3O SCIENTIFIC RECREATIONS. 



stops. In laboratories, wooden cylinders are often to be seen which will 

 ascend an inclined plane without any impulsion. This appears very surprising 

 at first, but astonishment ceases when we perceive that the centre of gravity 

 is close to the end of the cylinder, because of a piece of lead, which has 

 been fixed in it. 



Fig- 23 gives a very exact representation of a plaything which was 

 sold extensively on the Boulevards at Paris at the beginning of the New 

 Year. This little contrivance, which has been known for some time, is one 

 of the most charming applications of the principles relating to the centre oi 

 gravity. With a little skill, any one may construct it for himself. It con- 



Fig. 23. Automatic puppets. 



sists of two little puppets, which turn round axles adapted to two parallel 

 tubes containing mercury. When we place the little contrivance in the 

 position of fig. 24, the mercury being at #, the two dolls remain motionless, 

 but if we lower the doll s, so that it stands on the second step (No. 2) of 

 the flight, as indicated in fig. 25, the mercury descends to b at the other end 

 of the tube ; the centre of gravity is suddenly displaced ; the doll R them 

 accomplishes a rotatory movement, as shown by the arrow in fig. 25, and 

 finally alights on step No. 3. The same movement is also effected by the 

 doll S, and so on, as many times as there are steps. The dolls may be 

 replaced by a hollow cylinder of cartridge paper closed at both ends, and 

 containing a marble ; the cylinder, when placed vertically on an inclined 



