Whilst on the question of wheel resistance, it may be well to note that the small front wheels of a waggon cause considerably more harm to a macadamised road than the larger hind wheels. In the smaller diameter any loose stone or obstruction is pushed along in front for a considerable distance, often tearing up the surface of the road, whereas in the other case the stone is forced into its place or crushed as under a roller. On the question of “safety” to traffic, Mr. Haywood, the eminent Surveyor of the City of London, has caused several most complete observations to be made from time to time, the results of such observations being detailed by him in various reports. Amongst other useful information compiled by him, he has ascertained that a horse will travel 446 miles upon a roadway paved with blocks of wood without a fall, 191 miles upon asphalte, and 132 miles upon granite setts. I cannot do better than give verbatim his remarks upon this point:-- “Slight rain makes both asphalte and wood more slippery than they are at other times. On asphalte the slipperiness begins almost immediately the rain commences, wood requires more rain before its worst condition ensues. The slipperiness lasts longer upon wood, on account of its absorbent nature, than it does upon the asphalte; when dry weather comes after the rain, when asphalte is in its most slippery state, and the horses fall on it very suddenly, _on wood their efforts to save themselves are more effectual_; wood also is frequently in that peculiar condition of surface in which horses slip or glide along it without falling. A small quantity of dirt upon asphalte makes it very slippery, wood requires a large quantity. Slipperiness can be temporarily cured on both pavements; on the asphalte by sprinkling it with sand, on the wood by sprinkling it with gravel. The result in both cases is dirt. _The sand thrown on asphalte helps to wear it out, the gravel thrown on wood tends to preserve it._ When a horse falls on asphalte it has difficulty in getting up; on wood it rises more readily.”[11] In streets crowded with traffic, the constant stopping and starting, especially on any surface that is slippery, is very trying to horses. Attention has lately been directed to this point with a view to the storage of some power in a vehicle, either by the compression of a spring in stopping or by some other mechanical means, in order that in starting the driver may at will liberate this power so as to assist the horse in overcoming the inertia of his load. These trials, however, have not at present met with much success.