PNEUMATICS. 147 



tube, so as to cause the water to exhibit the ap- 

 pearance of stars, wheels, &c. 



THE BAROMETER. 



This is an instrument for measuring the pres- 

 sure of the air. It was invented by Torricelli, in 

 the following manner. — Having suspected that the 

 pressure of the atmosphere was the cause of the 

 ascent of water in pumps, and supposing that mer- 

 cury, being fourteen times as heavy as water, the 

 air could only support a column of mercury -/* 

 part of the height of one of water, he took a glass 

 tube four feet long, and closed at one end: this he 

 filled with mercury, and kept his finger tight on 

 the open end; then, inverting the tube, he im- 

 mersed the open end, with his finger still in it, 

 under the surface of the mercury. As he expected, 

 the mercury subsided in the tube so as to stand 

 only about 27i- above the surface of the basin, or 

 equal to T ' T of the height of water in pumps. He 

 therefore inferred, that the water and the mercury 

 were kept up by the same cause, and that the differ- 

 ence between the heights of the two columns was 

 owing to the difference of the specific gravities 

 of the substances. 



It was afterwards observed, that the height at 

 which the mercury stood in this tube was not con- 

 stant, and, consequently, that the pressure of the 

 atmosphere was variable ; and also that this change 

 of height was connected with alterations in the 

 weather. 



The simplest and most common construction of 

 the barometer is shown, Plate 7. %• 2. It consists 

 of a glass-tube, secured from injury by being fixed 

 to a wooden frame ; the tube is closed by being 



l 2 



