THE PRINCIPLES INVOLVED IN THE CONSTRUCTION OF THE TELESCOPE. By Thomas Nolan, B.S. I. (Illustrated). Written for Van Nostrand's Magazine.. 68 FILTRATION THROUGH SPONGY IRON.. RAILWAY NOTES.-Railway Accidents in Germany; Funicular Railway, 83; A New Traction Engine: Railway Communication between Paris and Constantinople; Train Blown Over a Precipice, 84. VAN NOSTRAND'S ENGINEERING MAGAZINE. NO. CXLV. JANUARY, 1881.-VOL. XXIV. IMPROVEMENT OF RIVERS AND HARBORS ON THE ATLANTIC COAST OF THE SOUTHERN STATES. Abstracts of Reports of General Q. A. GILLMORE.* The character and magnitude of the works in progress along our Southern Atlantic border attract attention only when the brief summary of the year's work is published, and then excite a momentary interest, only, in the many who do not know that an engineering problem of great importance is in process of solution. To aid, by artificial construction, in the establishment of a way permanently open to navigation, and free from the danger of accumulations of silt or sand, is the end sought. This requires, first of all, an intimate knowledge of the character and habits of all currents in the neighborhood, so that by properly directing them, they may aid, or at least shall not obstruct the end in view. I. kind, and, under the name of piers or sea-walls, they are found in many important commercial ports. The skill of the engineer is exercised when applying such a method, in adapting it to the special conditions which confront him in each case. A general plan of procedure, when a harbor is to be constructed at the mouth of a river, is to build piers or jetties out into the widened mouth of the river in such way as to continue the current of the stream, making the space between them as wide as the throat of the harbor or the waterway of the river; also to carry the jetties up to or above the surface of the water. One effect of this plan is to move the throat of the harbor to the outer extremity of the works, and to establish There is a widespread popular notion there the same conditions, so far as the that the method of such improvement deposit of a bar is concerned, as formerby jetties was first successfully applied ly existed nearer the mouth of the river. at the mouth of the Mississippi, whereas In Charleston harbor, where the more such means have been employed, with important of the works covered by the more or less success, for nearly a cen- report are in progress, Gen. Gillmore tury. The improvements at the mouth proposes to construct the jetties lower of the Danube, at Currachee in India, than usual, keeping them entirely below and the harbor of Dublin, are of this low water; also to make the width enclosed between them less than one-half the width of the natural throat of the harbor. The tide will therefore ebb and flow over the tops of the jetties. The Annual Report upon the Improvement of the Rivers and Harbors on the Coast of South Carolina, Georgia, and the Atlantic Coast of Florida, in charge of Q. A. Gillmore, Lieut.-Col. of Engineers, and Bvt. Maj.-General, U. S. A. Vol. XXIV.-No. 1.-1. object sought is to cut a narrow and nor more than six ship channels across deep channel through the bar without the bar at any one time. The greatest moving the bar. "The bar which stretches bow-shaped across the entrance into Charleston Harbor, from Sullivan's Island on the north to Folly Island on the south side, has not varied much in either location, general direction, or magnitude within the period covered by any trustworthy knowledge which we possess on the subject. depth of water has sometimes been found in one channel and sometimes in another, being rarely less than 12 feet or more than 13 feet at mean low tide. "The channels, whether four or more, have always existed in two groups or clusters, one in the northern and the other in the southern curved portion of the bar, and there has always been a deep and broad anchorage inside the straight reach of the bar abreast of Morris Island. "In the year 1852 a commission, composed of Prof. A. D. Bache, Superintendent of the United States Coast Survey; "This anchorage, sometimes called the Lieutenants J. N. Maffitt, M. F. Maury, 'main channel' and sometimes the 'outer and C. H. Davis, United States Navy; harbor,' varies in width from one-third and Lieut. J. D. Kurtz, United States to two-thirds of a mile between the 18 Army, in a report on Charleston Harbor foot curves, and in maximum low-water and Bar, made with special reference to depths from 20 to 45 feet. The direcworks of permanent improvement, stated tion of its central line is about north that a comparison of the chart of 1780, and south, and its length from the throat published in Des Barrés' Atlantic Nep- of the harbor between Morris and Sullitune, with those of 1821, 1825, and van's Islands to its southern terminus, 1851-52, 'shows that according to the where it spreads out in various channels earliest records the bar of Charleston has varied comparatively but little in extent, direction, or in distance from the mouth of the harbor.' and shoals in crossing the bar, is fully five miles. At the extremities of this outer harbor or basin, several miles apart, are found the two groups of channels already mentioned, the most northerly group being directly in front of the gorge of the harbor. "The bar is essentially a drift-and-wave bar, produced in part by the upheaving action of the waves when they approach the shore, and are converted by break ing into waves of translation, and in "Measured along its crest, or line of least depths, the bar is ten miles in length, its north end on Sullivan's Island being close up to the entrance or throat of the harbor, while its south end, resting on Folly Island, is six miles distant therefrom. Its average width between the 18 foot curves is about 13 miles. "In many places the highest points of part by drift material carried along the the bar are only 3 to 4 feet below the level of mean low-water, although the average depths along the crest are considerably greater. "The main central body of the bar, lying near due north and south, is nearly straight for a length of over five miles, has its crest parallel to the main shore south of the entrance and at a mean distance of about two miles from it, and is not at the present time, and, so far as we know, never has been traversed by practicable ship channels. "The northern and southern extremities of the bar are formed by rather sharp curves, which connect the straight portion already mentioned with the shore above and below the harbor. "So far as we can now ascertain there appear never to have been less than four coast by surf currents, especially by those produced by northeast storms. The peculiar location of the bar, largely to the southward of the gorge of the harbor, and the conditions under which a very large proportion of the ebb-flow is diverted from its most direct path, and forced to skirt the main coast for several miles before it can find a passage to the sea, indicate the controlling power of these storms. "The material composing the surface of the bar closely resembles that usually found on the sea shore between high and low water in that section of the country, being shells and fragments of shells, or silicious sand, or a mixture of them all. It is easily thrown into suspension by waves, and is moved by a moderate current. |