THE BANDILORK 103 



igth of the second strip will vary between O m> 8 and 2 m . The 

 threads are fixed either by pins or by drawing the ends of the threads 

 through small holes made in the paper, and tieing knots inside. 



Fig. 73 shows sections of two forms of the bandilore, a 

 >y once very common. It consists of two discs united 



the centre by a short thin cylinder. At A the central 



>rtion is thicker, but the form shown at B is better. 

 A small hole passes through the cylinder, through which 

 a fine cord is drawn, about l m long, and fastened by a 

 knot. The string is wound up so as nearly to fill the 

 groove between the discs, and the end held in the 

 hand while the bandilore is allowed to fall. The string 

 becomes unwound, and the bandilore revolves more and 

 more rapidly. The fall is slower than in a freely falling 

 body, because a portion of the work done by gravity is 

 expended in producing the rotatory motion. That 

 work becomes accumulated in this form, and the rota- 

 tory motion continues when the bandilore has reached 

 the end of the string. The string, therefore, becomes 

 wound up again, but in a reverse direction, and the 

 bandilore rises. It would rise again to the point of 

 starting, if part of the accumulated work were not lost 

 in various ways, especially by the friction of the string* 

 The loss may, however, be compensated by jerking 

 the hand which holds the string gently but firmly 

 upwards at the moment when the toy arrives at the end 

 of the string; it can thus be kept flying up and down 

 for any length of time. 



14. Equilibrium. Centre of Gravity. The Balance. 

 Cut any irregular figure, for example one like that in fig. 

 74, out of a piece of stout pasteboard, 20 or 30 cm square. 

 At a point near the edge, as a,-make a hole with the 



