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THOMPSON'S COAST PILOT. 131 hours, sudden but not lasting changes of weather may be expected. If the alteration should be gradual, probably in the proportion of two or three-tenths to twenty-four hours, the weather indicated will be likely to last. One-fifth of the variation of the barometer in any climate may be considered as an indication of sudden change. If wind should follow rain, the wind may be expected to increase. 7s Rain following wind is likely to lull it, and the wind may be | expected to abate. EXPLANATION OF THE ANEROID BAROMETER. The graduation and figuring on the dial represents the per- pendicular scale of the mercurial barometer. The falling of the barometer is understood to be the passage of the long or steel index from the higher to the lower number of figures, which movement comes under the same atmospheric change in which the mercury passes over part of its scale. The short pointer is simply a register, and is only movable with the fingers, to be placed over the steel index, thus enabling the observer to see more readily any movement of the index. RULES FOR BAROMETRICAL OBSERVATION. 1st. There is no point at which the barometer must stand to indicate rain or wind. 2d. The judgment must be governed by the rising or falling of the barometer. 3d. The falling of the barometer indicates the approach of a storm, the extent of which will be proportionate to the amount and rapidity of the fall. 4th. Showers.--The barometer falls previously from four to twelve-hundredths of an inch, varying in time, from one to three hours. The greater and more rapid the fall, the more violent will be the shower, accompanied more or less with wind.