Popular Science Monthly 



707 



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 



