KI.K.MKNTS OF GENERAL SCIENCE 



tilli-d. All of these common experiences, and many others, 

 teach us that air is a very real substance, even though it 

 is invisible, and that it occupies space as do other sub- 

 stances. This space-occupying property of air is useful to 

 mail in many ways. 



2. Working under water. In laying the foundations <>t 

 bridge piers and other structures which must rest below 

 water level (fig. 2), it is common for the laborers to work in a 



large steel chamber 

 which has no floor 

 and which rests 

 upon the bottom 

 of the lake or river 

 (fig. 3). This is 

 called a caisson. 

 The water is kept 

 out of the caisson 

 by the use of air 

 pumps which force 

 air into it with 

 sufficient pressure 

 to overcome the 

 pressure of the 

 water. The en- 

 trance and exit of 



workmen without allowing the escape of the air in the caisson 

 is made possible by means of a shaft and air lock. The shaft, 

 which contains a ladder, is a large, vertical, metal tube reach- 

 ing from above the surface of the water down to the caisson. 

 Within the shaft is a chamber, or air lock, with upper and 

 lower doors, the lower opening into the caisson. 



In case a workman wishes to enter the caisson, he first 

 enters the air lock through the upper doors, which are closed 

 after his entrance. Air is then forced into the air lock until' 

 its pressure is equal to that in the caisson. He then opens 



Fio. 2. A lighthouse upon a submerged foundation 



The foundation was constructed by men who worked 

 under water 



