Page:Popular Science Monthly Volume 88.djvu/735

 Popular Science Monthly

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��dredge. While the trench was being pre- pared, the structural steel tubes were in process of building over a slip about a mile away.

In launching each section nine flat- decked boats similar to canal barges, were uniformly distributed beneath the structure at low tide. As the tide rose the huge steel form was lifted clear of its supports; then tugs readily towed it out of the slip. Small valves in the bottom of these boats were simultaneously opened, the section slowly settling down into the w^ater until it floated on its own surfaces as a boat. The flat boats were ballasted with stone to overcome the buoyancy of the wood of which they were constructed and were readily pulled from beneath the structure. After the\' had been pumped out, they were available for use on the next section.

The flotation of the structure was made possible by the watertight wooden bulkheads which completely closed the ends of the outside tubes and the lower half of the ends of the center tubes. These bulkheads and tubes presented something of the appearance of four large submarines tied together, their ends cut off and boarded up. As the same essential principles are involved in their submersion, they might be termed, the "Subway Submarines." Their weight or displacement when entirely equipped was about seven hundred and fifty tons.

IIow the Tubes were Sunk

It is evident that, if the tubes are to be submerged, an enormous weight must be added to overcome the buoyancy that causes them to float. The admission of water suggests itself; but the scientist points out that this is a practical impossibility. Certainly it is a grave risk, to attempt to control and adjust the amount of water in so large a structure, especially where any tendency toward unequal settlement might cause the water to flow to the lowest points, and eventually plunge the whole struc- ture to the bottom a hopeless wreck. It is a well-known principle in physics ihat the resulting i)uoyancy-effect of a floating body (in other words, the weight which the floating body will carry and remain floating) is theoretically equal to the weight of a volume of water

��of the same dimensions as the floating body, less the actual weight of the body. In the light of that principle the use of the four steel air cylinders illustrated in place upon the top of the tubes is at once apparent; they furnish the neces- sary suspension while the tubes are being filled with water.

These cylinders, of light steel plate, were divided into three compartments (a small center one about fifteen feet long and two end ones about twenty-six feet long). Each compartment was fitted with separate valves for the admission of water and for the application of air pressure by which the water could be removed entirely from the cylinders, or from any compartment, or adjusted to any desired refinement. The cylinders had a combined floating effect seventy- six tons greater than the structure when submerged. Hence it was necessary to let in but nineteen tons of water to each of the cylinders to overcome their tendency to float. With the buoyancy- cylinders in place and four long steel location masts erected and carefully plumbed so that they were exactly over the center line at each end of the outer tubes, the section was read\' to be towed into position. Approaching the site, the scene presented was essentially that shown at the extreme right in the illustration.

Filling the Tubes with Water to Sink Them

In order to fill the outside tubes with water (the first operation in lowering a section), twelve-inch submerged valves in each of the end bulkheads were opened simultaneously. With the excess float- ing effect of the buo\anc\- cylinders in mind, it will be appreciated that it was relatively unimportant how fast the tubes filled with water as long as they maintained an even keel. Slowly the section settled, 'as it filled with water, until it became submerged. Gradually it transferred its weight to the buoyancy cylinders and pulled them down into the water until only about two feet six inches of the c>linders were \isible, a condition which followed shortly after that shown in the insert at the lower right-hand corner of the double-page illustration. Workmen standing upon

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