RIVETS. The rivets which fasten together plates of iron and other parts of mechanism are usually short thick pieces of iron or cop per, forged or drawn into the shape of a rod, and then cut off to the proper lengths. These are used either cold or red hot, according to circumstances; and when placed in 11 hole. the two ends of each rivet are exposed to such powerful blows (or blows on one end with pressure on the other) as to form a burr or head which clasps the plate tightly. The two millions of rivets in the Britannia Bridge present perhaps the most remarkable example of rivetting, yet known. ClIgNer IhuncES.] ROADS. The .Romans were distinguished by the vast extent and solid construction of their roads, of which several thousand miles were made in Italy alone ; while every country which was brought under their sway was more or less intersected by these excellent highways. The solidity of their construction was fully equal to the boldness of their design, a fact proved by the existence of many that have borne the traffic of nearly two thousand years without material injury. The Romans always gave a firm foundation to their roads, by ram ming down a layer of small stones and broken brick ; on this layer a pavement of large stones was laid, either squared to fit closely around one another, or cemented into a hard and firm causeway.
In this country very little was done towards a sound and scientific construction of roads until the beginning of the present century. The Highland roads have been made under a commission issued in 1803, and have been of incalculable benefit in opening districts which were before placed almost beyond the reach of commercial enterprise. Tho commissioners have constructed about 1000 miles of excellent road, and much more than that number of bridges. The Holyhead road improvements were commenced in 1815, under Mr. Telford; these comprise the establishment between London and Holyhead of the best coach-road in England, in which principles of sound con struction have been more attended to than in any other examples. The greatest angle or
ascent allowed in this road is 1 in 35 : a slope which may be ascended at a good rate of speed, and descended at twelve miles an hour without risk.
The Macadamised roads, named from the in ventor, exhibit a particular mode of applying small stones to the surface. Under the old metit,id, stones of irregular sizes and shapes were thrown down upon the soft earth of a road, and became mixed up with it in such a way as to form a yen( irregular road, Mr, M'Adam conceived that if small stones of nearly uniform size were used, they would tend to lock together into a hard and compact mass, forming a sort of crust nearly impervious to water. He objected to any kind of chalk or earth being mixed with the stone ; but the quantity of stone must depend on the quality of the ground beneath ; in many cases a layer three feet thick has been required. Mr. Tel ford employed the still more durable system of placing a layer of hand-laid larger stones beneath the smaller, so as to form, in fact, a sort of two-fold paving.
It has been found by experiment, that a layer of hard broken stone on a substratum of stone affords easier draught for horses than where the substratum is of earth; and that a gravel surface is more trying to horses than a broken stone surface. As a geneial rule, the hardest stone—such as basalt, granite, quartz, sienite, or porphyry—is found to be the best for road-metal, or the broken stones for the surface ; but this is not invariably the case, as some hard stones are found to wear more rapidly than softer but tougher stone. Lime stone, slaty stone, and sandstone, are all un fitted for roads. On some parts of the Holy head road small cubes of iron, about an inch square, are placed at intervals of four inches among the stones, and well packed round with small chips of stone; they have been found to produce a remarkably durable road.