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construction was accomplished in about seven years. The first three years were devoted mainly to the task of getting ready to work. During that time the thorough sanitation of the Canal Zone was accomplished, yellow fever was permanently banished, an operating plant was assembled, a working force gathered, living quarters erected and a food supply provided. Reservoirs were built which furnished an ample supply of pure water to the canal force, and to the cities of Colon and Panama, in which water and sewer-systems were built. An out-of-date railway- system was converted into an adequate one with a thoroughly modern equipment of rails, locomotives and cars. The first commission was in office only a year (1904-3) and accomplished little beyond formulating plans and ordering necessary supplies. The second, under the expert direction of John F. Stevens as chief engineer and afterwards as chairman also, reconstructed the railway-system, assembled the operating plant, collected an efficient working force and provided for it quarters, food and all necessary supplies. It also constructed wharves and docks at both terminals, and machine-shops in which the locomotives, cars, steam-shovels and other elements of the operating plant, which were shipped in parts from the United States, were put together. President Roosevelt paid a visit of three days to the Isthmus in Nov. 1906, inspecting the canal works at all points. His visit was notable as the first instance in which an American president had passed out of the U.S. territory while in office. Col. Goethals, of the Engineer Corps of the U.S. army, was appointed chairman and chief engineer of the third commission on the resignation of Mr. Stevens, and entered upon his duties April i 1907. This commission was composed of four army engineers, one navy engineer and two civilians. It had been in office only a few months when President Roosevelt, who had from the outset of the work been convinced that the best results could not be attained through an executive body of seven members, issued an executive order placing supreme power in the hands of Col. Goethals, abolishing the commission as an executive body, and making its members, who were heads of departments, subordi- nate to him. By this order Col. Goethals became the absolute autocrat of the Canal Zone, holding in his hands all civil, military and other powers. For issuing the order President Roosevelt was sharply criticized in Congress, on the ground that he had exceeded the authority conferred upon him by law, but his course was subsequently approved in the Act of 1912 for the government and operation of the canal after its completion.

Excavation and " Slides." Through the power thus given him, Col. Goethals pressed the work of construction forward with such vigour during the five years following the issue of the order that during that period nearly 75 % of the entire excavation of the canal was accomplished. The original plans of the canal contemplated a channel with a bottom-width of 200 ft. through the nine m. of the Culebra (afterwards called Gaillard) Cut section and estimated the excavation in the Cut at 54,000,000 cub. yd., and that of the entire canal at 95,000,000 cub. yards. The entire cost of construction, ex- clusive of $40,000, ooo paid to the French Canal Co., $10,000,000 paid to the Panama Republic, and the cost of sanitation and civil adminis- tration, was placed at about $190,000,000. The original plans also provided for locks 900 ft. long and 100 ft. wide. In 1906 President Roosevelt increased the length to 1,000 ft. and in 1908 the Canal Commission recommended and President Roosevelt approved changes in the plans which increased the bottom-width of the Cut channel to 300 ft. and the width of the locks to 1 10 feet. These and other changes increased the estimate of total excavation to nearly 175,000,000 cub. yd. and the estimate of total cost, including pay- ments to the French Canal Co. and to the republic of Panama, to $375,210,000. In this estimated excavation there was included an allowance of about 8,000,000 cub. yd. for " slides " or breaks in the walls of the Cut. These had been active during the French operations, and had begun to be troublesome to the Americans in the wet season of 1905. In Oct. 1907 a movement occurred which car- ried about 500,000 cub. yd. into the canal prism, completely filling it and stopping the movement of dirt trains for a month. In Jan. 1913 another movement occurred carrying about 2,500,000 cub. yd. into the prism. One in Feb. i92OcarriecVin about 2,ooo,ooocub. yd. more. During the period of construction and subsequently there were about 30 slides of different kinds, covering an area aggregating 220 ac., and compelling an additional excavation of about 30,000,000 cub. yards. Because of this and other developments, revised es- timates of the total excavation were made in 1912, 1913 and 1914, the last one placing it at nearly 240,000,000 cub. yd., or about 65,-

000,000 more than the amount on which the estimated cost of $375,- 000,000 had been based. When the canal was thrown open to com- mercial traffic on Aug. 14 1914, a total of 224,000,000 cub. yd. had been excavated, and when it was declared formally completed and opened by President Wilson July 12 1920, a total of about 240,000,- ooo cub. yd. had been excavated and yet the total cost of the canal up to that date was only $366,650,000, exclusive of expenditures for its military and naval defence. The excavation of slides alone cost about $10,000,000.

Gatun Dam. The Panama Canal is a huge water bridge rather than a canal, for its surface for the greater part of its length is 85 ft. above sea-level and is held in place by dams at either end. Of these dams the largest and most important is that at Gatun on the Atlan- tic side. It spans the northern and lower end of a deep valley through which the Chagres river formerly flowed to the sea. It is nearly ij m. long measured on its crest, nearly half a mile wide at the base, about 400 ft. wide at the water surface, about too ft. wide at the top ; and its crest is at an elevation of 105 ft. above sea-level. It is really two dams in one, for in its centre there is a natural hill of rock about no ft. in height. In this the spillway of the dam is constructed, and against its two sides rest the two sections of the great dam. The dam itself contains about 21,000,000 cub. yd. of material. Its outer portions are composed of rock and earth, mainly from the Cut, and its centre or core of material drawn by hydraulic process from pits above and below the dam. This material is a natural mixture of sand and clay which in a watery condition flowed into the interstices of the rock and clay of the structure, making the whole at the centre a rubble-wall firmly cemented together and thoroughly impervious to water. Of the entire length of the dam only 500 ft. are exposed to the maximum waterhead of the lake, which is 85 to 87 feet. In 1919, after an exceptionally light rainfall, the maximum level of the lake was 87-16 ft. in Dec.; and in May 1920, 81-65 feet. The spillway is a concrete-lined channel, 1,200 ft. long and 285 ft. wide, the bottom being 10 ft. above sea-level, sloping to sea-level at the lower end. Across the lake-opening of the channel is a concrete dam in the form of an arc of a circle, making its length 805 ft., although it closes a channel with a width of only 285 feet. The crest of this dam is 69 ft. above sea-level, or 16 ft. below the normal level of the lake. On the crest are 13 concrete piers with their tops 115-5 ft- above sea- level and between these are regulating gates of the Stoney type which move up and down on roller trains in niches in the piers. The gates permit a discharge of water greater than the maximum known dis- charge of the Chagres river during a flood. Near the N. wall of the spillway is a hydro-electric station capable of generating, through turbines which are supplied with water from the lake through a fore- bay, sufficient electricity to meet all demands, including the lighting of the canal and all Canal Zone towns and buildings; the machinery of the locks, the machine shops, dry-dock and coal-handling plant; and the telephone and telegraph systems. There is an emergency electric plant at Pedro Miguel, operated by steam.

Dams on the Pacific Side. Dams much smaller than that at Gatun were erected on the Pacific side, one with one lock at Pedro Miguel and one with two locks at Miraflores. In both instances the lock structures themselves form the main portion of the dam. That at Pedro Miguel maintains the level of the water in Gatun Lake and in Culebra Cut, and is placed at the lower or southern end of the channel through the Cut. That at Miraflores holds back the water of a small lake which furnishes the supply of the locks at that point. The Pacific dams are constructed in part like that at Gatun and in part of concrete. That at Miraflores has a small spillway in it.

The Locks. All locks of the canal are in duplicate, are constructed in the same manner, and their chambers, with walls and floors of concrete, have the same usable dimensions 1,000 ft. long and no ft. wide. There are six pairs, making 12 in all. The side walls are from 45 to 50 ft. wide at the surface of the floor, are vertical on the chamber side, and narrowed on the outside from a point 24$ ft. from the floor, by means of a series of steps each 6 ft. long, to a width of 8 ft. at the top. A culvert 254 sq. ft. in area of cross-section, about the area of the Hudson river tunnels of the Pennsylvania railway, extends the entire length of each middle and side wall, and from each of these large culverts, smaller culverts, 33 to 44 sq. ft. in area, ex- tend under the holes in the floors. Fifteen ft. above the top of the culvert in the middle wall there is a space much like the letter U in shape, 19 ft. in width at the bottom and 44 ft. at the top. This space is divided -into three storeys or galleries: the lowest for drainage; the middle for wires that carry the electric current to operate the gates and valve machinery installed in the centre wall ; and the upper a passage-way for the operators. All lock walls are approximately 81 ft. high, except in the lower pair of locks at Miraflores, where they are 82 ft. to meet the requirements of the extreme tidal oscillation of about 21 ft. in the Bay of Panama. In the walls at Gatun there are about 2,000,000 cub. yd. of concrete, and in those on the Pacific side about 2,400,000 cub. yards. All lock walls rest on rock founda- tions. The approach wall at the N. entrance at Gatun, 1,031 ft. long, rests upon piles driven from 35 to 70 ft. into the earth ; that at the S. or lake entrance, 1,009 ft- long, rests on piles reaching to rock, in some places over lop ft. below sea-level. Cellular form of rein- forced concrete is used in all approach walls except those of the lower locks at Gatun and Miraflores where mass concrete is used because of the effect of salt water on steel reinforcement.