PHI AND THETA 41 



life, have been elucidated by the instantaneous and 

 successive photographs of Marey. 1 One serious 

 difficulty in the way of any explanation consists in 

 the circumstance that a fulcrum, or point of resist- 

 ance, is required for any movement of rotation, and it 

 is not easy to see what fulcrum the falling cat can 

 employ. It had been previously conjectured that the 

 cat, at the moment of letting go its hold, might use 

 the supporting object as a fulcrum in order to rotate 

 its body as required. This was never a very likely 

 explanation, considering that the cat is usually dis- 

 lodged by surprise, and that the rotation caused by a 

 sudden shove-off would be pretty sure to continue too 

 long or not long enough. A second hypothesis 

 attributed the turning of the body of the cat to 

 the resistance of the air, but this is disposed of, like 

 the first, by examination of the photographs. The 

 cat, while falling, brings the feet round towards the 

 ground, first the fore feet, and then the hind feet. 

 This is accomplished by the twisting of the body. In 

 order to twist, one end of the body must be fixed, or 

 at least retarded in its revolution. When suddenly 

 let go, the cat gathers up her fore legs, pressing them 

 against her neck, and as near as possible to the axis of 

 the body. In this position their moment of inertia is 

 a minimum, that is, they are as free as possible to 

 rotate. But the hind legs are extended, so as to make 

 their moment of inertia a maximum, that is, to oppose 

 the strongest possible resistance to rotation. The 

 hind legs become for an instant a fulcrum, or 



1 Comptes Rendus, CIX., p. 714 (1894). The figures are 

 reproduced in Nature, Nov. 22, 1894. 



