134 LECTURES TO SCIENCE TEACHERS. 



rock or beside it. And in one way I consider that the depths 

 given by this instrument are more valuable than the others. 

 You do not want to know what is the accidental depth 

 if there is a big stone just under the ship ; but you want 

 to know the depth of the ocean, and this gives it more 

 correctly than a sounding-line. 



I have now endeavoured to give you a short account of 

 this instrument, and before concluding I would add a few 

 words regarding its possible application. I have at the 

 outset observed that my attention was directed to the matter 

 from the necessity I felt for getting even approximate 

 indications of the depth below a cable-laying ship ; but 

 would not such an instrument be also of considerable use on 

 board an ordinary ship ? Suppose that we knew the con- 

 figuration of the bottom of an ocean say of the Atlantic 

 more accurately than we do at present, and that a ship 

 with such an instrument on board were to start from 

 America to come to this country ; suppose gales supervened 

 and that the state of the clouds made astronomical obser- 

 vations impossible, and this continued to be the case for 

 days together, such an instrument would give the navigator 

 at all times an account of the depth below him, and if he 

 traced his line over the ocean according to his dead-reckon- 

 ing, coupled with observations of this instrument,' it would 

 be almost impossible to lay down two lines that would 

 answer to the two observations. The variations of depth 

 together with the direction of the compass and the dead- 

 reckoning would give him with very considerable precision 

 the position in which the ship was at any moment, because 

 if he assumed that gales had driven him away from his 

 course, it would be impossible to find in any other latitude 

 the soundings correspond with the indication of the instru- 

 ment ; and more particularly in approaching the opposite 

 shore the navigator would obtain from such an instrument 

 a very fair indication, not only of his distance from 

 shallower waters, but, through the ratio of variations 

 shown by the instrument, the exact position at which he 

 approached the land. It is on that account that I believe 

 this subject is well worth following further in order to make 

 the instrument perhaps more compact and more perfect in 

 every respect, so as to bring it within the compass of the 

 navigator ; and if by this means the safety of navigation 



