BAROMETER. The word is derived from two Greek words. which signify the measurer of weight. This, the most valuable instrument for meteorological observations in the farmer's pos session, was invented about the middle of the 17th century, by Torricelli, an Italian philoso pher. It was found by Torricelli, that a column of water of about thirty-two feet exactly balanced the weight of the atmosphere which surrounds our earth, and that this was equal to the weight of a column of mercury of about twenty-60a inches. This column of mercury under various forms is the barometer. As the pressure of the atmosphere commonly varies with approaching changes in the weather, the consequent rise or fall of the mercury merely marks its amount; one end of the mercurial tube is hermetically sealed and is void of air, so that the mercury rises or falls in it unresisted ; but the other end of the tube is open, and the atmosphere forces the mercury through this, by pressure on the surface of the fluid mercury in the cistern. Thus, the atmosphere operates by its varying pressure. When, therefore, the mercury rises, the atmos pheric pressure is increasing; when it falls, the pressure is diminishing. The more dense the atmosphere, the higher the mercury will rise in the instrument. It is a popular notion that the atmospheric pressure must be greater when the air is thick and cloudy. The term density, when applied to the condition of the atmosphere and its relations with the barometer, means specific weight, without reference to its clearness or cloudiness. Vapor or moisture in the air always
lessens its weight, and the more vapor, whether this be invisible, or in the condensed states con stituting fogs and clouds, the less the weight or density and pressure upon the barometer. It is more from this rising and falling of the barome ter, observes Mr. Forster, that from its height or lowness, that we are to infer fair or foul weather. In very hot weather the falling of the mercury indicates thunder; in winter, the rising indicates frost; and in frosty weather, if the mercury falls three or four divisions, there will follow a thaw; but in a continued frost, if the mercury rises it will snow. When foul weather laappens soon after the falling of the mercury, it will not con tinue; and, on the contrary, you may expect, if the weather becomes fair as soon as the mercury rises, that it will be of short duration. In foul weather, when the mercury rises much and high, and so continues for two or three days before the foul weather is quite over, then expect a con tinuance of fair weather to follow . There are many catch-penny, cheap barometers of late years. To buy them is money thrown away. If the farmer wishes a barometer, he had better get a mercurial instrument at once, since the hrst cost will be amply re-paid in the end.