Measuring the Velocity of Rotation. 249 
equals the number of vibrations per second of the fork, and 6 
equals the observed number of undulations in a single turn of 
the disk, then v=4. 
_A single wave, or even half of one, is sufficient for determina- 
tion if the length be measured in degrees, in which ease if d= 
the length of one wave in degrees, the formula will stand Vea 
If the rotation be very rapid the quickest possible touch is 
f 
white paper can be pasted upon it and smoked without burning, 
end it answers every purpose. To the certainty and ease o 
this method may be added another advantage, that the slightest 
touch needed for this cannot sensibly retard the motion of the 
a as any mechanical fixture attached for such a purpose 
ust do, 
Physical Laboratory, Bethany College, Bethany, W. Va.. January 19. 
