192 THE POPULAR SCIENCE MONTHLY. 



fingers slightly bent ; or with a light elastic hammer called the per- 

 eussor (Fig. 4). The finger of the other hand or a solid, flattened 

 disk, the pleximeter, must be held firmly against the chest to receive 

 the stroke and to educe the proper resonance. The percussion-sound, 

 though apparently unmusical, must have its intensity, be high or low, 

 and have its peculiar timbre — all requiring acuteness of hearing and 

 judgment in interpretation. 



The spirometer is of use for measuring in cubic inches the maxi- 



FiG. 4.— Flint's Percussor and Pleximeter. 



mum amount of respirable air, of which each individual has his normal 

 quantity, but which is subject to changes from organic affections of 

 the lungs. 



The cyrtometer is used for delineating the external contour of the 

 chest and for exact comparison of one side with the other. 



Numerous other instruments are of real utility, only one of which, 

 however, will be mentioned. 



The sphygmograph is an instrument of somewhat complicated mech- 

 anism. It is used to " feel the pulse " and to record its impressions. 

 It will give its frequency and rhythm, its varying tension and strength, 

 the condition of the heart and certain valves, with a delicacy and ex- 

 actitude which, compared with the results obtainable by the most sen- 

 sitive finger, are like the perfect work of photography compared with 

 the attempts of the juvenile charcoal artist. With its touch upon the 

 heart or its vessels, and its pen apparently in sympathy and vital con- 

 nection with them it will record in delicate but infallible tracery the 

 diagnosis, and mayhap the prognosis, of the subject under examination, 

 which may be read with trembling expectation. 



In this instrument the impulse of the blood-movements is commu- 

 nicated to the pen by water contained in flexible tubes. The oblong 

 receptacle, also containing water, is connected with one of these tubes. 

 It has on one side an elastic projection which is to be securely fixed 

 upon the pulse to be examined. All vibrations received by it are 

 transmitted by the water through the tube to the chamber. On the 

 upper surface of the chamber is a delicate membrane which receives 

 the vibrations with every requisite as to quality and exactitude. The 

 movement of its wave is, however, microscopic, and, in order to render 

 it visible and legible, an exceedingly light and sensitive lever termi- 



