MUSEUM OF COMPARATIVE ZOOLOGY. 179 



that the 8crew-cap can be got out of this second position only by hand, 

 for the lugs and slots are bevelled, and the lugs can clutch the screw- 

 cap only in one direction. The resistance of the water which would 

 reverse the propeller at a stoppage on the ascent would also lift the 

 propeller clear of the screw-cap. If each propeller were regulated to 

 close down the screw-cap in passing through equal distances in water, 

 each cup would be locked when the cup had passed through that dis- 

 tance after the commencement of the ascent. The screw-cap is found 

 screwed down so tight in coming out of the water that it can be set no 

 tighter without endangering the thread. 



