ENGINEERING. 



333 



vided for the purpose. An extensive system 

 of brick sheds and warehouses affords the 

 needed accommodation on the docks. When 

 fully completed, the works in progress will 

 provide about 88 acres of quayage, and sev- 

 eral miles of rails will bring the entire harbor 

 into direct communication with the great Cale- 

 donian and Glasgow and Southwestern "Rail- 

 ways. The entire cost of the dock- works is 

 placed at $4,550,000. 



The Alexandra Dock, at Hull, England, was also 

 completed in May. It is 2,300 feet long by 

 1,000 feet wide, and contains 46| surface acres 

 of water, with a depth of 34J feet. The en- 

 trance is bell-mouthed, with a lock 550 feet 

 long and 85 feet wide. The whole area of the 

 dock is 192 acres, of which 152 acres have 

 been reclaimed from the Humber. Two large 

 dry-docks are connected with the main dock. 

 One of these is 500 feet and the other 550 feet 

 long, with a width of 65 feet. One peculiar 

 feature of the dock is that it will be filled with 

 fresh water, thus avoiding the mud deposits, 

 which entail an expense of $50,000 a year up- 

 on the other Hull docks. The source of the 

 water is the Holderness drain, a large culvert 

 connecting the dock with this canal. 



The Tilbury Docks. The entrance to these 

 docks, now approaching completion, is opposite 

 Gravesend, on the Thames, below London. 

 This entrance is 100 yards wide, and leads into 

 a tidal basin, where vessels can enter at all 

 stages of the tide. The entrance-lock to the 

 dock proper is 80 feet wide, and consists of a 

 lock with wrought-iron gates. The walls are 

 Portland cement and concrete. In connection 

 are graving-docks with hydraulic pumping ma- 

 chinery. There is to be a large hotel near the 

 river-bank, for the accommodation of passen- 

 gers by ocean-steamers. The work is carried 

 out under contract for the East and West In- 

 dia Dock Company. 



The Davis Island Dam, On Oct. 8 the Davis 

 Island Dam, six miles below Pittsburg, on the 

 Ohio river, was declared finished. This is de- 

 signed to maintain the river at a navigable 

 depth the year round, and, if it proves practi- 

 cally successful, will doubtless be the forerun- 

 ner of many similar structures. The plan is 

 not altogether new, but some of its details 

 are novel. Similar dams exist in Europe, and 

 on the Monongahela and other slack- water 

 rivers in this country. The Davis Island Dam 

 was begun in 1878, and the work has been 

 prosecuted with various interruptions, owing 

 to failure of appropriations and the like. The 

 Government has expended about $1,000,000 on 

 the work. It consists, in effect, of 300 small 



. dams or " wickets," each of which is hinged so 

 that it may lie flat on the river-bed when there 

 is plenty of water, or be raised during the dry 



i season so as to deepen the channel and raise 

 the up-stream level. The line of wickets is 1,223 

 feet long, extending entirely across the river. 

 Of this, a span of 559 feet is given to the navi- 



i gable opening, which is available to all craft at 



ordinary stages of the water. When necessary, 

 however, a lock 110 feet wide and 600 feet long 

 is brought into service, the gates of which are 

 operated by turbines driven by the surplus 

 water. This lock, it is expected, will at all 

 times admit the passage of ordinary river-tows. 

 To fill or empty the lock requires but four min- 

 utes. To raise the wickets that close the navi- 

 gable pass, a " mano3uvring-boat " is brought 

 into play, while for the others, or " weir wick- 

 ets," as they are called, a sectional bridge is 

 used. When the wickets are raised they are 

 held in position by props, and the surplus wa- 

 ter escapes through two-inch spaces between 

 the wickets. These spaces, however, can be 

 closed with battens in case of need. 



Control of Sand-Danes. Some of the most no- 

 table achievements of engineering are accom- 

 plished through judiciously taking advantage 

 of the forces of nature. A case in point is af- 

 forded by the moving sand-dunes of Gascony, 

 France. In the last century these were a per- 

 petual menace to the country bordering the 

 ocean. In their slow but inexorable advance 

 they buried whole villages, and covered fertile 

 fields beyond hope of reclamation. After a 

 long series of experiments, M. Breniontier suc- 

 ceeded in covering the moving dunes. Some 

 45,000 acres in area were covered with a heavy 

 forest growth. This was successful so far as 

 the then existing dunes were concerned, but 

 new dunes formed along the shore and threat- 

 ened to become as formidable as their prede- 

 cessors, which, indeed, they began to encroach 

 upon, in spite of the forest. It was determined, 

 as reported by M. Chambrelent, Inspector ot 

 Public Works, to favor the formation near the 

 ocean of a reversed dune, which should, as it 

 were, react upon itself. The principle upon 

 which a sand-dune advances is well known. 

 Its windward slope is gradual; up this the wind 

 drives the sand. The leeward slope is steep ; 

 the sand, of course, settling at its natural angle 

 of inclination. This steep slope forms the ad- 

 vancing front of the dune. In order to favor 

 the formation of the reversed dune, a row of 

 palings, with somewhat wide interstices be- 

 tween the boards, was erected about 130 yards 

 from the sea. The sand that passed between 

 the boards flew a short distance before falling, 

 while that which struck the boards fell at the 

 foot of the fence. The result was as had been 

 anticipated, a reversed dune, with a steep slope 

 to windward and a gentle one to leeward. Of 

 course the sand soon reached the top of the 

 fence, but this had been provided for, and the 

 fence was raised by special machinery, and the 

 operation went on until the seaward slope be- 

 came so steep and high that the sand fell back 

 toward and into the ocean faster than the wind 

 could drive it up the incline. The final height 

 of the reversed dune is thirty to forty feet, ac- 

 cording to location. The inland slope is fixed 

 by planting with the Arundo arenaria, whose 

 roots penetrate the sand twelve or fifteen feet, 

 and which always keeps its head above the 



