The manner of laying down this cemented mass, and constructing the road, is fully detailed in Mr.idacneill's evidence before a select Committee of the House of Commons, in May 1830 :— "Are you the resident engineer under Mr. Telford! Yes.—You conducted the work at the Highgate Archway road! 1 did.—Will you explain to the Committee the expense of the cement composition which was laid on the foun dation f The expense of the cement delivered was 2s. a bushel; it was mixed with eight times as much of washed gravel and sand.—What distance of ground would a bushel, so made, extend! Laying on the cement six yards wide, and six inches in thickness, came to 10s. a running yard, part old gravel ; if new gravel had been purchased, it would have coat from 12s. to 15s.: that included the forming the bed of the road, which was done with very great care. There were four longitudinal drains, and secondary drains, running from those to the side channel drains, and those again to drains outside the footpaths, covered with brick : they all communicated with each other, and discharged the water into proper outlets. Ou the prepared centre of six yards in widtb, alter it had been properly levelled, the cement was laid on, mixing it first into • box with water, gravel, and sand, in certain proportion& In fifteen minutes it became so hard that we could stand upon it : an about four minutes after being laid, a triangular piece of wood, sheeted with iron, was indented into it, so as to leave a track or channel for the stones to lie and fasten in. This indent had an incli nation or fall from the centre to the side of the road of three inches ' • this allowed the water that percolated through the broken stones to run off the oemented mass into the drains.—What time of year was the composition laid on the road! Two hundred yards were done in the winter, all the rest in July, August, and September.—It has been on the road through the last winter There has been part of it on since June 1828,—nearly two years.—Have you examined it, to see what effect the weather has had on it! I examined it fre quently during the frost, almost every six weeks; I have found it perfectly bard in every case, and not injured by the frost, nor by the working of the carriages over it. ' The success of this experiment has placed beyond a doubt the correctness of the rule laid down by Mr. Telford, that wherever the substratum is weak, spongy, and elastic, it must, at whatever expense be rendered firm and dry, for the reception of the surface materials. If this be attended to, the road metal will remain unmixed with the earth of the sub-soil, and unaffected by the changes of wet and frost to which such mixture would expose it ; if this be neglected, no outlay of money, or expenditure of material, will ever produce a firm, dry, and hard road.
"Well-made roads, formed of clean, hard, broken stone," observes Mr. Macneill, "placed on a solid foundation, are very little affected by changes of atmosphere ; weak roads, or those that are imperfectly formed with gravel, flint, or round pebbles, without a bottoming, or foundation of stone pavement or concrete, are, on the contrary, much affected by changes of the weather. In the formation of such roads, and before they become bound or firm, • con siderable portion of the sub-soil mixes with the stone or gravel, in a of the necessity of putting the gravel on in thin layers : this or clay, in dry, warm seasons, expands by the heat, and makes the road loose and open ; the consequence is, that the stones are thrown out, and many of them are crushed and ground into dust, producing considerable wear and dimi nution of the materials. In wet weather, also, the clay or earth mixed with the stones absorbs moisture, becomes soft, and allows the stones to move and rub against each other, when acted upon by the feet of horses or wheels of carriages. This attrition of the stones against each other wears them out sur prisingly fast, and produces large quantities of mud, which tend to keep the road damp, and by that means increases the injury."
To ascertain the draught of carriages, and the comparative merit of roads, Mr. Macneill employs an instrument which- he calls a road indicator. The very important purposes to which this instrument can be applied, and the accurate results given by it, entitle it to attention, as supplying the means of subjecting the state of roads to an infallible test. Several inventions to effect the same object have been, at various times, proposed and brought into use (see the article DYNAMOMETER); but the improvements added to it by Mr. Macneill have materially increased their utility. In the Appendii to Sir Henry Parnell's Treatise is inserted an elaborate paper furnished by Mr. Macneill, describing the road indicator, and the purposes for which it may be used. We can, however, only afford space for a concise description of the instru ment, and refer our readers to the paper itself for further information.
The framework is of wrought iron, about two feet six inches long, and eighteen inches wide. In this frame, a dynamometer and brass cylinder are placed ; the dynamometer is connected by its arm to one side of the frame by trunnions, which are cast on it, and which turn in a circular hoop or belt, firmly screwed to one side of the frame, and a bar running across it. The dyra mometer, or weighing-machine, which forms part of the instrument, was im: proved, some years ago, by Mr. Marriott, and is now generally used as a substitute for the common steel-yard. On applying the weighing-machine in its simple form, to measure the draught of carriages, the index vibrated so quickly, and over so large an arc of the circle, that it was impossible to observe the point indicating the force of draught; for a horse exerts his power by a succession of or strokes of his shoulders against the collar, at every step he makes, and not by a constant uniform pull, as is generally supposed. To remedy this inconvenience, and do away with the vibrations, a piston, working in a full of oil, is connected with the dynamometer in such a manner, that when any power or force is applied to it, so as to carry round the index, the piston is at the same time moved through the fluid. The connexion of the dynamometer with the cylinder, is by means of a lever working on a pivot ; the arms of the lever are of unequal length ; the tail-piece of the dynamometer is connected with the short arm, at a distance of two inches from the centre, or fulcrum, by means of a pivot-joint at precisely the same distance from the fulcrum ; a flat bar of iron is connected with the longer arm, by a joint similar to that by which the tail-piece is connected with the short arm, so that any )lower or weight applied to the bar will produce the Beale effect on the index u if the power was applied directly to the tail-piece of the dynamometer ; this bar passes over a friction roller, and to it the power of the horses is applied when in use, by means of traces, and a bar, as in the ordinary mode of draught. At the extremity of the long arm, the piston rod is connected by a joint similar to the others ; the piston rod, after passing through a stuffing-box in the cap of the cylinder, is screwed into a piston, or circular plate of thin brass perforated with small holes ; and out of one part of the circumference a square notch is cut, the use of which will be hereafter deicribed. By this construction, the resistance of the fluid to the piston, which acts at the extremity of the long arm of the lever, prevents its turning round the fulcrum to the extent it otherwise would do, when it is acted upon by any sudden impulse applied to the bar; it will, however, move over a space proportioned to the intensity of the force applied; and if the pulls follow each other in rapid 'succession, the piston will move slowly out, and the index will turn round steadily and uniformly until the power is balanced by the spring of the dynamometer, at which time the index will point out on the dial very nearly the weight or power which is equivalent to the draught.