MEASUREMENTS 



311 



the adherence of the valves to the seatings ; these seatings are 

 made narrow to diminish their adherence to the valves. 



The external air which the subject is to breathe must be pure, 

 composed of 21% of oxygen and 79% of nitrogen by volume. 

 He should therefore be placed in the middle of a large room, 

 well ventilated, or, better still, fresh air from outside, taken at a 

 certain height from the ground, is brought in by a glass or rubber 

 tube (see fig. 137). 



In experiments on locomotion the preceding installation is not 

 suitable if the subject covers any great distance. Therefore the 

 study of walking requires special arrangements, to which further 

 reference will be made. 



240. When the valve is in place, the expired gases are conducted 

 by a rubber tube to a meter called a spirometer. The ordinary 

 spirometer indicates, in decalitres, litres and centilitres, the air 

 expired in a given time. Two readings, taken before and after 

 the experiment, give the volume of the air that has circulated 

 in the lungs. A deep expiration through the spirometer gives 

 what has been called the vital capacity ( 79). The rhythm of 



Ordinary spirometer. 



the respiration can be ascertained by counting the number of 

 movements of the large pointer irj. a minute. 



The ordinary spirometer is not generally accurate, nor is its 

 error constant. If a known volume of air, V, is passed through 

 it and the dial indicates V -f a, the quantity a will equal the 



