IXERTIA. 47 



other so as to form a small perpendicular column. 

 If the lowest man is pushed gently along, the whole 

 column will move forward ; the friction between the 



men being sufficient to com- I 



municate the motion from each I _{ 



man to the next above. But if i T 



the lowest is pushed somewhat r^ J-* 



more quickly, the second does jA r 



not acquire the same velocity, p- S 

 but will move more slowly, the J . 



next still more slowly, and so on, 



J 7 FIG. 42 (^ real size). 



till the column is upset (fig. 42). 



Finally, if the lowest man be rapidly struck with a thin 

 but heavy body, for example the back of a dinner-knife, 

 it will be seen to fly away, while the column remains 

 undisturbed and merely falls perpendicularly by a 

 height which is equal to the thickness of the removed 

 piece. The experiment succeeds best if the knife be 

 placed entirely on the table, so as to move it in a 

 perfectly horizontal direction. With a little practice 

 pieces may thus be removed from the middle of a column 

 without disturbing it. If a playing-card be placed upon 

 the mouth of a bottle, and a coin sufficiently small to fall 

 through the neck he laid upon it, the playing-card may 

 easily, in the same manner as the draughtsman be set into 

 a rapid motion by a sudden jerk with the finger, while 

 the coin drops into the bottle. Here the lighter card 

 receives sufficient velocity from the finger, while the 

 heavier draughtsman requires the heavier knife, that is 

 a body which is more capable of accumulating work. 



