Page:Elementary Text-book of Physics (Anthony, 1897).djvu/143

§ 114] density and the height of the liquid column, the pressure exerted by the fluid may be measured.

114. The Barometer.—The instrument which illustrates these principles, and is also of great importance in many physical investigations, is the barometer. It was invented by Torricelli, a pupil of Galileo. The fact that water can be raised in a tube in which a complete or partial vacuum has been made was known to the ancients, and was explained by them, and by the schoolmen after them, by the maxim that "Nature abhors a vacuum." They must have been familiar with the action of pumps, for the force-pump, a far more complicated instrument, was invented by Ctesibius of Alexandria, who lived during the second century B.C. It was not until the time of Galileo, however, that the first recorded observations were made that the column of water in a pump rises only to a height of about 10.5 metres. Galileo failed to give the true explanation of this fact. He had, however, taught that the air has weight; and his pupil Torricelli, using that principle, was more successful.

He showed, that if a glass tube sealed at one end, over 760 millimetres long, were filled with mercury, the open end stopped with the finger, the tube inverted, and the unsealed end plunged beneath a surface of mercury in a basin, on withdrawing the finger the mercury in the tube sank until its top surface was about 760 millimetres above the surface of the mercury in the basin. The specific gravity of the mercury being 13.59, the pressure of the mercury column and that of the water column in the pump agreed so nearly as to show that the maintenance of the columns in both cases was due to a common cause,—the pressure of the atmosphere. This conclusion was subsequently verified and established by Pascal, who requested a friend to observe the height of the mercury column at the bottom and at the top of a mountain. On making the observation, the height of the column at the top was found to be less than at the bottom. Pascal himself afterwards observed a slight though distinct diminution in the height of the column on ascending the tower of St. Jacques de la Boucherie in Paris.