GRAVEL FOR SURFACE COVERING. 293 



any hollows or rakiug down any bunches that may make their 

 appearance. 



The object of covering the surface with gravel is to fill the 

 crevices between the fragments of stone, to help hold them fast 

 in place, and to reduce the shock upon the wheels from striking 

 the angular points of the stones, by preventing them from falling 

 into the cavities. When finislied, the surface should have 

 solidity and smoothness sufficient for a horse and carriage to 

 trot over it with ease. 



The same process is required for covering a road-bed which 

 has no artificial foundation upon it, except that the thickness of 

 the layers should be increased, or the number greater. Gravel 

 consists of small fragments of stone, more or less rounded, and 

 as found inland, is generally mixed with sand or loamy earth, 

 while that obtained from sea-beaches is usually free from extra- 

 neous matter. It varies greatly in quality, and it is difficult to 

 find, in its natural state, suitable for road-covering. For this 

 use it should contain no pebbles of more than one and one-half 

 inches, or less than three-quarters of an inch in diameter. It 

 should be free from sand, but must contain a proportion of 

 binding material of a loamy or clayey character. 



It is generally necessary to prepare it for use by screening, to 

 separate from it both the larger pebbles and some of the finer 

 stuff, when that is in excess. When gravel is of excellent 

 quality, or it is properly prepared, it makes a superior road-cov- 

 ering, for which purpose it should be spread in layers and con- 

 solidated in the same way as that described for broken stone. 



The covering materials should be kept moistened, by sprink- 

 ling or by the state of the weather, while being compressed 

 under the rollers, but the rolling should not be done when they 

 are saturated with water. 



It is usual to admit the travel upon the loosely spread mate- 

 rials, and make it do the work of binding them into place, which 

 it will never do so perfectly as might be done by rolling. The 

 wheels act as wedges, forcing themselves between and displacing 

 the fragments which are crushed and abraded under the loads ; 

 mud and dust works into the interstices, so as to prevent the 

 parts from wedging or being bound together ; horses' feet are 

 liable to injury from loose stones ; the rims of wheels are rap- 

 idly worn ; riding in carriages, too, is very uncomfortable, and 



