[65] U. S. FISH COMMISSION STEAMER ALBATROSS 67 



may be lifted and the water discharged. When the screw cap is press- 

 ing upon the upper valve the threads inside the former are engaged 

 with the threads of the shaft, but on screwing up the cap, when its 

 lower thread clears the upper thread of the corresponding series on 

 the shaft, the cap is uncoupled, which preveuts any mistake being made 

 at this point by the person handling the cup; afterwards the screw 

 cap may be turned in the same direction indefinitely without jamming 

 or changing its position on the shaft. 



" With the screw cap up and the propeller in any position, the cup is 

 automatic, and may, if desired, be lowered into the water with no 

 other preparation ; yet it is a good practice first to screw up the pro- 

 peller by hand to observe if the threads are in perfect working order. 

 Assuming the propeller to be low down on the shaft, or even resting 

 upon the screw cap, the action of the water is as follows : 



" As it descends, the valves are lifted and held up by the resistance 

 of the water; by the same agency the propel er is revolved and carried 

 upward until, like the screw cap, it is uncoupled, after which it revolves 

 freely on the shaft, impinging against the German silver sleeve i. If 

 the propeller hub is allowed to come iu contact with the sleeve while 

 the screw threads are still engaged, it may remain impacted during the 

 subsecpaent ascent. To insure uncoupling at the proper time the guide 

 cap which fits over the top of the hub must be set well home in its posi- 

 tion, when the propeller is fitted to its shaft. It will be noticed that 

 the blades of the propeller are bent along their upper edges. With the 

 blades thus bent, and all parts of the propeller made very light in 

 weight, it has been found experimentally that the alternating movement 

 of translation imparted to the submerged cup by the vessel's motion in 

 a sea-way will cause the propeller, wheu engaged with the threads on 

 the shaft, gradually to screw up rather than down. This shows that 

 stoppages in the descent, whether to attach additional cups to the rope 

 or wire, or for any purpose whatever, may be made with safety if the 

 vessel is kept idle in the water, that is, without headway or sternboard. 

 Were the blades not bent it is evident that the propeller would gradu- 

 ally screw down by the same alternating movement, since its weight 

 would assist its action in screwing down, but resist the opposite motion. 

 Even thus experiments have shown that with the alternating move- 

 ment continued for a longer time than would probably be occupied by 

 any stoppage, the propeller would screw down on the shaft only a small 

 proportion of the distance to the screw cap. It is plain that in the 

 event of such action the propeller would rise and uncouple each time the 

 descent was continued. However, the bending of the blades insures 

 safety, and the valves are left' free to open during the whole descent. 

 At any stoppage in the descent each cup contains within its cylinder a 

 specimen of the water from its locality at the time being, allowing a 

 margin of 1 or 2 feet. 



"As soon as the ascent is begun the valves of each cup are pressed 



