78 Henry Riddell on 



psychology, the attempt to reconcile the known character of the 

 m;in with his statement thus put on record. 



From the description the wheel would seem to depend on 

 the same principle as that shown in the print. When this wheel 

 moves as each weighted lever reaches a certain point in its path it 

 falls into a position practically extending a diameter, but as the 

 wheel reaches another point in its rotation the same lever drops 

 into a position almost tangential. Hence, said the numerous 

 inventors of wheels of this class, the weights being much farther 

 from the centre during their descent than during their ascent, the 

 descending side of the wheel always overbalances the ascending, 

 and rotation becomes continuous and perpetual. A variation of 

 this design is frequently found in which the weights were replaced 

 by masses of quicksilver held in recesses so shaped that the same 

 change of position occurred as in the case of the weight carrying 

 levers. 



The other historical example I have referred U-. is the wheel 

 invented and constructed by Orphyreus, who was a councillor of 

 the Landgrave of Hesse Cassel. This machine caused a wonder- 

 ful sensation on the continent, and owes a great part of its 

 historic interest to the examination made of it by Gravesande, and 

 to his letter to Newton expressing his belief that the wheel was a 

 solution of the problem. In this letter Gravesande descnl^es the 

 wheel as a very light construction of wood closely covered with 

 canvas to prevent the interior from being seen. It was about 

 twelve feet in diameter and fourteen inches broad on the face. 

 When Gravesande moved it gently it always stopped as soon as 

 he took his hand off, but when he gave it any considerable 

 velocity in either direction, it immediately proceeded, gaining its 

 full velocity in one or two revolutions and continuing at about 

 twenty-five turns a minute, needing to be stopped by force when 

 required to cease moving. He tells us that the motion had con- 

 . tinued for two months in an apartment whose entrances were 

 locked and sealed to prevent any communication. After that 

 period, lest the machinery should wear out, the door had been 

 opened by order of the Landgrave, and the wheel stopped 



