52 
CRUISE OF H.M.S. CHALLENGER. 
book provided for that purpose. These intervals 
gradually increase in duration as more line is run 
out, the weights having to overcome the friction 
of the line in the water, which becomes greater 
with the amount run out. The intervals are found, 
however, to extend in regular proportion, so that 
when four minutes are taken up by one interval, 
the weights have reached the bottom, or a depth of 
between 2000 or 3000 fathoms has been obtained. 
The time intervals having informed us that the 
weights are at the bottom, the line is brought to the 
engine, and hove in, gently at first, but faster as the 
quantity out decreases ; care being taken to keep the 
ship still in her position over the line, as, if allowed 
to fall off, the line has not only to bear its own fric- 
tion, and that of the attached rod, water bottle, and 
thermometers, but also the additional friction of the 
drift of the ship. Eventually the rod, water bottle, 
and thermometers reach the surface, the thermometer 
is carefully read and registered, the water bottle is 
sent down to the laboratory, where the specific gra- 
vity of the water is taken, and the contents of the 
sounding rod are examined to ascertain the nature of 
the bottom, after which they are dried and bottled. 
The soundings having been obtained, and the line 
hove in, the next proceeding is to register the tem- 
perature of the ocean from the surface to the bottom. 
This is done by attaching thermometers with equal 
spaces between them to the sounding line ; a cup-lead 
