THE CIRCULATION OP THE BLOOD. 



201 



from the top of which a strong silk thread (.T) passes downward in-to the 

 groove on the pulley. 



This thread (j), after being twisted several times round a small pin 

 at the side of the lever, enters the groove in the pulley from above down- 

 ward, and then passes to be fixed to the lower part of the curve on the 

 piston-rod as shown in the smaller figure. 



The rise and fall of the lever (c) is controlled by the resistance to 



Fig. 170. Apparatus for recording the endoca; dial pressure. (Rolleston.) 



torsion of the steel ribbon (E), to the middle of whi;ih one end of the 

 lever is securely fixed by a light screw clamp (F). At some distance 

 from this clamp the distance varying with the degree of resistance 

 which it is desired to give to the movements of the lever are two hold- 

 ers (G.G') which securely clamp the steel ribbon. 



As the torsion of a steel wire or strip follows Hooke's law, the tor- 

 sion being proportional to the twisting force the movements of the 

 lever point are proportional to the force employed to twist the steel strip 

 or ribbon in other words to the pressures which act on the piston (B). 



To make it possible to record satisfactorily the very varying ventric- 

 ular and auricular pressures, the resistance to torsion of a steel ribbon 

 adapts itself very conveniently. 



This resistance can be varied in two ways, 1st, by using one or more 

 pieces of steel ribbon or by using strips of different thicknesses; or 2d, 



