PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. 423 



vertical position, then placed the lower end into a basin of mercury, and 

 withdrew his finger from it, when the column fell about a foot and a-half, 

 and remained stationary at a height of about twenty -nine inches. His next 

 experiment was to pour water into the basin containing mercury, and raise 

 the open end of the tube above the mercury; the result was the falling of 

 the mercurial column and the filling of the entire tube with water. The in- 

 ference drawn by Torricelli from this simple and beautiful experiment was, 

 that the force which raised the water and mercury columns was due to the 

 pressure of the air upon the liquids in the basin. 



Three years after this, Pascal, of France, commenced his experiments 

 with the Torricellian tube, which resulted in proving that the density of 

 the atmosphere diminishes in proportion to its elevation above the earth. 

 In 1053 he directed his brother-in-law, Perier, who resided in the mountain- 

 ous district of Avergne, to prepare two barometers and ascend the hio-hest 

 peak in his neighborhood, with one of them, leaving the other at its base 

 Arriving at the summit, Perier observed the column stood at the height of 

 24. T inches; on descending to the base, the columns in each barometer 

 stood at 28 inches. This instrument has been of great service in measur- 

 ing the altitude of mountains, especially on this continent; but its most im. 

 . portant use is in ((bserving the constant changes in the density of the air 

 Since the discovery that the falling column indicates rain, and the rapid 

 falling a coming storm, the barometer has been considered indispensable to 

 the navigator, and has been often of great benefit to the intelligent farmer 

 especially during the growing season. 



Objections have been raised to the mercurial barometer, owino* to its 

 size, and the care required in carrying it from place to place. These have 

 been obviated by the metallic barometer. The most noted of this class is 

 that invented by Vidi, of France, called the Aneroid barometer, from the 

 Greek word aneromai, [1 enquire). It consists of two corrugated disks 

 of brass, united to a ring, so as to form a chamber, from which the air is 

 exhausted. The centres of the disks approach each other in proportion as 

 the atmospheric pressure increases, and recede as the pressure diminishes 

 This motion of the disks is changed by ingenious mechanism, so that the 

 axis of motion is perpendicular to the face of a graduated circular scale, 

 over which moves an index hand. The whole is compact, not exceeding ' 

 live inches in diameter and an inch and a-half iu depth. Another was in- 

 vented in 1850, by Bourden, depending on the action of a concave and a 

 convex thin plate, cut in the shape of a segment of a circle. The manufac- 

 ture of metallic barometers has been carried on quite extensively in this 

 country, by Mr. Beaumont, specimens of which will be presented for your 

 inspection this evening. 



Mr. V. Beaumont here exhibited different sizes of his Aneroid Barometers 

 and explained as follows: 



The Aneroid barometer was invented in 1793 by M. Conte near Paris in 

 France. In making balloon ascensions for the army he found the mercurial 

 barometer very difficult to read on account of the violent oscillation of the 

 balloon. lie therefore invented a metallic air tight vacuum box similar to 

 a watch case, between the flat sides he put a spring to press the lid out 

 after the atmosphere became lighter; to the lid was attached a suitable 



