animals, by the processes going on in their bodies, must have some notion of time. This would give the "how long" element in the problem. The "how fast" might be given by such an instrument
Flight of six pigeons, covered cage.
as we find in the otoliths resting on the sensory hairs of the maculæ acousticæ; these hairs bending under the weight more or less, according to rate of movement. The "which way" could