DEEP-SEA THERMOMETERS.
=89. On Sixe's Principle.=--Thermometers for ascertaining the temperature
of the sea at various depths are constructed to register either the
maximum or minimum temperature, or both. The principle of each instrument
is that of Sixe. There are very few parts of the ocean in which the
temperature below is greater than at the surface, except in the Polar
Seas, where it is generally found to be a few degrees warmer at
considerable depths than at the surface. When the instrument is required
to register only one temperature, it can be made narrower and more
compact--a great advantage in sounding; and with less length of bulb and
glass tube, so that the liability of error is diminished. Hence, the
minimum is the most generally useful for deep-sea soundings. These
thermometers must be sufficiently strong to withstand the pressure of the
ocean at two or three miles of depth, where there may be a force exerted
to compress them exceeding three or four hundred atmospheres (of 15 lbs.
to the square inch).
Many have been the contrivances for obtaining correct deep-sea
indications. Thermometers and machines of various sorts have been
suggested, adopted, and eventually abandoned as only approximate
instruments. The principal reason for such instruments failing to give
correct or reliable indications, has been that the weight or pressure on
the bulbs at great depths has interfered with the correct reading of the
instruments. Thermometers have been enclosed in strong water-tight cases
to resist the pressure; but this contrivance has only had the tendency to
retard the action, so much so as to throw a doubt on the indications
obtained by the instrument so constructed.
The thermometers constructed by Messrs. Negretti and Zambra for this
purpose do not differ materially from those usually made under the
denomination of Sixe's thermometers, except in the following most
important particular:--The usual Sixe's thermometers have a central
reservoir or cylinder containing alcohol; this reservoir, which is the
only portion of the instrument likely to be affected by pressure, has
been, in Negretti and Zambra's new instrument, superseded by a strong
outer cylinder of glass, containing mercury and rarefied air; by this
means the portion of the instrument susceptible of compression, has been
so strengthened that no amount of pressure can possibly make the
instrument vary. This instrument has been tested in every possible manner,
and the results have been highly satisfactory, so much so as to place
their reliability beyond any possible doubt.
The scales are made of porcelain, and are firmly secured to a back of oak,
which holds in a recess the bulb with its protecting shield, and is
rounded off so as to fit easily and firmly in a stout cylindrical copper
case, in which the thermometer is sent down when sounding (see fig. 69).
The lid of the case is made to fit down closely, and water-tight. At the
bottom of the case is a valve opening upward; and the lid has a similar
valve. These allow the water to pass through the case as the instrument
sinks, so that the least amount of obstruction is offered to the descent.
At the lower end of the case is a stout brass spring, to protect the
instrument from a sudden jar if it should touch the bottom while
descending rapidly. As the instrument is drawn up, the valves close with
the weight of water upon them, and it arrives at the surface filled with
water brought up from its lowest position. The deep-sea thermometers used
in the Royal Navy are of this pattern.
[Illustration: Fig. 69.]
=90. Johnson's Metallic Deep-Sea Thermometer.=--The objection to the
employment of mercurial thermometers for ascertaining the temperature of
the ocean at depths, arising from the compression of the bulbs, which was
of such serious consequence previous to the modification made in the
construction of the instrument by Messrs. Negretti and Zambra, led to the
construction of a metallic thermometer altogether free from liability of
disturbance from compression by the surrounding water; which, however, is
certainly not so sensitive to changes of temperature as mercury. This
instrument is the invention of Henry Johnson, Esq., F.R.A.S., and is thus
described by him:--
"During the year 1844 some experiments were made by James Glaisher, Esq.,
F.R.S., on the temperature of the water of the Thames near Greenwich at
the different seasons of the year; when that gentleman found that the
indications of temperature were greatly affected by the pressure on the
bulbs of the thermometers. At a depth of 25 feet this pressure would be
nearly equal to the presence of three-fourths of an atmosphere. These
observations demonstrate the importance of using in deep-sea soundings an
instrument free from liability of disturbance from compression by the
surrounding water, and have ultimately led to the construction of the
thermometer now to be described.
"The instrument is composed of solid metals of considerable specific
gravity, viz. of brass and steel, the specific gravity of these metals
being 8.39 and 7.81 respectively. They are therefore not liable to
compression by the water, which under a pressure of 1,120 atmospheres, or
at a depth of 5,000 fathoms in round numbers, acquires a density or
specific gravity of 1.06. In the construction of this instrument,
advantage has been taken of the well-known difference in the ratios of
expansion and contraction by heat and cold of brass and steel, to form
compound bars of thin bars of these metals riveted together; and which
will be found to assume a slight curve in one direction when heat has
expanded the brass more than the steel, and a slight one in the contrary
direction when cold has contracted the brass more than the steel.
"The indications of the instrument record the motions under changes of
temperature of such compound bars; in which the proportion of brass, the
more dilatable metal, is two-thirds, and of steel one-third.
[Illustration: Fig. 70.]
"Upon one end of a narrow plate of metal about a foot long, _a_, are fixed
three scales of temperature, _h_, which ascend from 25 deg. to 100 deg. F., and
which are shown more clearly in the drawing detached from the instrument.
Upon one of these scales the present temperature is shown by the pointer,
_e_, which turns upon a pivot in its centre. The register index, _g_, to
the maximum temperature, and the index, _f_, to the minimum temperature,
are moved along the other scales by the pin upon the moving pointer, at
_e_, where they are retained by stiff friction. At equal distances from
the centre of the pointer are two connecting pieces, _d d_, by which it is
attached to the free ends of two compound bars, _b b_, and its movements
correspond with the movements of the compound bars under variations of
temperature. The other ends of the bars are fastened by the plate, _c_, to
the plate, _a_, on which the scales of temperature are fixed. The
connection of the bars with both sides of the centre of the pointer
prevents disturbance of indication by lateral concussion. The case of the
instrument has been improved at the suggestion of Admiral FitzRoy, and now
presents to the water a smooth cylindrical surface, with rounded ends, and
without projection of fastenings.
"In surveying expeditions, this instrument would be found useful in giving
notice of variation of depth of water, and of the necessity for taking
soundings. A diminution of the temperature of water has been observed by
scientific voyagers to accompany diminution of depth, as on nearing land,
or approaching hidden rocks or shoals. Attention would also thus be
attracted to the vicinity of icebergs."
This thermometer might easily be modified to serve for several other
important purposes, such as the determination of the temperature of
intermittent hot springs, and mud volcanoes.
[Illustration: Fig. 71.]
The principle of this thermometer is not altogether new; but the duplicate
arrangement of the bars, which effectually prevents the movement of the
indices by any shaking, and the application are certainly novel. Professor
Trail, in the _Library of Useful Knowledge_, writes:--"In 1803, Mr. James
Crighton, of Glasgow, published a new 'metallic thermometer,' in which
the unequal expansion of zinc and iron is the moving power. A bar is
formed by uniting a plate of zinc (fig. 71), _c d_, 8 inches long, 1 inch
broad, and 1/4 inch thick, to a plate of iron, _a b_, of the same length.
The lower extremity of the compound bar is firmly attached to a mahogany
board at _e e_; a pin, _f_, fixed to its upper end, plays in the forked
opening in the short arm of the index, _g_. When the temperature is
raised, the superior expansion of the zinc, _c d_, will bend the whole
bar, as in the figure; and the index, _g_, will move along the graduated
arc, from right to left, in proportion to the temperature. In order to
convert it into a _register thermometer_, Crighton applied two slender
hands, _h h_, on the axis of the index; these lie below the index, and are
pushed in opposite directions by the stud, _i_,--a contrivance seemingly
borrowed from the instrument of Fitzgerald," a complicated metallic
thermometer, described by the Professor previously.