128 
NWATORER Ee 
from shocks) to a measured=depth in the sea. This idea is 
worthy of consideration, especially if the gauge be made to 
register by means of a silvered tube. The only probable cause 
of error in such a case would be the breaking of the mercury 
column by a jerk, and to this all other forms are at least equally 
liable. 
XIII. £xternal Pressure Gauge.—But it was necessary not 
merely to measure accurately the pressure applied, but also, for 
the sake of the thermometers, to provide that the pressure should 
not be carried too far ; and for that purpose it was indispensable 
to have an exterior indicator of pressure. This was furnished 
by a thin cylindrical steel tube inclosed in a cavity bored in a 
large block of iron, the interior of the steel tube being full of 
mercury and the narrow space between it and the large iron 
block also full of mercury, ‘This exterior space was connected 
with the pressure apparatus. The pressure then throughout the 
whole of the space exterior to the steel cylinder was the pressure 
inthe pump. The steel cylinder was therefore compressed from 
the outside. In the neck of the steel cylinder, which was 
screwed into the surface of the block, there was luted a vertical 
glass tube. It was exposed to no pressure, but the mercury in 
it rose, by the compression of the steel cylinder, and the height 
to which it rose could be easily measured. Comparative experi- 
ments were made several times by putting one of the glass 
gauzes, whose scale had been carefully ascertained, inside the 
apparatus, while this newly-described gauge was also connected 
with it. In this way the external gauge was accurately calibrated. 
Bur, le-t an accident should happen to one of the gauges, or to 
its index (as sometimes was the case) no experiment was made 
without the presence of at least three gauges. The way in 
which these worked together during the whole course of the 
experiments is the best possible proof of their value. This form 
of gauge, also, is greatly improved by inserting a glass tube 
closed at both ends into the bulb; for the temperature changes 
produced by pressure in mercury are greater than those in water 
at ordinary temperatures. 
XIV. Results of the Experiments, The True Correction for Pres- 
sure is very small.—As soon as I applied pressure to the C/a/- 
lenger thermometers I found I reproduced pretty nearly the 
results obtained by Capt. Davis. I had already seen one proof 
that at least a large part of the result was in all probability not 
due directly to pressure. The experiment with the long thermo- 
meter tube showed that my theorctical calculations had been 
correct. The question thus became :—Is this a pressure effect of 
any kind ; and, if so, how does it originate? and if it is not a 
direct pres-ure effect, to what is itdue? There are many ways 
of answering such questions, One answer was furuished by one 
of the thermometers (A 3), whose degrees (especially on the 
maximum side) are very short. ‘The whole effect (in degrees) on 
this thermometer was not very markedly greater for a given 
pressure than on the others, as it would certainly have been had 
the effect been entirely due to pressure directly. Another. is, if 
it be not a direct pressure effect it must bea heating effect. With 
Sir Wyville Thomson's permission I got from Mr, Casella, the 
maker of the Challenger thermometers, a couple of others of 
exactly the same form and dimensions, but with the bulbs 
plu.ged after the manner of the gauges already described, so as 
to diminish their susceptibility to changes of temperature. When 
I put one of these into the pressure apparatus along with one of 
the Challenger thermometers, I fuuad the effects on the new 
form very much smaller than on the old. Thus it was at once 
proved that the effect could not be due to wry-neckedness pro- 
duced by the fitting on of the protecting bulb; which would 
have been an effect due to pressure directly ; but that it must be 
an effect due to heat. That is to say, it was now completely 
established that the large results obtained by Capt. Davis are 
due in the main to causes which can produce no effect when the 
therwometers are Jet down gradually into the deep sea; they are 
due to causes connected with the thermometers, and perhajs 
also with the pump, but solely under the circums'ances of a 
laboratory ex;eriment. 
XV. Sources of the large Effict obtained in the Press.—Now 
comes the question (no longer important to the Challenger work, 
but of great scientific interest), What are these various source, 
and how much of the effect is due to each? First of all we have 
seen that the water in the press is heated when pres-ure is 
applied: Using Sir William Thomson’s formula I found the 
amount of that heating should be about 0°05 F. at 43° F., 0°16 
at 50°, and only 0°°3 at 59°, for one ton of pressure, [These 
nuuibers are rather too small. We do not yet know to what 
extent the temperature of the maximum density point of water is 
= 
lowered by pressure.] These cannot be expected to be fully 
shown under the circumstances of the experiments, and even if 
they were fully shown the greatest of them represents only about 
one-half of the whole of Capt. Davis’ result ; there must there- 
fore be some other cause. [Prof, Tait then gives details of the 
various experiments by which he traced the sources of the large 
effect obtained. 
‘Thus it appears that there are no less than five different causes 
which contribute each its share to Capt. Davis’ result. Of 
these, one is independent of the others, aud would produce its 
full eftect even if they were not present. The other four give 
effects which are not cumulative, and it would be very trouble- 
some to try to assign to each its exact share of the result when 
two or moreact together. Fortunately, it will be seen that we 
do not require to attempt to solve this problem. 
(1.) First is the direct effect of the external pressure upon the 
expo ed part of the thermometer tubes. This, in general, will 
be found very small, except in tubes where there are large 
aneurisms, The whole effect of 3 tons pressure on a Challenger 
thermometer without aneurisms, at temperatures near freezing 
point, so far as the minimum index is concerned, would be only 
about 3 one-thousandths of 30 degrees or so, that is 90 thou- 
sandths or at most o'r of a degree for 3 tons pressure. That is 
an amount which, in consequence of the necessary errors of 
reading the thermometers, may be entirely neglected, and, unless 
there are large aneurisns, there will be little need for pressure 
corrections even in six miles of sea. ; 
The other parts of the observed effect were 
(2.) Heating of water. This 1 observed to follow very nearly, 
according to Thomson’s formula, the original temperature of 
the water. By comparing the pressure effects on the same 
thermometers during summer, and during winter (for which 
latter the late continued frost was of particular service, and 
enabled me to work for many days at the temperature of the 
maximum density of water), I found the results to vary in 
accordance w.th calculation. 
(3.) Heat due to frictionduring pumping. This from its very 
nature was unavoidable unless we could have got an apparatus 
into which (by enormous pressure) the plug could have been 
forced directly. This could not, however, have been done in 
my laboratory, even if the apparatus had been adapted to such 
a form of exjeriment. But it was very easy to calculate the 
extreme pos-ible amount of this effect. ‘ 
(4.) The peculiar heating effect due to the valcanite mounting. 
I verified this effect-of vuleanite by taking a thermometer which 
had no vulcanite about it and measuring the effect produced 
upon it by a definite pressure, and then putting loosely round 
the bulb (ina test-tube, which had itself been previously experi- 
mented on) a small quantity of vulcanite in thin plates, I found 
that so little as 8 grammes of vulcanite round the protecting bulb 
raised the effect produced by a pressure of 3°2 tons weight from 
os F.to 11 F. The vulcanite was in thia strips about a 
millimetre and a half in thickness. The effect of the vulcanite 
on the Challenger thermometers (in the hydrostatic press) must, 
from the mode of their construction and mounting, in all cases 
be considerably greater than this. 
Under these ci-cumstances, we might without farther inquiry 
fairly attribute the whole outstanding effects to the massive 
vulcanite slabs on which these thermometers are framed. But 
there still remains 
(5.) The most difficult question of all, the temperature effect 
produced by pressure upon the protecting bulb, which is under 
different circumstances altogether fron the vu'canite; for the 
vulcanite is simply compressed, while the glass sheath is under 
pressure oa one side and not on another, and is therefore 
subject to shear as well. In its interior the glass is extended in 
a radial and compressed in a tangential direction, Nobody has 
yet made any approximation to an answer to the question what 
effect in the way of heating or cooling will be -produced by 
deformation which consists partly of compression and partly of 
change of form, We know tbat in indiarubber a cooling effect 
is produced by traction, and il may happen that a similar change 
of form in glass also produces a reductioa of temperature. This 
is a question, however, which is not capable of answer by the 
help of my present apparatus ;—though it will probably be 
answered by experiment before theory is able to touch it, The 
results of my experiments on the thermometers with plugged 
bulbs show that, on the whole, a heating effect results from the 
combined compression and shear in a bulb exposed to external 
pressure only. This has been verified by cutting down a ther- 
mometer, an exact counterpart of the C/ad/enger thermometers 
