The layers of atmosphere extend to about 50 miles from the surface of the Earth.

Mercury is 13.596 times heavier than pure fresh water, and 13:251 times heavier than ordinary sea water.

DESCRIPTION OF THE BAROMETER DIAGRAM.

The Barometer Diagram is one of the most useful records that can be kept at sea. It is ruled vertically with heavy lines for every day at noon, and light lines for every four hours; it is also ruled horizontally, with heavy lines for every inch and half-inch, and light lines for every twentieth of an inch: thus, each space represents one-twentieth of an inch of the barometric scale taken horizontally, and four hours taken vertically.

In this form the daily movements of the mercurial column should be recorded by a dot at the place corresponding to its height and the time of each observation. These dots being joined together, result in an irregular line, which shows at a glance what has been the range of the column during the immediately preceding days, by which, and not merely by the last observation, a judgment may be formed of the weather to be expected. If closely attended to, this diagram will prove to be of considerable utility, and will in some degree reward the trouble of keeping an accurate and regular record.

For practical purposes, the diagram should be ruled to the scale of one-tenth of an inch to every four hours, for a month of 31 days, and one-tenth of an inch to every twentieth of an inch of the barometric scale from 28 to 31 inches inclusive; thus, after allowing space for the last day of the preceding month, and end columns for figures of side scale, the dimensions of the diagram ought to be about 20 inches long by 6 inches deep.

INDICATIONS OF THE BAROMETER.

In the Temperate Zones at the sea level, the column. of mercury ranges from about 28 to 31 inches, while in the British Islands the average height is about 29-9 inches. If the mercury RISES steadily above its mean height, while the weather gets colder and the air becomes drier; north-westerly, northerly, north-easterly winds, or less wind, less rain, or snow, may generally be expected. If, on the contrary, the mercury FALLS, while the weather gets warmer and the air becomes damper; wind and rain may be looked for from the south-east, south, or south-west.

If the weather gets warmer while the mercury is high and the wind north-easterly, a shift of wind to the south may be looked for. If, on the other hand, the weather becomes colder while the wind is south-westerly and the mercury low, a sudden squall or perhaps a storm from the north-west, with a fall of snow if it be winter time, may be looked for.

The wind usually shifts with the sun, viz., from left to right in the northern hemisphere-a change in this direction is called VEERING. (In the southern hemisphere motion with the sun is, of course, from right to left.) Thus an east wind shifts to west through south-east, south, and south-west, and a west wind shifts to east through north-west, north, and north-east. If the wind shifts the opposite way, viz., from west to southwest, south, and south-east, the change is called BACKING, and it seldom occurs unless when the weather is unsettled.

In WINTER, after a prevalence of easterly winds, if the barometer begins to fall, and the thermometer to rise, the sky being densely clouded, a gale which commences to blow from south-east will veer to south-west, while the barometer continues to fall. As soon as the wind passes the south-west point, the barometer begins to rise and the thermometer to fall suddenly; then a heavy

shower of rain falls, and a strong west-north-west or north-west wind may follow, with a clear sky.

If the wind backs from the north-west towards west and south-west, the bad weather is very likely to continue. In WINTER, if the barometer suddenly rises very high and a thick fog sets in, it is a sure sign that the northeast and the south-west winds are fighting each other. Neither of them can make head against the other, and there is a calm; but there is great danger of a severe gale succeeding such a state of things.

The barometer begins to rise occasionally before the conclusion of a 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 a heavy fall of rain or snow.

A fall of half a tenth (05) or more in AN HOUR is a sure warning of a gale, which rises from the southward and veers to the westward as the mercury rises.

A sudden rise of the barometer is very nearly as bad a sign as a sudden fall, because it shows that the equilibrium of the atmosphere is unsteady.

Allowance should always be made for the state of the column during some days, as well as some hours previous, because it may be affected by remote causes as well as by changes in the immediate vicinity. As a general rule, the longer a change of wind or weather is foretold by the barometer, the longer will the presaged weather last; and conversely, the shorter the warning the less time will such weather continue. Sometimes severe weather from an equatorial direction, not lasting long, may cause no great fall of the mercury, because followed by a duration of wind from the polar regions; and at times the mercury may fall considerably with polar winds and fine weather, because a continuance of equatorial wind is about to follow.

The column of mercury in a good barometer usually

stands, on an average, some tenths of an inch higher with or before polar and easterly winds than it does with or before equatorial and westerly winds, of an equal strength and moisture, in all parts of the ocean. Thus, with a low but rising glass, finer weather is USUALLY expected, while instead of that, a strong wind perhaps accompanied by rain, hail, or snow, rises from the polar direction. Conversely, with a high but falling glass, wind or rain or both, are USUALLY expected, yet neither follow to any remarkable degree; only a change of wind from one quarter to another taking place. In the Tropics, at the sea level, a few tenths above or below 30 inches are seldom exceeded, except before Hurricanes, or in the Bay of Bengal during the respective Monsoons, when the mercury sometimes falls below 28 even to 27 inches. On some coasts the barometer is differently affected by the wind, as it may happen to blow from the sea or from the land, the mercury rising on the approach of the sea breeze, and falling previous to the setting in of the land breeze. The tides are so much affected by atmospheric pressure that a rise of one inch in the barometer (indicating an INCREASE of one-thirtieth part of the atmospheric pressure) will have a corresponding fall in the tides of about one foot. Conversely, a fall of one inch in the barometer (indicating a DIMINUTION of one-thirtieth part of the atmospheric pressure) will have a corresponding rise in the tides of about one foot.

Continued observations have proved the inter-tropical barometric tides, or diurnal rise and fall of the mercury. The column rises from about 4 a.m. till about 10 a.m., when it reaches its highest point; then falls till about 4 p.m., when it reaches its lowest point; it again rises till about 10 p.m., then again falls till about 4 a.m., and so If, in any inter-tropical region the mercury moves contrary to this rule, comparatively unfavourable weather may be expected. The rise and fall, or diurnal range, is

on.