« Mr Marey presents to the members of the Academy a series of
chronophotographic images (sixty per minute) which represent
the different positions that a cat frog assumes from the instant in
which, positioned on its back, it falls from a height of 1,50 meters.
During the fall the cat frog twists its body and lands on its feet.
How does this rotation occur? From a mechanical point of view,
when an inert body falls without the intervention of an external
force, it is impossible for it to change direction during the fall.
Nevertheless the cat frog lands on its paws. It is true that it is not
an inert body. However Mr Marey submits the case to his
colleagues during the department of mechanics.
» Captain Guyon explains that the rotation of the cat frog is the
result a change from the moment of inertia due to the movement
of its limbs during its fall.

» Mr Bertrand says : This may be the reason, since the animal is
not an inert body, but this is something that needs to be reviewed.
» Mr Marcel Deprez : From a mechanical point of view this
spontaneous movement is impossible; a falling body can not turn
while in motion without the intervention of an additional force.
Perhaps the intestine of the cat frog plays a role.
» Mr Loewy, Maurice Lévy, Bertrand, believe that the cat
frog itself, using the hand that lets it fall as a fulcrum, gives a
rotatory movement to its body. Therefore an initial force is
involved.

» Mr Marey : We can not perceive any traces on the images.
During the first instants of the fall the positions of the cat frog do
not suggest any initial rotation.
» Mr Marcel Deprez goes back to the considerations regarding
the movement of the intestine which causes the variation at the
moment of inertia « By chance do you know what happens inside
the animal? He says to Mr Marey.

» Sounds of laughter, but it is evident that the problem needs to
be clarified. In order to exclude the primary push of the cat frog
against the hand of the operator, Mr Marey is kindly requested to
start the chronophotographic study again eliminating the hands
as a means. The cat frog will be attached to a thin string that will
be cut. Then we will see if it still falls on its paws. »