168. =Amount of Atmospheric Pressure.=--If, instead of the jar _a_, in Fig. 114, you have a tube thirty-four feet high, and closed at the top, situated as the jar _a_ is, it will remain full of water. If the tube be longer the water will stand only at thirty-four feet, leaving a vacuum above it. It makes no difference what the size of the tube is; the result will be the same in all cases.[2] That is, a column of water thirty-four feet high can be sustained by the pressure of the atmosphere. It is easy, therefore, to estimate the weight or pressure of the air. The pressure of the column of water is found to be fifteen pounds to the square inch of its base, and this, of course, is the amount of pressure or weight of the atmosphere which it balances. Mercury is thirteen and a half times as heavy as water, and therefore the air will sustain a column of it only about thirty inches in height. [Illustration: Fig 116.] 169. =Barometer.=--The weight of the atmosphere varies to some extent at different times, and the barometer is an instrument for measuring these variations. It is constructed on the principles developed in the previous paragraphs. In Fig. 116 is a representation of the instrument. A B is a glass tube about 34 or 35 inches long, closed at one end. It has been filled with mercury, and then inverted in a cup of the same liquid, C. The vacuum above the mercury is called the Torricellian vacuum, from Torricelli, an Italian, who first developed the principles of the instrument. The mercury generally, as stated in § 168, stands at about the height of thirty inches. But it varies from this with the weather. When the weather is bright and clear the air is heavier than this, and, pressing upon the mercury in the vessel, forces it up higher in the tube. But when a storm is coming the air is apt to be lighter, and therefore pressing less strongly on the mercury in the vessel, the mercury in the tube falls. The barometer is of great service, especially at sea, in affording the sailor warning of an approaching storm. An incident is related by Dr. Arnot which strikingly illustrates its value in this respect. He was at sea in a Southern latitude. As the sun set after a beautiful afternoon the captain foresaw danger, although the weather was perfectly calm, for the mercury in the barometer had suddenly fallen to a remarkable degree. He gave hurried orders to the wondering sailors to prepare the ship for a storm. Scarcely had the preparations been made when a tremendous hurricane burst upon the ship, tearing the furled sails to tatters, and disabling the masts and yards. If the barometer had not been observed the ship would have been wholly unprepared, and shipwreck, with the loss of all on board, would have been the result.