Dec. 23, 1875] 



NATURE 



145 



VI, 



WL\i 



by the graphic method, the moment of origin and the con- 

 formation of the wave produced in every part of an india- 

 rubber tube distended with fluid. A glance at the accom- 

 panying figure (Fig. i.) will indicate the method employed. 

 The wave is produced by the movement of the piston of 

 the pump at the left side of the figure. 

 It is transmitted along the continuous 

 horizontal tube in the foreground, 

 which is made to record the move- 

 ments of six equidistant spots twenty 

 centimetres apart, on the revolving 

 drum, covered with smoked paper, 

 in the background, by means of the 

 delicate system of double " tam- 

 bours,'" or elastic-covered drums and 

 levers which have been introduced 

 and so much employed by the author. 

 The tubes connecting the tambours, 

 being of the same material and of 

 equal length, any error from irregu- 

 larity in the rate of transmission 

 along them is avoided. A pair of 

 forceps, as in the figure, close the 

 experimental tube just beyond the 

 point of attachment of the last of the 

 tambours. 



When a positive wave, in other 

 words, one of compression, is trans- 

 mitted along this tube, thus arranged, 

 it is seen that the levers rise one after 

 the other, beginning at that nearest 

 the piston ; and that immediately the 

 last one begins to rise, a second wave 

 commences in the opposite direction. 

 There is still more to be learnt from 

 the curves recorded on the smoked 

 cylinder, which are reproduced in 

 Fig. 2, from an actual experiment. 

 In this figure the six undulations are 

 those of the six levers, the lowest 

 being that of the portion of the tube 

 nearest the piston, and the highest 

 that of the furthest end. The trace 

 of a chronograph vibrating fifty times 

 a second is given below the lowest 

 of the curves. Perpendiculars pro- 

 jected from the summits of each of 

 the curves upon the chronograph 

 trace would be separated by equal 

 intervals if, during the different parts 

 of its course, the rapidity of trans- 

 mission of the wave were uniform. 

 But it is seen from the figure under 

 consideration that, although in tra- 

 versing each 20 centimetres of the 

 tube the wave takes about one-fiftieth 

 of a second, and so travels at the rate 

 of about 10 metres a second, nevertheless its rapidity is 

 not absolutely uniform, being at its maximum at its orifice 

 of entry, and after it has become slower again, slightly 

 increasing in velocity in the neighbourhood of the closed 

 end. There is therefore a double change in the velocity 

 of the wave. 



When it has reached the extremity of the tube the wave 

 takes a reverse course, and returns through each of the 

 recording drums to the place from which it started. This 

 reflected wave is indicated by the down-turned arrows in 

 Fig. 2 ; the direct one and its secondary companions having 



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Fig. 



upturned arrows above them. By varying the distance of 

 the occluding forceps from the last recording tambour 

 the time of commencement of the reflected wave can be 

 similarly varied. 



It is interesting to observe' that in a paper on the 

 movement of the pulse-wave in the human arte- 



