Book II. ROADS OF VARIOUS ENGINEERS. 591 



pebbles may, with ease, be brought on the block with a ring. The ring should be about 

 five or six inches in diameter, an inch and a half in breadth, and a little thicker than 

 hoop-iron, with a short handle to it : this instrument is used for confining the stones on 

 the block, while going through their operation. The trough has four feet to support it, 

 two of which (those nearest to the breakers) are no longer than what is necessary to allow 

 the stones to come upon the block : the other two are placed at a little distance from the 

 block, and should be somewhat longer, in order that the far end of the trough may be 

 higher, say four or five inches ; by which means the person who breaks the stones will, 

 with ease, pull them up on the block ; and, as he must always be in a sitting posture, 

 it is requisite that he should get all the advantage over them he can. The trough is, in 

 form, like a washing tub, except that the end next the block is much narrower than 

 the other, and left open ; at the bottom of it the end next the block should be fixed a 

 grate, to let through the dirt or sand which is shovelled up with them when put into the 

 trough. It will sometimes be of great advantage to gravel, when clay, earth, or other 

 matter, adheres to it ; for, by constantly removing it about, and being frequently ex- 

 posed to frost, wet and dry weather, the dirt becomes tender and moulders into pieces, 

 which the grate will readily separate, without any hinderance to the breaker or waste in 

 the stone. A blacksmith's anvil is the best block ; and a box or trough, made as just 

 described, must be framed so as to agree with it." (Fall's Surveyor's Guide.) 



3661. Breaking by machinery. On a new line of road, between Bury and Bolton, in 

 Lancashire, a rotatory steam-engine is attached to a machine similar to a stone-mill, but 

 considerably stronger, which breaks the stones to cover the road at the astonishing rate 

 of seventy or eighty tons in ten hours. The engine is movable on wheels, so that it can 

 be removed to any part of the road without being taken to pieces. (London Journal of 

 the Arts, ^c. Sept. 1822.) 



3662. M' Adam's criterion for size is weight. On being asked by the road com- 

 missioners to mention the dimensions, he stated, that there was very little difference in 

 the weight of the stones used in road-making. " I did imagine," he says, " that a dif- 

 ference existed ; but having weighed six ounces of different substances, I am confident 

 there is little difference in appearance, and none in effect : I think that none ought to 

 exceed six ounces ; I hold six ounces to be the maximum size. If you made the road 

 of all six- ounce stones, it would be a rough road ; but it is impossible butthat the greater 

 part of the stones must be made under that size." " Do you find a measure or ring 

 through which the stones will pass, a good method of regulating their size ? " That is 

 a very good way ; but I always make my surveyors carry a pair of scales and a six-ounce 

 weight in their pocket, and when they come to a heap of stones, they weigh one or two 

 of the largest, and if they are reasonably about the weight, they will do ; it is impossible 

 to make them come exactly to it." 



3663. With respect to the size of stones, Paterson disapproves of six ounces being 

 made the maximum, as proposed by M'Adam. " I find," says he, " there are many 

 under the weight that are yet of a very improper shape and size ; even from three to 

 four inches between the extreme points. Besides, scales for weighing are not so portable 

 nor convenient as gauging-rings for the size. The ring I generally use is two inches 

 and a half in diameter ; and the stones should be broken so that the largest may pass, 

 in any direction, through it. On this plan you have the materials smaller, more equal, 

 and more square in shape, than on his plan. An inexperienced person, on the first view 

 of it, may think otherwise ; but it is a fact, that taking my ring as a gauge, you will 

 not have five stones ih a thousand that will exceed four ounces in weight, and none of 

 improper shape or dimensions : while on Mr. M'Adam's plan you will have more than 

 twenty in a thousand that will not pass longitudinally, even through a three-inch ring. 

 It is now nearly three years since I first heard of his standard weight. During that 

 time I have had people both working to it, and also to my ring-gauge ; but I have 

 uniformly found that mine are so much smaller, that they cost about a ffth more in 

 breaking than his. Upon the whole, then, I would recommend the ring as every way 

 preferable to the scales : and I have no doubt that it would be an improvement even to 

 reduce the ring a little, where the ground under the road is completely dried by the 

 method I have described." 



3664. JVith respect to the depth of metals, Marshal mentions twelve inches: but 

 Edgeworth considers an average of nine inches as sufficient for any road on a good 

 basis ; and two thirds of the quantity, he says, will make an excellent road at a distance 

 from any great town. 



3665. The depth of materials, according to Walker, depends so much upon the soil 

 and the nature of the materials themselves, that it is impossible to lay down any general 

 rules for them. The thickness ought to be such that the greatest weight will not affect 

 more than the surface of the shell ; and it is for this purpose chiefly, that thickness is 

 required, in order to spread the weight which comes upon a small part only of the road 

 over a large portion of the foundation. 



