226 THE DEPTHS OF THE SEA. [chap. v. 



a descent of 225 fathoms. Where greater depths 

 are required it is only necessary to add another 

 dial and index. This sounding instrument answers 

 very well in moderately deep water, and is extremely 

 valuable for checking soundings by the ordinary 

 method, where deep currents are suspected, as it 

 ought to register vertical descent only. It is not 

 satisfactory in very deep water, and its uncertainty 

 is shared apparently by all instruments involving 

 metal wheel-work. It is difficult to tell the reason. 

 The machinery seems to get jammed in some way 

 under the enormous pressure of the water. 



The ' Massey's sounding-machine ' in common use 

 is somewhat different from the ' shield ' instrument 

 described and figured above. It is constructed on 

 precisely the same principle, but it is bolted to a 

 special form of sounding lead, and is thus somewhat 

 more cumbrous. 



Besides the increasing attention which has been 

 paid of late years to all subjects of scientific interest, 

 and especially to those connected with physical geo- 

 graphy, the conditions of the depths of the sea, the 

 nature of the bottom, the force and direction of deep 

 currents, the temperature at great depths, and, in 

 fact, all the conditions affecting the sea bottom, 

 have lately acquired great practical importance in 

 connection with telegraphic communication by ocean 

 cables. 



The Atlantic Ocean, with the accessible portions 

 of the Arctic Sea, has naturally, from the relation in 

 which it stands to the first maritime and commercial 

 nations of the present period, been the most carefully 

 surveyed ; and as it appears to contain depths nearly 



