Since writing this chapter my attention has been directed to a stone-breaking machine which is said to substitute a “knapping” for that of the usual crushing motion which is so generally the great defect in these machines: I allude to that known as “Baxter’s patent knapping-motion stone breaker,” by which a rapid jerk or blow is given instead of the slow crushing movement, thus (it is contended) causing less waste from dust and chippings, and also less strain of the machinery and less power to drive it.
[26] Toughness is not all that is required. Leather would be very difficult to break with a hammer, but it would not make a good road metal.
[27] I have tried this experiment, but without success, except on such soft stones as were evidently unfitted for use as a road metal.
[28] Many hundreds of miles of roadways in this country are made with limestones; they often make an excellent surface, as they possess a considerable power of binding together, but weather and very heavy traffic affect them considerably: as they all have a strong affinity for water, their very power of thus cementing themselves together causes a quantity of dust in dry, and mud in wet weather.
[29] A flinty or quartzose stone seems to harden with exposure. This is notably the case in pebbles; old pebble paving taken up and broken makes a most hard and durable road metal.
[30] ‘Roads, Streets and Pavements,’ by Q. A. Gillmore, p. 10.
[31] Ibid.
[32] Vide ‘The Maintenance of Macadamised Roadways,’ by Thomas Codrington, p. 33, a most excellent work upon this subject.
[33] Mr. W. Bold considered a hammer weighing 1¹⁄₄ lb. of an elliptical form, pointed at the ends, the area of each end being about ¹⁄₁₀₀th of a square inch, to be the most suitable to break hard stones. Vide ‘Minutes of Proceedings, Institution of Civil Engineers,’ vol. i. (1840) p. 50.