THE MYOGRAPH. 55 



In order to make the calculation, the rate of rota- 

 tion of the cylinder must be accurately known, as 

 uniformity in the time of reofistration of vibrations is 

 not effected by the apparatus. As we have already 

 seen, the rate of rotation of the cylinder is not uniform, 

 but increasing ; owing, however, to the weight of the 

 disc B and of the immersion in oil, the increase is very 

 gradual, and when a certain speed has been attained 

 the resistance offered by the oil is so great that no 

 further increase occurs and the speed remains constant. 

 By means of the hand on the face d this speed can be 

 determined ; and it is easy to cause the cylinder to 

 make exactly one revolution per second by adjusting 

 the oil vessel of the apparatus. 



The desired speed having been attained, it is only 

 necessary to know the circumference of the cylinder in 

 order to calculate the time value of that which is 

 marked on the cylinder. In order to facilitate the 

 measurement of the separate portions of the curve, 

 the cylinder, after being carefully removed from its 

 axis, must be fastened into a suitable forked handle 

 (such as is represented in the left-hand lower corner of 

 fig. 17, where it is marked E), and the cylinder must 

 then be rolled on a sheet of moistened gelatine paper. 

 The whole layer of soot adheres to the sticky gelatine ; 

 and the whole must then be fastened with the blackened 

 side downward on to a white ground. The described 

 curves will then appear in white on a black ground, 

 and will admit of easy measurement. 



Fig. 18 is accurately copied from a curve described 

 in this w^ay by the calf-muscle of a frog. The point at 

 which the irritation occurred is marked z. It will at 

 once strike the observer that the rising of the indicator 



