218 PNEUMATICS, 



At 1000 feet above the sea, the column falls to 28.91 inches. 

 2000 " " " » " 27.86 " 



3000 » " " " " 26.85 " 



4000 " " " " " 25.87 " 



5000 " " " " » 24.93 " 



1 mile " " " " 24.67 " 



2 " " " « u 20.29 " 



3 " " « " " 16.68 " 



4 " u u u u 1373 u 



5 " " " " u 11.28 " 

 10 " " " " " 4.24 " 

 15 " " " " " 1.60 " 

 20 " " " " '< 0.95 » 



At the level of the sea, the barometer falls about 

 one hundredth of an inch for a rise of nine feet, or a 

 little more than the tenth of an inch for a rise of one 

 hundred feet. At a height of one mile it requires about 

 eleven feet rise to sink the mercury a hundredth of an 

 inch. 



In selecting land in mountainous districts of the 

 country, w^here degrees of frost increase with increased 

 altitudes, and .where the height of one portion above 

 another has an important relation to the cost of draw- 

 ing loads up and down hill, the barometer might be- 

 come of much practical value. 



THE SYPHON. 



The si/phon operates on a principle quite similar to 

 that of the pump ; but, instead of pumping out the air 

 of the tube through which the water rises, a vacuum 

 is created by the weight of a column of water, in the 

 following way : Fig: 181 represents a syphon, which is 

 nothing more than a tube bent in the form of a letter 

 U inverted. Now, if this be filled throughout with 



