tion attended with considerable risk to the tube and operator

and for this reason is seldom carried into practice; and in the majority of barometers made, it is altogether neglected. The diameter of the mercurial column averages four-tenths of an inch, so that, altogether, instruments of the greatest efficiency will

be obtained.

A

great change has also been effected

in the old system of making the scales with Fair,' 'Change,' 'Rain,' &c., words which in reality have often a tendency to mislead, and to throw discredit on barometrical indications.

The plan to be adopted in these barometers is shewn

in the accompanying engraving. This

The Fitzroy Barometer.

has been arranged systematically by the late Admiral Fitzroy, F.R.S., who devoted his life to studying the indications of the

barometer, and their true explanation. The meanings of the contractions marked on the dial of these barometers is explained by the Admiral as follows:

Moisture or dampness in the air (shewn by a hygrometer) increases before rain, fog, or dew.

Add one-tenth of an inch to the observed height of the barometer, for each hundred feet it is situated above the mean sea-level.

The average height of the barometer in England, at the sealevel, is about 29.94 inches; and the average temperature of air is nearly fifty degrees.

The thermometer falls about one degree for each three hundred feet of elevation, more than fifty feet, above the level of the ground.

DIRECTIONS FOR POSITION AND USE.

A barometer, for a coast weather-glass, should be placed where it may be seen at any time, in a good light, by fishermen, boatmen, seafaring persons, &c., and should be set regularly by a duly authorised person about sunrise, noon, and sunset.

The barometer at a fixed station should be suspended against a post or wall, in a well-lighted situation, at such a height that the middle of the scale shall be level with the eye of an observer, or about five feet above the ground. When possible, the barometer should be fixed, so that light may be seen through

* These rules apply to northern latitudes. For southern hemisphere, substitute S. for N., allowing E. and W. to remain unchanged.

the tube, or a piece of white paper, or of ivory, should be placed behind the glass, at the back of the scale. When the barometer is suspended by a strong nail or screw, take the milled-head key-that used for moving the index or vernier-and with it unscrew, to its fullest extent, the square brass pin at the lower end of the instrument, by turning it gently from right to left. The mercury will then descend in the tube to its true level, and the true height or length of the column will then be shewn by the scale and vernier.

To Read the Barometer.-With the milled-head key move the index at the side of the tube, until it is level with the top of the mercury; then note the number of inches and tenths of an inch on the scale, and the hundredths on the vernier at which the column stands; this, compared with the previous observation, will shew if the mercury has risen or fallen. If it is desired to remove the barometer, it may be rendered portable by inclining it gradually until the mercury flows nearly to the top of the tube; then, with the key screw up slowly the brass pin until the mercury fills the tube, avoiding forcing the screw, otherwise the tube or the cistern may be injured. When screwed up, the barometer may be carried with safety either with the cistern end upwards, or in a horizontal position.

The Marine Barometer, instead of being fixed as described above, should be suspended in gimbols like a compass, to prevent, as much as possible, the tendency of the mercury to oscillate by the motion of the ship; in other respects, the directions given above apply equally to it, except that unscrewing at the bottom will not so soon take effect on account of the contraction in the tube.

The barometer should be set regularly by an authorised person twice each day, say at nine o'clock in the morning and three o'clock in the afternoon.

On taking the reading every morning, a dot should be placed on a ruled scale, corresponding to the reading, and a line drawn from the last dot to the new one, and thus successively day by day from one point to the next, indicating

DAYS OF THE MONTH.

decrease about the point 29.5 inches till the end of the month.

OF WINDS AND THEIR CAUSES.

Wind is air in motion relatively to the surface of the earth, and is generally caused by the varying temperature of the air in different places, produced chiefly by the heating power of the sun's rays. When the air at any place becomes heated, its volume expands, and it becomes specifically lighter, and ascends (the barometer falls). The colder and denser air then rushes in from the neighbourhood to restore the equilibrium, thus producing wind. If the partial vacuum, formed as stated above, be of large extent, the air rushes in from various quarters, and the currents, striking each other obliquely, cause a rotation of the air round the point of least density, which appears to be the manner of formation of those circular storms frequently encountered at sea, and now generally known by the name of Cyclones.

Since the earth revolves on its axis, and the perpendicular distance of the earth's surface from this axis is nothing at the poles, and increases from there to the equator, if a wind blow in the direction of a parallel of latitude, all the parts of the earth over which it passes will have the same rotatory motion, and consequently will have no tendency to cause the wind to change its direction; it will therefore blow directly east or directly west. But if a wind proceed from the neighbourhood of the north or south pole towards the equator, it will pass over portions of the earth's surface, whose velocity towards the east is becoming greater at each advance; hence, as it advances towards the equator, it will gradually veer more and more towards the east, until it arrives in the vicinity of the equator; it will then blow directly from the east, and in this manner is formed the two belts of easterly wind, one on each side of the equator, called the Trade-winds.

The sun being always vertical to some portion of the torrid zone, the air there becomes much heated and rarefied, which causes it to ascend; and as the air from the north and south is