THE SIMPLE LEVER 25 



friction wheel, by means of clockwork or a running cord, which can be thrown 

 into contact with a second wheel on the axis of the drum (see fig. 87). For 

 slow rates the most satisfactory method is to rotate the drum by means of a 

 tangent screw. The drum represented in fig. 30 combines these two, so that 

 one or the other plan can be used by simply changing the position of a 

 lever H. The figure represents the base of the drum only, the cylinder and 

 upper fittings being the same as those seen in fig. 37. The axis of the drum 

 rests on the steel point of a short vertical rod round which a brass disc d 

 rotates in a collar. On the disc is a little upright e which is placed in 

 contact with a bar f screwed into the drum- spindle A. F and e are kept in 

 contact by a brass spring, so that the drum is rotated by the brass disc. The 

 coned prdley c is driven by a running cord, and on the same axis is a smaller 

 coned pulley K and a brass ring r covered with rubber. "When R is brought 

 into contact with the edge of the disc D the latter is set in movement, and 

 thus gives a rapid movement to the drum. The coned pulley K by an endless 

 cord drives a second pulley G on a second axis, the end of which is a screw. 

 The screw fits into a toothed projection on the rim of the wheel d, so that when 

 s and D are in contact, as in the figure, the rotation of s gives a slow move- 

 ment to the disc d. These two axes are fixed on a base pivoting about a 

 point hidden in the drawing by pulley c. When the handle h is carried over 

 to the left, the rubber disc r comes into contact with the disc d, and the screw 

 s is removed. "When, on the other hand, the handle H is to the right, R is 

 removed and s brought into contact with the disc. 



As a general rule, the various movements we have to record are 

 small in extent, and it therefore becomes necessary to magnify them 

 at the time we record them. This is usually effected by employing 

 some form of lever, the extremity of which is made of a writing-point, 



Fig. 31. — Muscle held in Muscle-forceps f and attached to Simple Lever l. 



and which is fixed, at a point near its axis, to the muscle or other 

 tissue whose movements are to be recorded. The degree of magnifica- 



