238 LECTURES TO SCIENCE TEACHERS. 



What has become of it ? It has been lost in distending 

 the elastic wall of the tube. Let us next consider what is 

 the state of things immediately after the impulse, i.e. 

 between one stroke of the heart-pump and the next. At 

 that moment the rate of efflux exceeds that of influx. Hence 

 motion has reappeared. Where has it come from 1 ? It 

 has come from the distended elastic wall of the tube, which 

 is now repaying to its contents that which immediately 

 before it borrowed. So long as the pump continues to 

 work, this process, by which during the stroke motion is 

 lent by the contents to the wall, and repaid by the wall to 

 the contents during the interval, goes on. 



FIG. 4. Instrument for recording the progress of a wave of distension along an 

 elastic tube containing liquid. A may be described as a box two inches long 

 and one inch high, without sides ; it contains the receiving tympanum. B is the 

 recording tympanum and its writing lever. The tympanum t is a metal box 

 without a top, of such a size and shape as to contain a florin piece. Over this 

 , box a sheet of indiarubber is extended and fixed air-tight. To this membrane 

 a thin metal plate adheres, which is connected by a jointed vertical rod with 

 the long lever. The joint is very near the axis on which the lever moves. The 

 horizontal rod by which the tympanum is supported is tubular and communi- 

 cates with the cavity of the tympanum. To its end is adapted a flexible tube 

 d, by which the two tympana communicate, and which may be several feet in 

 length. The receiving tympanum is of the same form and construction as the 

 other, but is upside down, its bottom being screwed to the under surface of the 

 top of the box. If an elastic tube, along which liquid is being driven in an in- 

 termittent stream by a pump, is placed between the two brass pieces p p' shown 

 in the figure, the increase of diameter of the tube which occurs at each pulse 

 produces a vertical motion upwards of the membrane of the receiving tympanum, 

 and a corresponding movement in the same direction of the other. This last 

 motion is written on the blackened cylinder by the style at the end of the lever. 

 For investigating the course of a distension wave along a tube at least three 

 pairs of tympana are required. In using them the recording tympana must be 

 placed on the same vertical rod, one above another. 



This process of alternate distension and relaxation is 

 betrayed to us by visible motions, and my next object will 

 be to show you how we may obtain a graphical record of 

 these motions, and to explain to you their characters. 

 Let us first fix our attention on the instantaneous dis- 

 tension which is produced at the near end of the tube, i.e. 

 at the end next the pump, at each stroke, in consequence of 

 the injection of a certain quantity of liquid. If we arrange 



