Bibliothèque mondiale du cheval

« The utmost speed of a horse is the result or effect of a certain power inherent in the animal, being his moving or impelling force. He possesses this power in degrees varying much according to his age. At three years old it is much less than at four; at four less than at five, and at five less than at six, when he is generally considered at his best; but the increase of this power in each of the two latter years is not so great as in the year next preceding it.
This power of the animal must be considered as a constant and uniform force for the time being, whatever his age may be. It is proportionate to his animal powers in whatever manner exerted; that is, whether mechanically or chemically, or both ways. It is not material for our purpose to inquire how this power is exerted. Our object is to ascertain what is the quantum or value in numbers of his power, as the means indeed the sole means) of ascertaining his speed, which term we shall always use in the following remarks as meaning his utmost speed. By well-known and established mechanical principles, founded on the undeviating laws of uniform motion, the velocity or speed of a body, impelled by any impulse, whether applied externally (as it is when a horse draws a carriage, as to the carriage), or internally (as it is to a ridden horse and his rider, taken together), varies directly according as the impulse or moving power is more or less; and inversely according as the weight impelled is greater or less.[…] » Présentation de l’éditeur (1840)