6 
THE VOYAGE OF H.M.S. CHALLENGER. 
over into the position Fig. 9. In getting into this position the weight drags the sling 
out of the notch h, and it falls into the notch c. 
Here it remains as long as the tube is at the 
bottom, exerting all its weight in pushing it into 
the ground. On heaving in, the tumbler is drawn 
into an upright position, when the sling slips free 
and the tube is brought up without the sinker. 
When it has been brought to the surface, it is 
found that the mud tube B is filled with a com- 
pact cylinder of mud, which by its weight has 
kept the india-rubber valves closed by drawing 
them tight down on their seats, and has therefore 
insured that the water enclosed at the bottom has 
not been contaminated by admixture with other 
water on the way up. 
The localities, even in mid ocean, where the 
bottom is “ hard ground,” are by no means rare, 
and if the tube just described be dropped on it 
with a 50 Ib.^ sinker, the mud tube will be much 
disfigured ; but if there be any loose material at 
all, such as gravel or coral, a little of it will 
probably be entangled behind the comb valve. 
In the absence, however, of a mud plug, the 
bottom water will be valueless. As a rule, the 
bottom of the deep sea consists of mud sufiiciently 
soft and tenacious to fill the mud tube throughout 
the greater part of its length with a compact plug, 
and if the tube B be screwed water-tight into the 
lower part of the tul^e A, it is retained in it just as 
a liquid is retained in a pipette. In soft ground, 
clays and most Globigerina Oozes, it is better 
to discard altogetlier the comb valve L, because 
it always oflTers some resistance to the entrance 
of the mud, and is not wanted to keep it in. 
The instruments are fitted with mud tubes of two 
sizes, namely, the smaller 1 inch^ in diameter, and 
the larger inches® in diameter. In the ordinary 
routine work of running a line of soundings the smaller size should be used and without 
• 22'8 kilognuiimeit. * 25‘4 iiiiii. ^ 44‘46 mm. 
Fio. 11. — The Slip Water-Bottle. 
A, braM cyllixler sliding ap and down the metal shank 
B, sttacherl by a wire to the slipping arrangement E, 
fixed to the «>d of the itxl F. When the ai>paratus 
reaches the bottom the cylinder is released and falls 
down to the lower j*rt, where it rests on the lower of 
two sernrstely ground valves C and D ; the water 
cacloMd Is removed by the tap U. 
