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continuous surface with the upper wall of the horizontal tube. In 

 order to effect this continuity of surface, the press (Fig. 5) must 

 be curved correspond- 

 ingly to the wall of the 

 horizontal tube; and 

 it must be of such size 

 as exactly to fit the 

 piece which is want- 

 ing where the two 

 tubes, horizontal and 

 perpendicular, communicate. The whole of this mechanism is constructed 

 of brass. The piston must be removable from its perpendicular square 

 tube, so as to admit of the index ball, which I shall presently describe, 

 being attached to its press. When an observation is to be made, the piston 

 is to be taken out, together with the lid of the perpendicular square tube. 

 To the under surface of the press is then attached lightly the index ball, 

 which is made of gelatine. The arrangement is then placed in the position 

 as seen in the drawing (Fig. 2), and the blood is now brought through the 

 glass tube, the above preparations having been, of course, completed first. 

 When the time has come for making the observation, after the blood 

 has been circulating for a few seconds, the piston is pressed down, the 

 spring which kept it raised is overcome, and another spring is engaged 

 which keeps it down. The index ball is thus suddenly propelled into 

 the current, and, being of a certain size, is carried along the tube with 

 the same velocity as that with which the blood itself moves ; the lower 

 end of the press of the piston has at the same time formed the wanting 

 part of the upper wall of the horizontal tube, and the blood courses 

 without interruption. The index ball has not gone far before it comes 

 up to a scratch (Fig. I, i) on the glass index tube. As it passes this 

 point, the operator glances at the chronometer, and notes the time ; sub- 

 sequently, the ball passes another scratch (Fig. which is at a 

 known distance from the former one, and immediately as it does so, 

 the operator again consults the chronometer, notes the time, and turns 

 the stopcock to prevent the ball from passing into the peripheral end 

 of the artery. The observation is now completed, the index ball having 

 passed through a certain distance in a certain lapse of time, and thus 

 the velocity of the movement of the blood has been computed. 



For a long time I was puzzled how to obtain balls of such lightness 

 as would not affect the movement of the blood. After some consideration, 

 I adopted the following process : — A warm concentrated watery solu- 

 tion of gelatine is made, and the polished extremity of an iron rod is 

 rolled about in it, having been previously dipped in oil. The rolling 

 having been continued for some seconds, the rod is taken out of 

 the solution, and a thin coating of gelatine adheres to it. This 

 coating has now to be removed from the mould, which is done by 

 very gentle manipulation, until it is coaxed off. A small " ex- 

 hauster," furnished with a sharp extremity, and with a stopcock, is now 

 introduced into the open mouth of the gelatine ball, and, by working 



