The barometer is used for measuring the pressure of the atmosphere. The
principle of this instrument was first discovered by Torricelli, a pupil
of Galileo, the great Italian philosopher and scientist, in 1643. Many
and various types of instruments have been made, but the two most
generally used, especially where accurate indications are desired, are
the mercurial and aneroid barometers. Either of these instruments are
quite sensitive to changes in the weight or pressure of the earth’s
atmosphere, and from their variations we are able to draw conclusions
relative to changes in the weather. Figs. No. 41 and 42 illustrate the
standard mercurial and aneroid barometers used most extensively today. A
description of these barometers will serve to make the photographs
clearer to the readers of this text.