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LECTURES TO SCIENCE TEACHERS. 



of the elastic tube, the distension progresses from its start- 

 ing point until it gets to the further end, and from thence 

 it returns again. In the one case as in the other, the 

 difference of time between the first wave and its repetition 

 depends, as you will readily understand, on the distance 

 from the terminal point of the system to the point at 

 which the observation is made. 1 



Having got thus far having before us an actual case, 

 a motion of the kind which presents itself in the living 

 organism, an apparatus by which I have translated that 

 motion to paper I will proceed by explaining to you the 

 principles of its construction, and how they may be applied to 

 the investigation of other vital movements. The apparatus 



FIG. 5. Graphical representation of the characters of a distension wave in its pro- 

 pagation from the near to the far end of a closed elastic tube and back again, 

 a, near commencement of tube ; 0, near the middle ; c, near the end. 



illustrates (1) the mode of immediate transmission commonly 

 used, and (2) one of the most important modes of indirect 

 or mediate transmission. In the apparatus you have had 

 before you, the motion of the tube which represents the 

 artery is communicated first to an indiarubber membrane. 

 (See Fig. 4.) Then the motions of that membrane are 

 written on the paper. Let me first explain how this more 

 simple result is attained. The simplest of all arrangements 

 to magnify and inscribe a linear motion of small extent 

 consists of a lever of the second order, of which one end 

 is pierced by a fixed axis, on which it can rotate freely, 



1 It is proper to note that in the schema described, the motions of 

 the arterial wall are only partly represented. In the arterial system 

 there is no terminal point or obstacle by which the wave is reflected. 



