by skeleton devices of steel mounted on rails so that the pair of them, though separated by 500 feet of space, spanned by the sagging cables, could be moved in unison. Out on the swinging cables ran the loaded cars or buckets, filled with concrete and dumped with a crash and a roar at the chosen place. Giant mixers ground up rock from Porto Bello, sand from Nombre de Dios, and cement from divers states of our union into a sort of Brobdignagian porridge with which the hungry maws of the moulds were ceaselessly fed. Men wig-wagged signals with flags across gaping chasms. Steam whistles blew shrill warnings and cryptic orders. Wheels rumbled. Pulleys creaked. It seemed that everything a man could do was being done by machine, yet there was an army of men directing, correcting and supplementing the mechanical labor. [Illustration: _Photo by Underwood & Underwood_ TRAVELLING CRANES AT WORK Mounted on rails these cranes carry the heaviest burdens. Those shown are placed for delivering concrete to the forms. One crane will cost $60,000] Into the locks at Gatun will go 2,000,000 cubic yards of concrete if the original estimate is adhered to. A statistician estimates that it would build a wall 8 feet wide and 12 feet high and 133 miles long--which would just about wall off the state of Delaware from the rest of the Union. The side walls of each of the locks are practically monoliths, constructed of concrete poured into great steel frames or moulds where it hardens into a solid mass. They are based in the main on bed rock, though it was found on making tests that the bed rock was not of sufficient extent to support the guide walls as well, so one of these is therefore made cellular to lighten its weight, which rests on piles of 60 feet long capped and surrounded with concrete. This wall was built by slow stages and allowed to stand in order that its settlement might be uniform. An examination of the picture below will make clear the method of constructing the lock walls, for in it are shown the completed monoliths and a steel form half completed with men preparing it for the concrete therein. Col. Sibert describes the details of the work thus: “The locks proper are founded on rock and the heavy masonry is completed. This rock foundation was not of sufficient extent, however, at available elevations, for supporting the guide walls. Under that guide wall extending into the lake the underlying rock at the south end is about 150 feet below sea level, and the overlying material is soft. This wall is cellular in construction. It is composed of four longitudinal walls about 2 feet thick with cross walls about 17 feet apart, all built of reinforced concrete. [Illustration: _Photo by Underwood & Underwood_ BUILDING A MONOLITH]