MOTION, MECHANICS, ETC. 



145 



ing a ball, which by a hole 

 is made to slip easily up and 

 down. In the centre beneath 

 is a third ball, resting loosely 

 on the table. If the ball A 

 falls, it would strike the ball 

 c, and drive it to D ; if B falls, 

 it would drive it to E, but if 

 both A and B are suffered to 

 fall at exactly the same time, 

 the ball c would obey the 

 impulse of both at once, and 

 take an intermediate course ; 

 that is, be propelled across the board to E. 



Price, $5 



Fig. 152. 



Rocking Horse. (Fig. 152.) 

 This is an amusing figure, represent- 

 ing a horse and rider, and sometimes 

 used to illustrate the effect of plac- 

 ing the centre of gravity very low. 

 The figure is a light toy in the form 

 of a horse, having a wire and heavy 

 ball fastened to it, as represented 

 in the cut. If the hind feet of the 

 horse be placed on the table with- 

 out the ball being attached, the 

 centre of gravity will be beyond 

 the table, and the horse will of 

 course fall. But if the ball be at- 

 tached the centre of gravity will be below, and the line of 

 direction fall within the table ; the horse will consequently 

 be supported, and may be vibrated in an amusing manner. 



Price, 75cts. 



Also a variety of blocks, wheels, and figures, illustrating 

 the centre of gravity. Price, 50cts. to $1.50. 



The Mechanic Powers. The following instruments are 

 included under a set of mechanic powers: 1. Several sys- 

 tems of various kinds of pulleys. 2. The various kinds of 

 levers. 3. Inclined plane and carriage. 4. Wheel and 

 axle. 5. Wedge. 6. Screw. Also a set of weights to 

 use with them. 



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