TEXT D The ability of falling
cats to right themselves in midair and land on their feet has been a source of
wonder for ages. Biologists long regarded it as an example of adaptation by
natural selection, but for physicists it bordered on the miraculous. Newton’ s
laws of motion assume that the total amount of spin of a body can- not change
unless an external torque speeds it up or slows it down. If a cat has no spin
when it is released and experiences no external toque, it ought not to be able
to twist around as it falls. In the speed of its execution, the
righting of a tumbling cat resembles a magician’ s trick. The gyrations of the
cat in midair are too fast for the human eye to follow, so the process is
obscured. Either the eye must be speeded up, or the cat’ s fail be slowed down
for the phenomenon to be observed. A century ago the former was accomplished by
means of high - speed photography using equipment now available in any pharmacy.
But in the nineteenth century the capture on film of a falling cat constituted a
scientific experiment. The experiment was described in a paper
presented to the Paris Academy in 1894. Two sequences of twenty photographs
each, one from the side and one from behind, show a white cat in the act of
righting it- .self. Grainy and quaint though they are, the photos show that the
cat was dropped upside down, with no initial spin, and still landed on its feet.
Careful analysis of the photos reveals the secret: As the cat rotates the front
of its body clockwise, the rear and tall twist counterclockwise, so that the
total spin remains zero, in perfect accord with Newton’s laws. Halfway down, the
cat pulls in its legs before reversing its twist and then extends them again,
with the desired end result. The explanation was that while no body can acquire
spin without torque, a flexible one can readily change its orientation, or
phase. Cats know this instinctively, but scientists could not be sure how it
happened until they increased the speed of their perceptions a thousand
fold. Why arc the photographs mentioned referred to as an "experiment"
A．The photographs were not very clear. B．The purpose of the photographs was to explain the process. C．The photographer used inferior equipment. D．The photographer thought the cat might be injured.