driven off an extension of tlie worm drive shaft by two separate scotch 

 yokes similar to but smaller than that previously described. The action 

 of the plungers caused a sinusoidal pressxire difference between the two 

 channels and an oscillatory permeability flow into and out of the sand 

 bed surfaces and through the connecting gap below. 



7. Durability . 



The materials in the apparatus which are in contact with water 

 (Lucite, anodized aluminum, brass, PVC) are noncorrosive in that element. 

 During modification of the tunnel, spots of a scaly deposit were found on 

 the inner walls of the cylinders, and were removed by brushing. These 

 spots may have been calcium deposited during a time before distilled 

 water was used. Lucite immersed in water for long periods will expand, 

 with an ultimate extension in length of about 0.33 percent. Allowance 

 for this expansion was made in the construction. No parts of the appa- 

 ratus are subject to significant wear except the piston seals which can 

 be replaced. 



V. OPERATIONAL SEQUENCE 



A typical sequence of operation of the water tunnel follows. It is 

 assumed that the sand is already in the tank, the system filled with 

 water, and the bubbles removed as described in Section IV, 4. The 

 flywheel is disengaged, and the motor is turned off. The pistons have 

 been bled and covered with a shallow layer of water to ensure that the 

 seals are always completely wet. The sequence then continues: 



(a) The driving arms are moved into horizontal position 

 by manually turning the drive shaft, since only when 

 the amis are parallel to the slots in the yokes can 

 the sleeves holding the bearing pins be freely moved. 



(b) The amplitude S is then set on the driving arms. 



(c) The motor is then turned on and the speed gradually 

 increased until the period is reduced to its oper- 

 ating value, T. 



(d) If S/Sj^ is considerable, e.g., greater than 0.3, 

 y/yjjj is adjusted in increments during speedup, 

 according to equation (16). Otherwise, y/ym can 

 be set when steady operation is attained. 



(e) The flywheel turning freely on the drive shaft is 

 then engaged. 



At the end of the experiment, the flywheel is disconnected, the speed of 

 the motor is reduced while adjusting y/yu^ in increments if necessary. 

 When the motor is at low speed it is turned off. 



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