30 INTRODUCTION. 



heavier ones, as the instrument has to displace a larger bulk of liquid 

 in the lighter than in the heavier liquid in order to displace its own 

 weight. 



"Weight of gases. We have so far considered the gravity of solids 

 and liquids only, and the next question will be : Do gases also possess 

 weight are they also attracted by the earth ? The fact that a gas, 

 when generated or liberated, expands in every direction, might indi- 

 cate that the molecules of a gas have no weight, are not attracted by 

 the earth. A few simple experiments will, however, show that gases, 

 like all other substances, have weight. Thus a flask from which the 

 atmospheric air has been removed will weigh less than the same flask 

 when filled with atmospheric air or any other gas. 



Barometer. A second method by which may be demonstrated 

 the fact that atmospheric air possesses weight, is by means of the 

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

 earth with a layer some 50 or 100 miles in thickness, exerting a con- 

 siderable pressure upon all substances by its weight. The instru- 

 ments 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 contain- 

 ing 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 jpressure js equal 

 to about JJ) 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 mercury, but as the 

 height of the column must always bear an inverse proportion to the density of 



