66 THE ATMOSPHERE. 



surface, and thereby increasing its pressure. The ap- 

 plication of cold condenses the vapor ; and as the air 

 has been previously driven out by boiling, the pressure 

 is so much diminished, as to offer but little resistance 

 to the escape of vapor from the bottom of the fluid in the 

 form of bubbles. 



THE BAROMETER. 



The first instrument we shall describe illustrative of 

 the pressure of the air, is the Barometer. This name 

 is derived from the Greek, and signifies a measurer of 

 weight. With this instrument, the famous experi- 

 ment of Torricelli was made, which he communicated to 

 his friend Viviani, who repeated it in 1643. 



The barometer consists of a glass tube about thirty- 

 four inches long, sealed at one end, which, being filled 

 with quicksilver, is inverted in a vessel or cup of the 

 same material. The tube being now held perpendicu- 

 larly, the fluid will subside from the top, and stand at that 

 height by which it is balanced by a column of atmo- 

 sphere extending from the surface of the earth to its 

 utmost height. The average height of the quicksilver 

 is about thirty inches at the level of the sea. It is main- 

 tained at a certain elevation by the pressure of the air 

 on the surrounding fluid, while that portion over which 

 the tube stands has been relieved from the weight. If 

 water were substituted for quicksilver, it would be sup- 

 ported at the height of thirtytwo feet, because the quick- 

 silver is about fourteen times heavier. 



The barometer is commonly used as a weather-glass, 

 and as such, it gives evidence of the changes that are 

 about to take place. The plate connected with the upper 

 part of the tube, is divided into inches and tenths. A 

 movable point, called a vernier, subdividing this division 

 into tenths and hundredths, moves through the centre of 

 this plate perpendicularly. By placing the vernier at 

 the exact height of the quicksilver, we have the height 

 in inches, tenths, and hundredths. The words marked 

 on the plate are not so much to be regarded as the mo- 

 tion of the fluid ; for a deviation from the highest point 

 may be followed by rain, although the quicksilver may 



