerations show in some degree why the barometric changes usually precede, but sometimes only accompany, changes of weather: and, though very rarely, occur without any sensible alteration in the wind current of the atmosphere. An observer may be near a central point towards which the surrounding fluid tends,—or from which it diverges. He may be at the very farthest limit of the portion of fluid that is so influenced. He may be at an intermediate point—or he may be between bodies of atmosphere tending towards opposite directions.

It has been said, that "a whirlwind which sets an extended portion of the atmosphere into a state of rapid revolution diminishes the pressure of the atmosphere over that portion of the earth's surface, and most of all at the centre of the whirl. The depth of the compressing column of air will, at the centre, be least, and its weight will be diminished in proportion to the violence of the wind." Yet this has been controverted with respect to the general effect of air in horizontal motion, and the depth of the column in question.

Certainly there are two kinds of whirlwinds—one caused by rarefaction, tending to lighten vertical pressure under the vortex, though not, perhaps, under all the current drawn towards it; and the other, a consequence of opposing winds, which occasion huge eddies or whirlwinds of compression.

Some whirlwinds are accompanied by rushes from the upper atmosphere, from the colder regions, which, mingling with warmer and moister air near the sea, cause dense clouds. About their centre it sometimes happens that the barometer falls as much as two or three inches, showing a diminution of atmospheric pressure by nearly a tenth part; when it should be expected, from physical considerations alone, that very dense clouds would be formed.[24]

The column of mercury falls about one tenth of an inch for each of the first few hundred feet above the sea level, but varying when it becomes much more elevated.[25] Due allowance, therefore, should be made in observing, when on high land.

The tides are affected by atmospheric pressure, so much that a rise of one inch in the barometer will have a corresponding fall in the tides of nine to sixteen inches, or about one foot for each inch.[26]

Vessels sometimes enter docks, or even harbours, where they have scarcely a foot of water more than their draught; and as docking, as well as launching large ships, requires a close calculation of height of water, the state of the barometer becomes of additional importance on such occasions.

To render these pages rather more useful at sea, in any part of the world, a few words about squalls and hurricanes are here offered to the young seaman.

Generally, squalls are preceded, or accompanied, or followed by clouds; but the very dangerous "white squall" (of the West Indies and other regions), is indicated only by a rushing sound, and by white wave crests.

"Descending squalls" come slanting downwards, off high land,[27] or from upper regions of atmosphere. They are dangerous, being sometimes violently strong.

A squall cloud that can be seen through or under is not likely to bring, or be accompanied by, so much wind as a dark continued cloud extending beyond the horizon. How the comparative hardness or softness of clouds foretells more or less wind or rain, was stated in pages 13 and 14.

The expressions "hardening up," "softening," or looking "greasy," are familiar to seamen: and such very sure indications are the appearances so designated, that they can hardly be mistaken.

The rapid or slow rise of a squall cloud—its more or less disturbed look—that is, whether its body is much agitated, and changing form continually, with broken clouds, or scud, flying about—or whether the mass of cloud is shapeless and nearly quiet, though floating onwards across the sky—foretells more or less wind accordingly.

An officer of a watch, with a good eye for clouds and signs of changing weather, may save his men a great deal of unnecessary exposure, as well as work, besides economising sails, spars, and rigging.

In some of the "saws" about wind and weather, there is so much truth that, though trite and simple, their insertion here can do no harm.

To these short expressions—well known, in practice, to the experienced; a very concise but sure rule may be added, for avoiding the central or strongest part of a hurricane, cyclone, typhoon, tornado, or circling storm.

With your face towards the wind, in North latitude, the centre of the circling, or rotatory storm, will be square to your right. In South latitude, square to your left.

The apparent veering of the wind, and the approach or retreat of the dangerous central circle, depend on your position in the curvilinear whirl or sweep.

Draw a circle;—mark the direction of the rotation or circulation, by an arrow with the head towards the left hand (against the movement of a watch's hands) in North latitude; but towards the right (or with the hands of a watch) if in South latitude. The direction of the wind, and the bearing of the centre, show your position in the meteor, for such it is, though perhaps hundreds of miles in diameter; and the veering of the wind, or the contrary, and its change in strength, will show how the meteor is moving bodily—over an extensive region, revolving horizontally—or inclined at a certain angle to the horizontal plane.

If the observer be stationary, in North latitude, and the centre pass on his polar side, he will experience a change of wind from Southward by the West towards North; but if it pass between him and the Equator, the change will be from Southward by the East towards North; but otherwise in South latitude, as his place in circles sketched will show more clearly than words. The roughest sketch or diagram, indicating the various directions of wind, and the course of the meteor's centre, will show more plainly than descriptions—which must necessarily vary with each case, and are tedious.

Cyclonology, or really meteorology, is simple enough in these great characteristic effects; but their causes must be the philosopher's study, rather than that of the young practical seaman.

Were it not for this reflection, one might endeavour to show how all the great Easterly trade winds—the no less important anti-trades,[29] or nearly constant Westerly winds,—and their complicated eddying offsets, are all (on greater or smaller scales) breadths, or zones of atmosphere, alternating, or circulating, or crossing (superposed or laterally)—between which, at distant intervals, occur those strong eddies, or storms, called hurricanes—typhoons—tornadoes—or cyclones.

The great easterly and westerly movements—so clearly shown by philosophers to be the consequences of cold polar currents of air—warm equatorial currents—and diurnal rotation of the earth;[30] are grand ruling phenomena of meteorology—to which storms, and all local changes, occurring but occasionally, are subordinate and exceptional. Further investigations into electrical and chemical peculiarities will probably throw additional light, perhaps the strongest, on meteorological science.

In the previous observations, general reference has been made to mercurial barometers of the ordinary kind; but, excepting the construction of the instruments themselves, those observations apply to all barometers, wheel—aneroid—or metallic—and likewise, of course, to the sympiesometer, which is a modified barometer. But as these four last-mentioned instruments are scarcely so familiar as the simplest form of barometer, it may be useful to add a few words about each of them.

The Wheel barometer has a syphon tube, partly filled with mercury, on which, at the short or open end of the tube, a float moves, to which a line is attached that moves a wheel, carrying an index.[31]

Aneroid barometers, if often compared with good mercurial columns, are similar in their indications, and valuable; but it must be remembered that they are not independent instruments; that they are set originally by a barometer,[32] require adjustment occasionally, and may deteriorate in time, though slowly.

The aneroid is quick in showing the variation of atmospheric pressure, and to the navigator who knows the difficulty, at times, of using barometers, this instrument is a great boon, for it can be placed anywhere, quite out of harm's way, and is not affected by the ship's motion, although faithfully giving indication of increased or diminished pressure of air.[33] In ascending or descending elevations, the hand of the aneroid may be seen to move (like the hand of a watch), showing the height above the level of the sea, or the difference of level between places of comparison.[34]

The principle on which it is constructed may be explained in a few words, without going into a scientific or too minute detail of its various parts. The weight of a column of air, which in a common barometer acts on the mercury, in the aneroid presses on a small circular metal box, from which nearly all air is extracted; and to this box is connected, by nice mechanical arrangement, the hand visible over the face of the instrument. When the atmospheric pressure is lessened on the vacuum box, a spring acting on levers, turns the hand to the left, and when the pressure increases, the spring is affected differently, the hand being turned to the right. It acts in any position, but as it often varies several hundredths with such a change, it should be held uniformly, while read off.

The known expansion and contraction of metals under varying temperatures, caused doubts as to the accuracy of the aneroid under such changes; but they were partly removed by introducing into the vacuum box a small portion of gas, as a compensation for the effects of heat or cold. The gas in the box, changing it bulk on a change of temperature, was intended to compensate for the effect on the metals of which the aneroid is made. Besides which, a further and more, reliable compensation has lately been effected by a combination of brass and steel bars.[35]

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.