GRAVITATION. 29 



of the barometer. The atmosphere is that ocean of gas which 

 encircles the earth with a layer some 50 or 100 miles in thick- 

 ness, exerting a considerable pressure upon all substances by 

 its weight. The instruments used for measuring that pressure 

 are known as barometers, and the most common form of these 

 is the mercury barometer. It may be constructed by filling 

 with mercury a glass tube closed at one end, and about three 

 feet long, and then inverting it in a vessel containing mercury, 

 when it will be found that the mercury no longer fills the tube 

 to the top, but only to a height of about 30 inches, leaving a 

 vacuum above. The column of mercury is maintained at this 

 height by the pressure of the atmosphere upon the surface of 

 the mercury in the vessel; a column of mercury about 30 

 inches high must consequently exert a pressure equal to the 

 pressure of a column of the atmosphere of the same diameter as 

 that of the mercury column. 



As the weight of a column of mercury, having a base of one 

 square inch and a height of about 30 inches, is equal to about 

 15 pounds, a column of atmosphere having also a base of one 

 square inch must also weigh 15 pounds. In other words, the 

 atmospheric pressure is equal to about 15 pounds to the square 

 inch, or about one ton to the square foot. This enormous 

 pressure is borne without inconvenience by the animal frame in 

 consequence of the perfect uniformity of the pressure in every 

 direction. 



A barometer may be constructed of other liquids than mer- 

 cury, but as the height of the column must always bear an in- 

 verse proportion to the density of the liquid used, the length of 

 the tube required must be greater for lighter liquids. As water 

 is 13.6 times lighter than mercury, the height of a water column 

 to balance the atmospheric pressure is 13.6 times 30 inches, or 

 about 34 feet, which would therefore be the height of the 

 column of water required. 



Changes in the atmospheric pressure. The height of the mer- 

 cury column in a barometer is not the same at all times, but 

 varies within certain limits. These variations are due to a 

 number of causes disturbing the density of the atmosphere, and 

 are chiefly atmospheric currents, temperature, and the amount 

 of moisture contained in the atmosphere. 



