MUSEUM OF COMPARATIVE ۰ 179 
that the screw-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. 
