APPARATUS FOR PHYSIOLOGICAL USE. 245 



is a matter of some nicety, but of much practical impor- 

 tance, for by measuring in succession different diameters, 

 we can determine the enlargement of volume, with much 

 more accuracy than by any other method that is applicable 

 to the human body. The apparatus consists of a frame, of 

 which the shape resembles that of the Greek letter II. One 



FIG. 7. A, tympanum ; B, ivory knob : B', rod which carries the knob opposed to 



B ; C, T tube by which A communicates on the one hand with the recording 



tympanum, on the other with an elastic bag D. The purpose of the bag is to 



i enable the observer to vary the quantity of air in the cavity of the tympana at 



will ; the tube leading to it is closed by a clip when the instrument is in use. 



end of the II rests, in case it is the antero-posterior diameter 

 that is to be measured, by an ivory knob against a dorsal 

 spine, while between the other and the anterior surface of 

 the chest a receiving tympanum is interposed, by which the 

 motions of the sternum are transmitted to the recording 

 tympanum. A tracing is obtained in which the original 

 motion is represented, amplified about twenty times. 



