Baro'meter. n.s. [from βάρος, weight, and μἐτρον, measure.]
A machine for measuring the weight of the atmosphere, and the variations in it, in order chiefly to determine the changes of the weather. It differs from the baroscope, which only shews that the air is heavier at one time than another, without specifying the difference. The barometer is founded upon the Torricellian experiment, so called from Torricelli the inventor of it, at Florence, in 1643; which is a glass tube filled with mercury, horizontally sealed at one end; the other open and immerged in a bason of stagnant mercury; so that, as the weight of the atmosphere diminishes, the mercury in the tube will descend, and, as it encreases, the mercury will ascend; the column of mercury suspended in the tube, being always equal to the weight of the incumbent atmosphere. Many attempts have been made to render the changes in the barometer more sensible, in order to measure the atmosphere more accurately; and hence arose a great number of barometers, of different structures. Dr. Halley observes, in the Philosophical Transactions, that in calm weather, when the air is inclined to rain, the mercury is commonly low; in serene good settled weather, high. On great winds, though unaccompanied with rain, the mercury is lowest of all, with regard to the point of the compass the wind blows on. The greatest heights of the mercury are on easterly and north-easterly winds, cæteris paribus. After great storms of wind, when the mercury has been low, it rises again very fast. In calm frosty weather, it stands high. The more northerly places find greater alterations than the more southern; and within the tropicks, and near them, there is little or no variation of the height of the mercury. The rising of the mercury forebodes fair weather after foul, and an easterly or north-easterly wind; its falling portends southerly or westerly winds, or both. In a storm, the mercury beginning to rise, is a pretty sure sign that it begins to abate. But there are frequently great changes in the air, without any perceptible alteration in the barometer. The alterations of the weight of the air, are generally allowed to be the cause of those in the barometer; but philosophers cannot easily determine whence those alterations rise in the atmosphere.
The measuring the heights of mountains, and finding the elevation of places above the level of the sea, hath been much promoted by barometrical experiments, founded upon that essential property of the air, its gravity or pressure. As the column of mercury in the barometer is counterpoised by a column of air of equal weight, so whatever causes make the air heavier or lighter, the pressure of it will be thereby encreased or lessened, and of consequence the mercury will rise or fall. Again, the air is condensed or expanded, in proportion to the weight or force that presses it. Hence it is, that the higher from the sea, in the midland countries, the mercury descends the lower; because the air becomes more rarified and lighter, and it falls lowest upon the tops of the highest mountains. Harris.
Gravity is another property of air, whereby it counterpoises a column of mercury from twenty-seven inches and one half to thirty and one half, the gravity of the atmosphere varying one tenth, which are its utmost limits; so that the exact specifick gravity of the air cannot be determined when the barometer stands at thirty inches, with a moderate heat of the weather. Arbuthnot on Air.