Nov. 24, 1881] 
NATURE 
OI 
introduced, except after special cooling, into the press with the 
index suitably adjusted to the temperature of the water. The 
whole difficulty might have been avoided by putting an exceed- 
ingly small piece of iron or steel wire above the index, to be 
acted on by a sufficiently powerful magnet. 
Thus, although these instruments are absolutely perfect so far 
as regards immunity from pressure (and in other essential respects 
which will be mentioned later), it is not easy to work with them 
uader the circunstances of this investigation. 
IV. LZndividual Peculiarities of some of the ‘‘ Challenger” 
| Thermometers.—The Challenger thermometers are not all exactly 
similar to one another. Some of them have their degrees very 
much longer than others ; others have the extraordinary peculiarity 
that the degrees upon the maximum side are nearly half as long 
again as those on the minimum side, and sometimes it is the 
reyerse. Inone of the instruments which was occasionally used 
in the deep sea, the length of a single degree on the maximum 
side is only about three-fourths of a millimetre, and thus a reading 
to a tenth of a dezree is not tv be looked for. But on account 
of this unexpected peculiarity this particular instrument was of 
use, as will be seen later, in demonstrating that the effects pro- 
duced in the press were due partly to heating, partly to compres- 
sion. Several instances of useful peculiarities of a similar 
character were detected, and utili ed. 
In fact, the instruments cannot be said to do more than furnish 
roush and ready means of approximating to temperatures within 
about a quarter of a degree, or in the most favourable circum- 
stances a tenth of a degree Fahrenheit. Had they been more 
nearly what would be called ‘* scientific” instruments, they might 
have altogether failed on acc .u.t of the rough treatment to which 
they were necessarily subjected during ue. Letting them down 
into the sea presents in general no great difficulties, but when 
they have to be hauled on board again they are subject to jerks 
and shocks, and sometimes swing through large arcs at the end 
of the lead line. Such misadventures are unavoidable at sea, 
and are excessively unfavourable to accu-ate results, because the 
index is necessarily not fitted so tightly in the stem that it may 
not in a few oscillations be sensibly displaced. And there is a 
defect inseparable from the use of movable indices :—viz. that the 
reading of the mercury column is sensibly different according as 
the index is, and is not, in contact with it. The capillary con- 
vexity affects the maximum and minimum indices in offosite 
ways. 
Further, I may observe (though it does not affect my work) 
that in these thermometers the scale is at some distance from the 
mercury in the stem, and no provision is made for avoiding 
parallax or personal equition. By merely altering the position 
in which one holds the thermometer, it is possible to read the 
temperature whether by the mercury column or the end of the 
index next it, to an amount diff-reat in some of the thermometers 
by as much as a quarter of a degree, and in the great majority 
of them by as much asateath, Thus if we get readings con 
sistent within a tenth of a degree we get all that the instruments 
are capable of furnishing. I have therefore always read the 
thermometers in exactly the same position and (when so much 
accuracy was attainable) only to the nearest tenth of a dezree. 
And I have always made my comparisons between successive 
positions of the index; the only readings of the mercury directly 
being taken rouzhly to find whether any permanent temperature- 
change had been produced in the water of the press by pressure 
or otherwise, during the course of an experiment. 
A great many different materials were tried for the framing of 
the thermometers : and vulcanite was finally chosen, having been 
found to answer the purpose exceedingly well, Wood warped, 
and metal was; unsuitable for various reasons. It is rather 
curious to find, as will be seen below, that this substance was 
one of the mai: cau.es of the very large amount assigned to the 
pressure-correction. 
V. Capt. Davis’ Mode of Testing ; and his Correction for the 
Maximum Side.—\t is necessary to look somewhit closely into 
the mode in which Capt. Davis conducted his experiments, in so 
far at least as it differs from the one I afterwards employed ; in 
order that we may be able to form an idea how, wih nearly all 
the facts before him, he yet failed to get their proper interpreta- 
tion. Take, for instance, the way in which he attempted to deter- 
mine the correction which is due to the heating of water by 
compression. This, of course, affects the thermometers while in 
the hydrostatic press, but of when they are let down into the 
sea. When the water in the press is compressed with the ther- 
mometers in it, it becomes hotter as the pressure increases (so 
long at least as its temperature is above 4° C. or 39°'2 Fahr. that 
of its maximum density). This is quite analogous to the heating 
of air in a cylinder when a piston issuddenly forced down; when, 
as every one knows, tinder can be kindled by the heat developed. 
So water is heated by compression, but not to anything like the 
same extent. But it is necessary to remark that the amount of 
heating of water by a given compression depends in a very 
curious manner upon the original temperature of the water. For 
water taken at its maximum density is neither heated nor cooled 
by compression, but it is heated by compression if it is at a 
temperature higher, and cooled if it is at a temperature lower, 
than that of the maximum density. One set of Capt. Davis’ 
observations were made in water at temperatures near, but 
under, the maximum density point : in which, therefore, very 
little effect can be produced, even by very great pressure (and 
that little should be cooling, not heating), and he combined these 
with a number of other observations made at temperatures ap- 
proaching 55° F., in which a comparatively large amount of heat- 
ing is produced even by moderate pressures. The average of the 
results of these determinations was taken, but, unfortunately, 
Capt. Davis struck out before taking the average all those 
observations which appeared to give much larger effects than the 
others, taking them as being obviously erroneous. 
When we sift out from the observations all those made nearly 
at any one temperature we find they agree fairly enough with the 
theoretical result of the compression. But observations made 
at different temperatures were included in the group from which 
the average effect was deduced, Such an average has no physical 
meaning. 
Capt. Davis concluded from two sets of observations, one at 
55° F. and the other about 39° F., that little attention need be 
paid to the heating of water by compression, and thu; that the 
effect observed in the hydraulic press was due mainly to direct 
pressure, and would, of course, be experienced by the thermo- 
meters when they were let down into the sea. 
The officers who managed the thermometers of the expedition, 
were, in consequence, furnished with corrections for each ther- 
mometer, all of the order already indicated, z.e. about half a 
deszree for each mile under the surface of the sea. These 
corrections were, of course, for the maximum side of each 
instrament. 
VI. Conseguent Correction for the Minimum Side.—Looking 
at the thermometers, it seemed to me perfectly evident that this 
correction, if it was to be applied at all, must be applied in very 
nearly the same amount both to the maximum index, for which 
it was determined, and also to the minimum. Any difference 
between these two must be due solely to the effects of tempera- 
ture upon the column of mercury which lies between the two 
indices, and of pressure on the tube containing that mercury. 
Unless the heating effect were confined to the space between the 
indices, the former is provided for by the graduation of the 
instrument itself ; and it was quite certain that the two together 
could not produce an effect amounting to more than a small 
fraction of the degree and a half for three tons pressure, 
Therefore, as all the readings of the Chad/:2ger thermometers 
were taken from the minimum index, they were subject, accord 
ing to iy interpretation of Capt. Davis’ results, to a correction 
of very nearly half a degree Fahr. for every mile of depth. 
Now, even if the heating effect on the water in the press had 
been correctly determined, the result would have led to a deduction 
of at the utmost only about one-fourth of the whole correction, 
thu; still leaving a very formidable correction indeed. 
VII. Theoretical Determination of the Direct Effect of Pressure. 
Experimental Verification.—1 therefore calculated the effect of 
pressure on a thermometer tube, assuming the best data for the 
compressibility and the rigidity of glass. The result, so far as 
is required for the present argument, is that the internal capacity 
of a glass tube (whose walls are thick in comparison with the 
diameter of the bore) is reduced by about 1-1cooth part for each 
ton weizht (per square inch) of pressure applied from without ; 
the ends being closed. Hence, if sucha tube be partly filled 
with mercury, with an index above it; the index should be dis- 
placed by 1-1000th of the length of the column of mercury for 
each ton weight of pressure applied to the outside of the tube. 
I tried the experiment with a thermometer tube, the length of 
the mercury column being as nearly as possible a metre, and I 
found for every ton-weight of pressure to which the tube was 
exposed the index was displaced by one millimetre, the 1-1o0oth 
part of the length of the column precisely, being far more nearly 
yban I had expected the result I had already calculated from 
