vin ATMOSPHERIC PRESSURE AND BOYLE'S LAW 115 



mercury in the open and closed tubes must be subtracted from 

 the barometric height and not added to it. Why ? 



Application of Boyle's Law to the Atmosphere. It has 

 been learnt that if we increase the volume occupied by a given 

 mass of a substance we decrease its density, or if we decrease 

 its volume we increase its density. Therefore, we not only 

 alter the volume of the enclosed air in the two preceding experi- 

 ments, but we also change its density. Increase of density and 

 increase of pressure are proportional to one another. It is not 

 difficult to apply these facts to the case of the atmosphere. 

 We have learnt that the pressure of the atmosphere decreases 

 as we ascend, and we are now able to add that its density 

 decreases also and at the same rate. Therefore the densest 

 atmosphere will be that at the surface of the earth, leaving out, 

 of course, the air of mines and other cavities below the surface, 

 where the air will be denser still. The air gets less dense or 

 rarer as we leave the surface, until eventually it becomes so rare 

 that its existence is practically not discernible. 



CHIEF POINTS OF CHAPTER VIII. 



Determination of the Weight of Air. This i.s done by weighing 

 a stoppered flask (1) full of air, (2) after the air has been driven 

 out. The difference of weight is the weight of the air which filled 

 the flask. 



Pressure of Air. In consequence of its weight, air exerts pressure. 

 On the earth's surface, under ordinary conditions, the pressure of 

 the atmosphere equals 15 Ibs. per square inch. 



A Barometer is an instrument used to measure the pressure of the 

 atmosphere. 



The Principle of the Action of a Mercurial Barometer. The column 

 of mercury balances a column of air extending from the surface of 

 the mercury in the cup to the limits of the atmosphere. The action 

 is therefore analogous to balancing columns of different liquids in a 

 U-tube. 



Some Points referring to Barometers. (1) Mercury is the liquid 

 usually employed because it is the heaviest liquid known, is not 

 very volatile, is easy to see, and does not wet the tube. (2) Water 

 and other liquids can be used in the construction of barometers, but 

 these barometers require to be longer in order to hold the longer 

 liquid columns required to balance the atmospheric pressure. (3) If 

 a crack or a hole is made in the top of a barometer, air enters 

 through it, and the mercury column falls to the level of the liquid in 

 the cup. (4) The height of the column of mercury is about 30 

 inches at sea-level, but it varies from time to time on account of 

 variations of atmospheric pressure. 



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