This section is from "The Horticulturist, And Journal Of Rural Art And Rural Taste", by P. Barry, A. J. Downing, J. Jay Smith, Peter B. Mead, F. W. Woodward, Henry T. Williams. Also available from Amazon: Horticulturist and Journal of Rural Art and Rural Taste.
In trotting, the horse moves its legs in pairs, diagonally. Thus, if the legs a d (Fig. 5) be raised, and advanced first, the legs b e will be raised the instant those designated by a d reach the ground. On the other hand, when the legs b e are raised before the legs a d reach the ground, there is a minute interval, during which all the legs are raised above the ground at the same time. In trotting, each leg moves rather more frequently, in the same period of time, than in walking, or nearly as 6 to 5; bat the velocity acquired by moving the legs in pairs instead of consecutively, depends on the circumstance that, in trotting, each leg rests on the ground a short time, and swings during a long one. In walking, the trunk oscillates laterally, whereas, in trotting, it oscillates vertically; but in each of these kinds of movement there appears to be a slight motion of the trunk of the animal both laterally and vertically.
It may be observed that the vertical line traversing the base of support, passes through the horse in such a manner as to leave by far the greater part of the weight of the body to be supported by the two fore legs.