We can measure Time in one way only--by counting repeated motions. Apart from the operation of the physical Law of Periodicity we should have no natural measures of Time. If that statement be true it follows that apart from the operation of this law we could not attain to any knowledge of Time.[11:1] Perhaps this latter proposition may not at first be readily granted. Few, probably, would hesitate to admit that in a condition in which our experience was a complete blank we should be unable to acquire any knowledge of Time; but it may not be quite so evident that in a condition in which experience consisted of a multifarious _but never repeated_ succession of impressions the Knowledge of Time would be equally awanting.[12:1] Yet so it is. The operation of the Law of Periodicity is necessary to the measurement of Time. It is by means, and only by means, of periodic pulsative movements that we ever do or can measure Time. Now, apart from some sort of measurement Time would be unknowable. A time which was neither long nor short would be meaningless. The idea of unquantified Time cannot be conceived or apprehended. Time to be known must be measured. Periodicity, therefore, is essential to our Knowledge of Time. But Nature amply supplies us with this necessary instrument. The Law of Periodicity prevails widely throughout Nature. It absolutely dominates Life. The centre of animal vitality is to be found in the beating heart and breathing lungs. Pulsation qualifies not merely the nutrient life but the musculo-motor activity as well. Eating, Walking,--all our most elementary movements are pulsatory. We wake and sleep, we grow weary and rest. We are born and we die, we are young and grow old. All animal life is determined by this Law. Periodicity--generally at a longer interval of pulsation--equally affects the vegetal forms of life. The plant is sown, grows, flowers, and fades. Periodicity is to us less obvious in the inanimate world of molecular changes; yet it is in operation even there. But it is more especially in the natural motions of those so-called material masses which constitute our physical environment that Periodicity most eminently prevails. Indeed it was by astronomers that the operation of this Law was first definitely recognised and recorded. Periodicity is the scientific name for the Harmony of the Spheres.