■Ill
ill-
A,, I» ;;:ì
B (tT
move slightly either up or down, and an observation
is in tliis way very frequently vitiated. In almost
every serial temperature-sounding one or two of
tbe thermometers were evidently adrift from some
sucli cause. Tbere ivas an excellent proof tbat tbese
eccentricities did not always depend upon differences
of temperature. Very frequently, especially at considerable
depths, where tbe differences were very
slight, tliermometers sent to greater depths gave
indications liigber than those above tbem ; there
may be no absolute reason ivliy underlying water
might not in some cases have a temperatnre higher
than that of the layers ahove it, hut the thermometer
is not constructed to show snch an anomaly ; having
once registered its minimum, it has no power of
amendment.
I have no hesitation therefore in saying that any
single indication ivitli a thermometer on Six’s principle
is not trustworthy, and that a fact in temperature
distrihution can only be established hy a series
of corroborative determinations.
Although the gross errors to which an unprotected
thermometer is liable from pressure may he said to
he got rid of hy the addition of the outer shell, a
certain amount of error in the same direction still
remains, probably from a slight compression of the
unprotected parts of the tube. This error, which is
one of slight excess, altbougb for pi*actical purposes
it migbt perbaps be safely regarded as tlie same for
all tliermometers, is in detail special to eaeb instrument,
and all our tliermometers were tested by
Captain Tizard, and tbeir individual errors tabulated
for everv 100 fatlioms.
Tbe following table, ivbich is given as an example.
is in Fabrenbeit degrees.
o -S
MS sB rt rt
0 1 O 2
03
04
0 5
06
07
08
II
0000
0
00
0
0-2
0-2
0-2
0-4
0-3
0-3
0-2 0-2
Oto
0-4
0-4
0-7 0-6
0-6
(14
0-4
0-7 0-6
0-6
0-8 0-8
0 8
0-6
0 6
0-9
0-8
0 8
0-9
0-9
0-9
0-7
0-8
1-1
1-0
1-0 1-1
1-1
1-1
0'9
1-0
1-4 12
1-2
1-1
1-2
1-2 1-i
1'2
Tbese particular tbermometers ivere part of a batch
sent out to us late in the cruise, specially strengthened,
and certainly of a better construction than those
which we had had before. By testing a large series
of the earlier instruments in a Bramah’s press, Captain
Davis had come to the conclusion that when snbject-
ted to a pressure corresponding to a depth of 2,000
fathoms, they gave, broadly, a uniform error of 1° -4 F.
in excess, and tbat tbis correction might he applied
proportionally to the depth at which the observation
is taken, i.e. 0° -7 F. for every 100 fathoms. Tbis
may probably bold as a rough rule for ordinary instruments,
where absolute accuracy is not required.
On reconsidering th is m a tte r since our re tu rn
home, a doubt has arisen Avhether Ave Avere justified
in applying to the minimum side of tb e thermometer
these corrections on th e scale prepared by Captain
Davis, and a ncAv set of experiments lias been
A'OL. n , „