370 POPULAR LECTURES AND ADDRESSES. 



a sudden decrease in the speed of rotation of the 

 wheel. The moment this is observed, a man 

 standing at the wheel grasps it with his two hands, 

 and stops it. Not more than three or four hundred 

 fathoms of wire having run out, the hauling-in is 

 easy. In following this process I have generally 

 found it convenient to arm the lead with a proper 

 mixture of tallow and wax, in the usual manner, to 

 bring up specimens from the bottom. The actual 

 depth is, of course, less than the length of wire run 



FIG. 45. Flying Soundings. 



out. The difference, to be subtracted from the 

 length of wire out to find the true depth, may be 

 generally estimated with considerable accuracy 

 after some experience. The estimation of it is 

 assisted by considering that the true depth is 

 always, as we see from the annexed diagram, 



greater than / a and less than </P a 2 , where / 

 denotes the length of wire out, and a the space 

 travelled by the ship, diminished by the space 



