188 



J. A. POLLOCK. 



Reference will first be made to forms of stethoscope in 

 which the mechanical action is obvious. Types of such 

 instruments are shown iu section in figures 1 and 2, about 

 one half natural size. 



Fig. 2. 



In figure 1 a is a plate attached to the case of the instru- 

 ment by a ring of elastic material e. Parallel to this is a 

 fixed plate b. A layer of air, in communication with the 

 ears through rubber tubes attached to the hollow arms c, 

 is thus formed between two plates, the lower one of which 

 is capable of movement relative to the other. When 

 in use the knob p is placed on the spot where it is desired 

 to determine the nature of the mechanical movements. In 

 figure 2 a is a base of thin flexible material. 



Such instruments possess two main features, very 

 familiar to seismologists, a comparatively large mass — the 

 case of the apparatus — and an elastic attachment connect- 

 ing the mass to the surface which supports it. 



The equations of motion appropriate todynamical systems 

 like these stethoscopes, when subject to vibration, are 

 similar to those discussed with reference to the theory of 



