between the succeeding two passes. The timer can be selectively acti- 

 vated whenever a contact on the rotating counterweight framework touches 

 any of four equally spaced contacts along its circular path. Since the 

 rate of rotation of the counterweight framework is twice that of the 

 driving arms, a timing is possible every one-eighth cycle, or 45° of 

 phase. 



4. Connecting Conduits and Spools . 



The curved conduits under the cylinders and reservoirs are of 

 1.0-millimeter (0.040 inch) brass sheet, soldered along the joints 

 (Figs. 2 and 4). The cross-sectional area along the length of the con- 

 duits remains nearly constant, and a vane has been inserted to minimize 

 flow separation. Just above the curved conduits in the lower 18 centi- 

 meters of the cylinders and reservoirs, are flow adapters between the 

 rectangular cross section below and the larger circular cross section 

 above. These adapters are also made of soldered, curved brass sections, 

 and resemble large four- leaf clovers (not shown in the figures) . 



Each of the horizontal spools between the brass conduits and the 

 test section is 41 centimeters long and made of 12.7-millimeter-thick 

 (1/2 inch) Lucite. Each spool has been broken and rejoined with rubber 

 tubing to form an expansion joint, containing a screen. Screens are 

 also inserted between the spools and the curved conduits next to honey- 

 comb flow straighteners placed just inside the conduits. To remove 

 bubbles under the long level top of the test section, hatches have been 

 cut in the tops of the spools and Lucite barriers erected around them. 

 The hatch covers are removed when filling and the water level raised 

 until about 2 or 3 centimeters over the spool top, filling the test 

 section and flooding the enclosed areas over the spools. The bubbles 

 can then be swept out by pulling a buoyant sponge from one hatch to the 

 other at the opposite end. The hatches are then secured, the flooded 

 areas drained, and the filling continues. 



5 . Test Section . 



The test section is a Lucite tank with 19. 1-millimeter-thick (3/4 

 inch) walls and ends and with a 25 .4-millimeter-thick (1 inch) top 

 (internal dimensions are given in the Table). The walls are stiffened 

 externally by a pair of steel channels extending the length of the tank 

 and held together by tie rods through the tank and under the surface of 

 the sand bed. 



6. Permeability Study Configuration . 



In the permeability experiments, performed before the tunnel was 

 modified, the mean depth of flow over the sand bed was usually 16.6 

 centimeters, about half the present value (Lofquist, 1975). The test 

 section was divided longitudinally into two channels connected only by a 

 gap extending along the bottom of the test section beneath the sand. A 

 pair of plungers in the reservoirs oscillated vertically and oppositely, 



22 



