Page:The American Cyclopædia (1879) Volume III.djvu/242

BREAKWATER by the great waves in the winter storms, col- lected and formed a solid and compact surface, at a new slope, of which the base was about quadruple the vertical height. It was observed that the lateral movement of the small stones by the storms, driving obliquely along the out- er face of the dike, caused them to collect at each extremity in a conical mound of the pre- cise configuration traced for the proposed ter- minal batteries ; but to prevent their extending into and obstructing the passes, it was found indispensable to face the whole exterior with blocks large enough to resist these oblique im- pulsions. In May, 1805, the battery on the cen- tral portion was armed with 20 pieces of heavy ordnance. In February and May, 1807, oc- curred two great storms, the effects of which upon this portion, as also of the unprecedent- edly severe storm of Feb. 12, 1808, are de- scribed in the " Memoir upon the Dike of Cher- bourg, compared with the Jetty or Breakwater at Plymouth," by the baron Cachin, inspector general of roads and bridges. In the last named storm the battery was submerged, the parapet was upset, and the barracks and gar- rison, with 60 men, were swept away. The large blocks of stone with which the dike was faced were by this storm arranged in new po- sitions, and so closely stowed that they appear- ed as if placed by the hand of man in positions of the most perfect stability. As thus arrang- ed, the outer side presents four slopes. At the upper part, reached only by the tops of the waves, the height is to the base as 100 to 185. Beneath this is the space between the high and low-water marks, which is exposed at all times of tide to the most violent action of the sea; its slope is the most inclined, the height being to the base as 100 to 540. Below the low- est spring tides is a space but little ex- posed to the action of the waves; the height of this slope to its base is as 100 to 302. The lowest part, which is always submerged, has a height of 100 to a base of 125. The slope on the inner side is of 45. From the experience of these two breakwaters, incomparably the greatest of their sort which have ever been undertaken, M. Cachin concludes with the ob- servation, that if man be strong enough to heap together rocks in the midst of the ocean, the action of the sea alone can dispose them in the manner most likely to insure their proper stability. This, it may be added, will necessa- rily vary in form with the specific gravity and by a foreshore of great blocks of stone on the outer side, which extend in a slope of 120 ft. to the depth of 21 ft. below low-water mark. This nearly vertical wall (the slope of its sides being ^ to 1) is 36 ft. 3 in. wide at base, and 29 ft. 3 in. wide at top. A parapet is raised to the height of 6 ft. upon its outer edge, which is 8 ft. 3 in. thick ; at top 8 ft. 6 in. wide. The altitude of 'the breakwater is given by the United States commission of engineers and na- val officers, who examined it in 1829, at 72 T 3 5 ft., the base of its sea slope being 228 T 5 T ft. ; and they state that similar proportions were adopted at the Plymouth breakwater, the alti- tude of which is 57 ft., and base 180 ft. The inner slope of this, however, was built at an angle of 32, although that of Cherbourg had stood perfectly well at 45. The adoption of the general plan of this work by the English and American engineers sufficiently proves the correctness of its principles, though by some English authorities the work is alluded to as a failure. The breakwater at Plymouth, Eng- land, to protect the inner harbor from the heavy sea that is driven in by southerly storms, was commenced in 1812. Its dimen- sions are only about one fourth of those of FIG. 1. Section of Cherbourg Breakwater. size of the stones used. In 1830 it was decided to raise the dike by building up a wall of rub- ble masonry faced with granite to the height of 6 ft. above highest water. This is protected FIG 2. Section of Plymouth Breakwater. the breakwater at Cherbourg, its total length being 1,700 yards, made up of a central portion of 1,000 yards, and a wing bending in from each end, at an angle of 120, of 350 yards. Its profile is 993 sq. ft. It was designed to have a base of 210 ft., breadth at top 30 ft, and height in the middle 40 ft. Its actual height exceeds this, but it is only about 3 ft. above the highest tides. It is built of large blocks of limestone, some exceeding five tons in weight, brought in vessels from the quarries at Catwater, about 24 m. up the harbor. The convenience of position of these quarries for loading the vessels, the facilities of quarrying the stone, and the judicious arrangements in- troduced, made the work of comparatively light expense. After some experience was had, the stone was quarried by contract at 2. 5d (58 cents) per ton, and transported for 34 cents ; and the total cost of the stone laid, including land purchases, salaries, buildings, &c., was estima- ted in 1816 at about 8s. l^d. per ton. In 1841 it was calculated that 3,369,261 tons of stone had been laid, at a cost of nearly 1,500,000; and the work was considered as being essentially completed. The 15 vessels kept employed in transporting the stone were furnished with two railways laid along in the hold, upon which were run the loaded cars from the quarries, entering through two stern -ports. These could be tight- ly closed when the vessel was loaded. On eacli