THE ATMOSPHERE 25 



air-pump or more simply by placiog some water in the flask 

 and heating il to boiling. The steam from the water drives 

 the air out of the flask, and if the pinch-cock be closed and the 

 flame removed from under the flask the steam within will 

 gradually condense. When quite cold the flask can be 

 accurately weighed. If the pinch-cock be then opened for a 

 moment air will be heard to enter the flask to take the place 

 of the steam which has condensed, and the flask will then be 

 found to have increased in weight, the increase being the 

 weight of the air which has entered. In this way it can be 

 shown that a litre of air weighs, under ordinary conditions, 

 about IJ grammes, or 1000 cubic feet weigh about 80 lb. 



In consequence of its weight, air is pulled down towards the 

 surface of the earth, and those portions nearest the surface are 

 compressed by the weight of those above; consequently all 

 bodies on the earth are subjected to the pressure of the air 

 above and around them, for air, like other fluids, transmits 

 pressure in all directions. The pressure exerted is very high, 

 amounting on the average at the sea-level to about 14| lb. 

 per square inch, or 1033 grammes per square centimetre. 

 This pressure is a direct measure of the weight of the air. 

 On every square foot of surface at the sea-lev«l, therefore, 

 there rests 14*75 x 144 = 724 lb. of air, or upon an acre the 

 total weight of air would be about 41,300 tons. 



The Barometer. — The pressure of the air is measured by 

 means of an instrument called a barometer. In its simplest 

 form it consists of a glass tube (preferably rather wide in bore) 

 about 32 or 33 inches long, closed at one end, open at the 

 other, filled completely with mercury and inverted into a 

 trough of mercury. It is then found that the mercury sinks 

 in the tube only a few inches, and comes to rest with the 

 mercury surface in the tube about 30 inches above that in the 

 trough. Equilibrium is reached when the weight of the 

 mercury column is equal to the weight of an air column of 

 the same sectional area whose base is at the mercury surface in 

 the trough or cistern and wliich extends to the utmost limits of 



