BAROMETER. A well-known instru ment for measuring the weight or pres sure of the atmosphere. The invention of the barometer was in some degree owing to an accident. Some workmen employed by the Duke of Florence to prepare a sucking-pump for a deep well, Mond to their surprise that notwith standing the care in forming and fitting the valves and piston, the water would not rise higher than 18 palms, or about 32 English feet. For an explanation of this unexpected difficulty they applied to the illustrious Galileo, then passing the evening of his life at his villa near Arcetri ; but the philosopher was not yet prepared with the true answer. In that age the doctrine of a plenam was an axiom in philosophy ; and the ascent of water in the barrel of the pump was uni versally ascribed to nature's horror of a vacuum. Galileo, either fearing to en counter further persecutions by pro pounding opinions at variance with the prejudices of the times, by the prevailing metaphorical modes of expression, evaded the difficulty by say ing that the power of nature to overcome a vacuum was limited, and did not ex ceed the pressure of a column of water 82 feet in height. That he was himself little satisfied with this explanation, is evident from the circumstance that pre viously to his death, which happened soon after, in 1642, he earnestly recom mended to his pupil Torrieelli to under take the investigation of the subject, which the infirmities of advanced age no longer permitted him to prosecute. Tor ricelli, suspecting the true cause of the suspension of the water, namely, the weight of the atmosphere, happify con ceived the idea of trying the experiment with mercury. He perceived that if the weight of the atmosphere forms a coun terpoise to a column of water of 32 feet, it must also counterpoise a column of mercury of about 28 inches in height, the weight of mercury being about 14 times greater than that of water. Hav ing accordingly procured a glass tube of about 3 feet in length and a quarter of an inch in diameter, hermetically sealed at one end, he filled it with mercury ; and covering the open end with the fin ger, hg immerged it in an open vessel containing mercury. On bringing the tube to the vertical position, and remov ing the finger, the mercury instantly sank, leaving a vacuum at the top of the tube, and, after making several oscilla tions, stood in the tube at the height of about 28 inches above the surface of that in the vessel. On covering the mercury in
the vessel with a portion of water, and rais ing the tube till the lower end came into contact with the water, the mercury all ran out, and the water rushed up to the top of the tube. This experiment, called after its author the Torrieellian etperiment, de monstrated that the mercury was sustain ed in the tube, and the water in the barrel of the pump, by exactly the same counterpoise, whatever the nature of it might be. Torricelli died shortly after, in the flower of his age, without com pleting his great discovery ; but the fhme of his experiment was soon carried into other countries, and the subject engaged the attention of the most eminent phi losophers; among others the celebrated Pascal. After a variety of ingenious ex periments on the subject, all of which tended to establish the pressure of the atmosphere, it at length occurred to Pas cal that if the mercurial column was really supported by atmospheric pres sure, it must be affected by the weight of the superincumbent mass of air, and consequently be diminished at considera ble elevations. In order to verify this conjecture, he requested his brother-in law, Perier, to try the experiment on the Pay de Dame, a lofty conical mountain in the province of Auvergne, which rises to the height of 500 toises. At the foot of the mountain Perier filled two tubes, and observed the mercury in each to stand at precisely the same height, nearly 2S English inches. Leaving one of them under the care of a person to watch its rise and fall, lie carried the other to the top of the mountain ; and on repeating the experiment there, the mercury stood at the of only 24•7 English inches. At two intermediate stations in his de scent, the mercury was observed succes sively to rise, and at the foot of the moun tain it stood at exactly the same height in the tube as at first. This experiment was decisive ; the result of it was com municated to Pascal at Paris, who, after confirming it by similar observations made successively on the ground, and at the top of a glass-house and the belfry of a church, proposed the barometer as an instrument for measuring the height of mountains, or the relative altitudes of places above the surface of the earth.