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 
