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130 THOMPSON'S COAST PILOT. In very warm weather, the fall of the mercury indicates | thunder. The barometer will descend sometimes as an indica- tion of wind only, and sometimes rise when the wind is to the north or east. A NE wind generally causes the barometer .to rise, and it is generally low with a SW wind. An extraordinary fall of the mercury will sometimes take place in summer previous to heavy showers, attended with thunder; but in spring, autumn and winter, it indicates violent winds. | The mercury is higher in cold than in warm weather, and lower at noon and midnight than at any other period of the day. . The mercury generally falls at the appraach of new and full - moon, and rises at the quadratures. | Before high tides, there is almost always a great fall of the mercury; this takes place oftener at the full than at the new moon. The greatest changes of the barometer commonly take place during clear weather with a north wind, and the smallest risings during cloudy, rainy or windy weather, with a south or nearly south wind. : The words generally engraved on the plate of the barometer rather serve to mislead than to inform; for the changes of weather depend rather on the rising and falling of the mercury, than on its standing at any particular height. When the mer- cury is as high as " fair," and the surface of it is concave (which is the case when it begins to descend), it very often rains; and, . on the contrary, when the mercury is opposite "rain," and the surface convex (which is the case when it begins to ascend), fair weather may be expected. These circumstances not being duly attended to, may be said to be the principal cause of many people not having a proper confidence in this instrument. For sea service, it would be as well to read off the barometer at least three times a day, viz., at 8 a. M., noon, and 8 p. M.; and oftener if bad weather. In Europe, if the alteration in the quicksilver should be in' as great a proportion as six-tenths of an inch to twenty-four