• Maneuver of security:

If a cat is retained by its four legs with belly upward, and it is dropped, it will turn in less than half second around his own axis and will cushion the blow against the ground with the prim legs. It gives the sensation of which, after that turn of 180 degrees, it will not change of position until putting the legs on the ground.
The animal has to act quickly. After half second, the speed of its center of gravity reaches the 18 Km./h. Whereas the speed of fall "only" grows proportionally with time, the kinetic energy of the cat long ago more quickly and, with this one, increases the danger that it is injured in an unfortunate landing.
What happens in so brief time interval happens with vertiginous rapidity before our eyes, incapable to catch the details. According to information of some biologists, the instinct to turn itself to fall on foot is shared with hares, dogs, rabbits and monkeys. I have not verified it.

• Public call:

From immemorial time the man has observed that cat ability, but only in 1894 he began to consider it a "scientific problem". The Sciences Academy of Paris summoned a public aid on the "physical explanation of how the cat is able to always land on four legs when falling from high altitude". To the experts in mechanics it seemed to them that the turn had to be from the push distributed to the animal when it was released, that thus it would obtain a little angular moment in one or another direction. The cat, during its fall, could only turn part of the body moving simultaneously another part in opposite sense, in such a way that angular moments were compensated both. Total angular moment always is conserved; if it was zero in the beginning, it could not appear from anything some moment. In addition, to put simultaneously the front and back legs on the ground, a whole turn of its body was needed, which, according to the observed, was not the case.
But that hypothesis of the push was rejected after meticulous experiments in which, before the fall, cords to the legs were tied separately. It is possible to admit that with the interchange with the air of the surroundings it is not extracted either a little angular moment enough: even though the cat was shaken with violence, the upward aerodynamic forces and of friction could not contribute to the push necessary to make the turn.

• Many questions:

Until today the prejudice persists that it obtains the turn throughout his axis rowing vigorously with the tail in opposite sense. It is not a preposterous idea, in an animal that uses that appendix for the balance movements. But in this case a considerable tail would have to rotate like a helix. The experiments made with cats without tail show that they turn with the elegance of which they have it.
And although all the physical questions had been solved, it is left the physiology of the acquisition of the impulse and the control of the movement. The biologists have dropped cats, with the eyes covered or in dark enclosures, from heights that the animals could not know beforehand. The healthy individuals always fell on foot. They only lost such ability when the labyrinth of the internal ear was extirpated to them surgically along with the organ of the sense of the balance.

• Photography of high speed:

In the mentioned year of 1894, Etienne Jules Marey displayed two sequences of images, from different perspective, of the fall of a cat. One was a space-temporary representation of the turn that undertakes moving parts of their body in opposite sense. Marey, pioneer of the cinematography applied to Biology, invented in 1890 a camera that could take sixty images per second and that was already used successfully in the flight of the birds. When it projected six times more slowly, "like a temporary magnifying glass", the sequence of images with "zoetrope", and the eye still perceived them without continuity solution, as a "film".
The projection of the film untied a storm in the Academy. Some physicists doubted of which they saw: by principle, it was impossible that a body that fell acquired a turn by itself.

• The turn in two times:

From those images, Marey supposed that the cat turned in two times. In first, it perpendicularly extended his back legs to the axis of the body (with which it increased the moment of inertia of back half of the body for the axial turn), whereas simultaneously the axis folded its front legs towards a (and it reduced the moment of axial inertia of front half of the body). If the cat turned in a direction its front half, then it will rotate its back half in opposed direction, but more slowly, in inverse relation at the moments of inertia.
In a second time the feline stretched the front legs cross-sectionally and gathered the back legs along, so that the back part turned with greater angle. The final result was that two halves had turned in identical sense approximately the same difference of angle.
The call of the Academy had an immediate effect. The problem even appeared in the manuals and induced the physicists to think on turns without moment about the space. From decades to this part, the subject has returned to put itself fashionable by its potential application in the sport (gymnastic jumps), in the circus acrobatics (plinton, trapeze) and in the space trips.

• The cat as a spin:

The cats dominate in addition other tricks to fall of four legs. In series of photographies of magazine Life, that in 1969 T. R. Kane and M. P. Scher took as it bases of his analysis, could not be recognized in the animal no turn of the leather during the fall. Rather, it doubled his espinazo to the height of the hip. The authors, in their mathematical model, assimilated the cat to two rotors that turned with the same angular velocity, whose spin axes formed an angle. If it were straight the spine and the axes therefore in a same line, the animal could not turn indeed thus, without a little external angular moment.
Imagine the other extreme case: the cat could double its body as if it was a pocket knife. It is folded at the beginning of the fall, gut with gut, defeat both halves one against the other average return, until they are back against back, is opened and already it has the legs downwards. If, in addition, both halves of the body have the same moment of inertia, their respective angular moments will be compensated.
The normal case is between these two ends. The angular moments are not compensated totally. In order to compensate the remaining angular moment an opposite turn appears then of the body of the cat that looks itself like the precession of one spins. If some loosen to the feline without turn, will begin to turn at the moment in which it starts up his rotors, and will finalize, with equal rapidity, as soon as it returns them to stop.