44 
CRUISE OF H.M.S. CHALLENGER. 
sudden jerks of the ship’s motion from bringing too 
great a strain on the lead line. At the bottom ot 
the accumulators, which are kept separated from 
each other by being passed through holes in a 
Fig. 2 . circular disk of wood, a 9-inch block is 
hooked, and through this block the lead 
line is rove. The end of the line is then 
secured to the sounding-rod, to which is 
attached the number of iron weights re- 
quired to sink it rapidly. A short distance 
above the rod the slip water-bottle is fas- 
tened, and above that a deep-sea thermo- 
meter. 
Two descriptions of sounding-rods have 
been in use during the cruise. The one 
first employed is known as the “Hydra” 
rod (Fig. 2), and consists of a brass tube 
1^ inch in diameter, and 42 inches in 
length, having at its extreme end a but- 
terfly valve, and at its top a sliding rod 30 
inches in length. On the upper part of 
this rod is a small stud, with a spring 
reaching to its head (when there is no pres- 
sure on it) ; to this rod the weights are attached, 
and, by means of the spring, disengaged, when at the 
bottom, in the following manner. 
The sinkers are of cast iron, and average one hun- 
dredweight each. They are cylindrical in form, having 
a hole through the centre ; through this hole the rod is 
