SELF-REGISTERING THERMOMETERS.
=69. Importance of Self-Registering Thermometers.=--Heat being apparently
the most effective agent in producing meteorological phenomena, the
determination of the highest temperature of the day, and the lowest during
the night, is a prime essential to enable an estimate of the climate of
any place to be formed. To observe these extremes by means of the ordinary
thermometer would be impracticable, from the constant watchfulness which
would be necessary. Hence, the utility and importance of self-recording
thermometers are evident. A thermometer constructed to _register_ the
highest temperature is usually called a _maximum thermometer_; one to show
the lowest temperature is termed a _minimum thermometer_; and if made to
record both extremes of temperature, it is designated a
_maximum-and-minimum thermometer_. We will, for the sake of method,
describe the instruments in use in this order.
It would carry us beyond our scope to explain in detail the methods of
dealing with temperature observations; but we may remark that half the sum
of the maximum and minimum temperature of each day of twenty-four hours,
is not what meteorologists designate the _mean daily temperature_,
although it very frequently approximates to it. The mean temperature of
the day is understood to be the average of twenty-four consecutive hourly
readings of a thermometer; and meteorology now supplies formulae whereby
this result can be deduced from two or three observations only in a day.
But we would observe that the actual mean temperature of any place has not
such an important influence upon life, either animal or vegetable, as the
abruptness and magnitude of the variations of temperature. Climate,
therefore, should be estimated more by the range of the thermometer than
by the average of its indications. The Registrar General's returns prove
that with a wide range of the thermometer, the mortality greatly
increases; and it is now becoming apparent to meteorologists that the
daily range of the thermometer marks the effects of temperature on the
health of men, and the success of crops, better than any other
meteorological fact of which we take cognizance. Now that self-registering
thermometers are constructed with mercury, the most appropriate of all
thermometric substances, not only for maxima, but likewise for minima
temperatures, the determination of the diurnal range of temperature is
rendered more certain, and observations at different places are more
strictly comparable.
MAXIMA THERMOMETERS.
=70. Rutherford's Maximum Thermometer.=--The maximum thermometer, invented
by Dr. John Rutherford, differs from an ordinary thermometer in having a
small cylinder of steel, porcelain, or aluminium, moving freely in the
tube beyond the mercury, so as to form an index. The stem of the
thermometer is fixed horizontally on the frame, which must be suspended in
the same position, as represented in fig. 53. The instrument is set by
holding it bulb downward, so as to allow the index to fall by its own
gravity into contact with the mercury. Increase of heat produces expansion
of the mercury, which consequently pushes forward the index. When the
temperature decreases, the mercury recedes from the index, leaving it so
that the extremity which was in contact with the mercury indicates upon
the scale the highest temperature since the instrument was last set.
[Illustration: Fig. 53.]
As it is easily constructed and is comparatively cheap, it is still
employed for ordinary purposes. Its disadvantages are, firstly, its
liability of soon getting out of order by the index becoming embedded in
the mercury, or fixed by oxidation, thus rendering it altogether useless;
secondly, the ease with which the index can be displaced by the wind
moving the instrument, or other accidental disturbance, so as to cause it
to give erroneous indications occasionally; and thirdly, its consequent
total unfitness for use at sea.
In the part of the tube beyond the mercury, a small quantity of air is
enclosed for the purpose of preventing the metal flowing freely in the
tube. This necessitates the construction of a larger bulb, which renders
the thermometer less sensitive. Moreover, as it frequently happens that
some mercury passes the index, particles of air insinuate themselves in
the metal, and cause separations in the column, which very often can be
removed only by a maker. To facilitate this re-adjustment, a small chamber
is left at the end of the tube, and the mercury being expanded into it by
heat until the index and air bubbles are forced into it, if possible, upon
the cooling down again, by a little management, the mercury will contract,
leaving the air and index behind. Yet sometimes the index cannot be moved
in the least from its place of fixture, so that the instrument must be
virtually reconstructed.
=71. Phillip's Maximum Thermometer.=--A maximum thermometer, better
perhaps in its action than Rutherford's, has been suggested by Professor
John Phillips, of Oxford. A small portion of air is introduced into an
ordinary thermometer, so as to cut off about half an inch of the mercurial
thread near its end in the tube. This forms a maximum thermometer, when
the stem is arranged horizontally. The isolated portion is pushed forward
by expansion, and is left in this position when the mercury contracts.
The end remote from the bulb shows on the scale the maximum temperature.
When made with a capillary tube so fine that the attraction arising from
capillarity overcomes the force of gravity, and prevents the mercury
falling to the end of the tube when the instrument is inverted, it forms a
very serviceable thermometer, quite portable and suitable for use on board
ship. In such a tube a smart shake from a swing of the hand is required to
bring the detached portion back to the column, so as to set the instrument
for future observation; no ordinary motion will move it. When the
thermometer has not this peculiarity, the mercury will flow to the end, if
held bulb downward; and in this state it is not at all a satisfactory
instrument, as the air is likely to be displaced, and a great deal of tact
is requisite to again get it to divide the column suitably. It has been
found in practice that the air bubble at different temperatures assumes
different lengths, and if very small it disappears in a few years by
oxidation and by diffusion with the mercury, so that the instrument
becomes defective and uncertain in action,--results which led to the
construction of the self-registering mercurial maximum thermometer,
invented and patented by Messrs. Negretti and Zambra. It has been before
the public about twelve years; we may therefore, now, safely speak of its
merits.
=72. Negretti and Zambra's Patent Maximum Thermometer= consists of a glass
tube containing mercury fitted on an engraved scale, as shown in fig. 54.
The part of the thermometer tube above the mercury is entirely free from
air; and at the point A in the bend above the bulb, is inserted and fixed
with the blow-pipe a small piece of solid glass, or enamel, which acts as
a valve, allowing mercury to pass on one side of it when heat is applied,
but not allowing it to return when the thermometer cools. When mercury has
been once made to pass the contraction, which nothing but the expansive
force of heat can effect, and has risen in the tube, the upper end of the
column registers the maximum temperature. To return the mercury to the
bulb, we must apply a force equal to that which raised it in the tube; the
force employed is gravity, assisted when necessary by a little agitation
of the instrument.
[Illustration: Fig. 54.]
The degrees are generally divided on the stems of these thermometers, but
their frames of course bear a scale as well. The makers have various
styles of framing in wood, metal, porcelain, and even glass. Each material
is eligible according to requirements. Porcelain scales, having the marks
_etched_ upon them by acid and permanently blackened and baked in,--by a
process for which the inventors have a separate patent,--will be found
very serviceable, as they do not corrode or tarnish by exposure to any
kind of weather; while any amount of dust and dirt can readily be cleaned
off.
The chief recommendation of this thermometer is its simplicity of
construction, enabling it to be used with confidence and safety. Of no
other maximum thermometer can it be said that it is impossible to derange
or put it out of order; hence, as regards durability, it surpasses all
others. Nothing short of actual breakage can cause it to fail. Hence it is
the most easily portable of all self-registering thermometers, an
advantage which renders it suitable for travellers, and for transmission
abroad. In the year 1852, the British Meteorological Society reported this
thermometer to be "the best which has yet been constructed for maximum
temperature, and particularly for sun observations." Since then eleven
years have elapsed, and it is still without a rival.
_Directions for use._ In using this thermometer for meteorological
observations, it should be suspended by means of two brass plates B, C,
attached for that purpose, in such manner that it hangs raised up a little
at C, and so placed that it is in the shade, with the air passing freely
to it from all sides; then, on an increase of heat, the mercury will pass
up the tube as in an ordinary thermometer, and continue doing so as long
as the heat increases. On a decrease of heat, the contraction of mercury
will take place _below_ the _bend_ in the tube, leaving the whole column
of mercury in the tube, thus registering the highest temperature, and
showing such till the instrument is disturbed.
To prepare the instrument for future observations, remove and hold it
perpendicularly, with the bulb downward, and then shake it. The mercury
will then descend in the tube, and indicate the temperature of the air at
that time; and, when again suspended, is prepared for future observation.
After the temperature has attained a maximum, there will be, with a
decrease of heat, a slight contraction of mercury in the tube--as well as
of that in the bulb--and hence doubts have arisen as to the accuracy of
the registration; but calculation shows, and critical trial has proved,
that the greatest daily range of temperature will not produce an error
large enough to be appreciable on the scale.
A very great advantage of this thermometer is that the mercury may be
allowed to flow to the end of the tube without the maximum temperature
attained during an experiment being lost. It can be employed with the bulb
uppermost. All that is necessary for reading the maximum temperature is to
slope the instrument so that the mercury flows gently towards the bulb. It
will then stop at the contraction so as to show the maximum temperature on
the scale. Afterwards the mercury is driven into the bulb by agitating the
instrument while held in the hand. Hence the instrument is invaluable as a
registering thermometer on board ship, as its indications are in no way
affected by the motions and tremors of the vessel.
For physiological experiments, such as taking the temperature of the
mouth in fever, this thermometer is the only one that can be used with
certainty, as it can be held in any position, without losing the maximum
temperature attained.
MINIMA THERMOMETERS.
=73. Rutherford's Alcohol Minimum Thermometer=, fig. 55, consists of a
glass tube, the bulb and part of the bore of which is filled with
perfectly pure spirits of wine, in which moves freely a black glass index.
A slight elevation of the thermometer, bulb uppermost, will cause the
glass index to flow to the surface of the liquid, where it will remain,
unless violently shaken. On a _decrease_ of temperature the alcohol
recedes, taking with it the glass index; on an _increase_ of temperature
the alcohol alone ascends in the tube, leaving the end of the index
_farthest_ from the bulb indicating the minimum temperature.
[Illustration: Fig. 55.]
_Directions for using, &c._--Having caused the glass index to flow to the
end of the column of spirit, by slightly tilting the thermometer, bulb
uppermost, suspend the instrument (in the shade with the air passing
freely to it on all sides) by the two brass plates attached for that
purpose,--in such manner that the bulb is about half an inch lower than
the upper, or the end of the thermometer farthest from the bulb; then, on
a decrease of temperature, the spirits of wine will descend, carrying with
it the glass index; on an increase of temperature, however, the spirits of
wine will ascend in the tube, leaving that end of the small glass index
farthest from the bulb indicating the minimum temperature. To reset the
instrument, simply raise the bulb end of the thermometer a little, as
before observed, and the index will again descend to the end of the
column, ready for future observation.
_Precautions._--1. By no means jerk or shake an alcohol minimum
thermometer _when resetting_ it, for by so doing it is liable to
disarrange the instrument, either by causing the index to leave the
spirit, or by separating a portion of the spirit from the main column.
2. As alcohol thermometers have a tendency to read lower by age, owing to
the volatile nature of the fluid allowing particles in the form of vapour
to rise and lodge in the tube, it becomes necessary to compare them
occasionally with a mercurial thermometer whose index error is known; and
if the difference be more than a few tenths of a degree, examine well the
upper part of the tube to see if any alcohol is hanging in the bore
thereof; if so, the detached portion of it can be joined to the main
column by swinging the thermometer with a pendulous motion, _bulb
downwards_.
3. The spirit column is sometimes much separated by jolting in travelling.
If the instrument is in such a condition when received, it should be held
by the right hand, bulb downward, and the frame tapped smartly, but
cautiously, against the palm of the left hand. The broken thread of spirit
will soon begin to join, and by continuing the operation a sufficient time
all the bubbles will disappear, and the thermometer become as good as
ever.
=74. Horticultural Minimum Thermometer.=--This instrument, represented in
fig. 56, is a special construction of Rutherford's minimum thermometer to
meet the requirements of horticulturists. It is desirable, if not
essential, that gardeners should have the means of ascertaining to what
temperature stoves and greenhouses descend on cold nights, especially in
winter. This thermometer is mounted on a strong cast zinc frame, with the
divisions and figures of the scale raised.
[Illustration: Fig. 56.]
The sunk surface of the frame is painted dark; the figures and division a
bright colour, so that observations can be made without a close inspection
of the instrument.
The directions for using are the same as those given in the preceding
section. It may be used as an ordinary thermometer, by simply hanging it
from the top loop, in which position, the coloured liquid will always
indicate the present temperature.
It was a source of annoyance with the ordinary boxwood and flat metal
scales, that after a time, exposure to a damp warm atmosphere favoured the
growth of confervae upon them, and obliterated the divisions; the plan of
raising the figures and divisions of the scale has been found to prevent
the destruction of the instrument in this way.
=75. Baudin's Alcohol Minimum Thermometer.=--This instrument resembles
Rutherford's thermometer in appearance; its indications are given by the
expansion and contraction of alcohol, and its minimum temperature is
likewise registered by a glass index being pulled back and left behind by
the alcohol, as in Rutherford's instrument. There is, however, a great
improvement in Baudin's instrument; for whilst Rutherford's thermometer
can only register in a horizontal position, Baudin's can be used either
horizontally or vertically, as necessity may require. This important
change is effected in the following manner:--Instead of the index in the
thermometer being loose and free to run up and down according to the
position in which the instrument is held, as in Rutherford's, the index in
the new instrument is made to fit the bore of the tube as nearly tight as
possible, so much so that in holding the thermometer even upside down, or
shaking it, the index will not shift from its position; but, inasmuch as a
minimum thermometer with an immoveable index could not be set when
required for observation, and would consequently be useless, the inventor
has introduced behind the index a piece of solid glass, about
one-and-a-half inch in length, which moves freely in the alcohol. The
addition of the weight of this piece of glass on the top of the index,
when turned upside down, forces the index down to the edge of the alcohol;
and it is there left, as in the case of the ordinary Rutherford's
thermometer. It is, therefore, by turning the thermometer upside down, and
letting the moveable piece of glass fall on the index, that the index is
driven to the end of the alcohol; after this operation the thermometer is
hung up either horizontally or vertically, and will then be ready for use.
The index, although immoveable _per se_, is by the alcohol drawn back, as
in the ordinary minimum, and its indications are read off on the scale
from the top of the index.
=76. Mercurial Minima Thermometers desirable.=--Alcohol does not expand
equally for equal increments of heat, consequently errors are likely to
exist in the scale indications unless the graduations are very
accurately--not necessarily equally--made. On this account, as well as
from the volatility of alcohol, and the intervention of gaseous partitions
in the tube, a good and thoroughly reliable minimum thermometer was for a
long time a desideratum. It was desirable to obtain a thermometer which
should register the lowest temperature by mercury, the fluid in general
use for meteorological thermometers. Several instruments have recently
been invented to meet this requirement, which are suitable and
satisfactory for land purposes, but one well adapted for use on board ship
is still very much wanted.
For very low temperatures, alcohol thermometers will always be required;
as mercury freezes at -40 deg. F, and contracts very irregularly much before
this point, while alcohol has never yet been frozen.
=77. Negretti and Zambra's Patent Mercurial Minimum Thermometer=,
represented by fig. 57, has a cylindrical bulb of large size, which, at
first sight, might induce the idea that the instrument would not be
sufficiently sensitive; but as length is given to the cylinder instead of
increasing its diameter, it will be found as sensitive as a globular bulb
of the same diameter, and much more so than an ordinary alcohol
thermometer.
[Illustration: Fig. 57.]
The reason for having the bulb large is to allow the internal diameter of
the thermometer tube to be larger than that generally used for
thermometrical purposes, so that a steel index, pointed at both ends, may
move freely within when required.
The tube is blown, filled and regulated in the usual way, 60 deg. of
temperature being about half-way up the tube. A small cylindrical bulb is
then formed at the upper end of the tube, and then is introduced a steel
needle pointed at both ends, that in contact with the mercury being
abrupt, the other more prolonged. The open extremity of the tube is now
drawn out into a fine capillary tube, and the bulb of the instrument
warmed so as to cause the mercury to fill the tube completely. When the
mercury reaches the capillary tube, the flame of a blow-pipe is applied;
the glass is dexterously melted, the superfluous part taken away, and the
tube left hermetically closed. During this operation, the steel index has
been embedded in the heated mercury. As the instrument cools, if held
upright, the mercury will recede and expose the needle, which will then
follow the descending column simply by its own gravity. In this condition
the thermometer resembles Rutherford's maximum, being a tube of mercury
with a steel index floating on its surface; but it possesses these
important advantages: it is quite free from air, so that the mercury can
move with perfect freedom; and the index is pointed at both ends, to allow
the mercury to pass, instead of being ground flat to prevent it.
_To use the Thermometer_, it is suspended perpendicularly (figure 57) with
the steel index resting on the surface of the mercurial column. As the
mercury in the cylinder contracts, that in the tube descends, and the
index, of its own gravity, follows it; on the contrary, as the mercury
expands and rises in the tube, it passes the index on one side, and in
rising, exerts a lateral pressure on the needle, and jams it to one side
of the tube, where it remains firmly fixed, leaving the upper point of the
needle indicating the minimum temperature. In this thermometer, the
reading is always from the upper point of the needle, and not from the
mercury itself.
_To extricate the Needle_ from the mercury, a magnet is used, when, if the
needle is embedded only a few degrees, it can readily be withdrawn without
altering the position of the instrument. Should the magnet not be
sufficient for the purpose, we simply turn the thermometer on its support
from the upright position, slightly elevating the bulb (fig. 58 (=2=)).
The mercury and index will then flow into the small reservoir. Should the
index not freely leave the tube with the mercury, assist it with a magnet,
and when the mercury and index are in the upper bulb (figure =2=), apply a
magnet outside, which will attract and hold fast the index; and whilst
thus holding it, again bring the thermometer to the upright position, when
the mercury will immediately fall back into the tube, leaving the index
attached to the magnet (figure =4=), with which it is guided down to the
surface of the mercury, ready for another observation.
[Illustration: Fig. 58.]
Care must be taken not to withdraw the magnet until the index is in
contact with mercury; for, if released before touching, it might plunge
too deeply, and give a false indication. The rule for re-setting it will
be to bring the needle-point in contact with the mercury, and then
withdraw the magnet, having previously ascertained that no particles of
mercury are attached to the index.
It may sometimes, though rarely, happen, that from the time a minimum
temperature is registered by the index, and by the time an observation is
made, the mercury may have risen so high in the tube as to completely pass
the index, as shown (figure =3=). Should it so happen, the space which the
index occupies will readily be observed, as it will be pressed to one side
of the tube, causing a different appearance in that part, although the
point of the needle may not be seen. If such be the case, apply a magnet
to the spot where you see the index is fixed: this will hold the needle
firmly. Then, by slightly tilting the thermometer bulb uppermost, the
mercury will flow into the top bulb, leaving the index attached to the
magnet, and quite uncovered. Having taken the reading, draw the needle
into the top bulb, and hold it there whilst you adjust the thermometer by
again bringing it to the upright position.
By contracting the bore of this thermometer, at the bend of the tube,
sufficiently to keep the mercury from flowing out of its bulb with too
much freedom by motion, the instrument becomes perfectly safe for
transmission abroad.
=78. Negretti & Zambra's Second Patent Mercurial Minimum Thermometer.=--In
this thermometer a principle is used that has been long known to
scientific men, viz. the affinity of mercury for platinum. If mercury be
placed in contact with platinum under ordinary circumstances, no effect
will take place; but if the mercury is once made to attack the platinum,
the amalgamation is permanent and the contact perfect, so much so, that
the principle was made use of in constructing standard barometers. A ring
of platinum was fused round the end of the tube, dipping into the mercury;
and the contact between the platinum and mercury became so perfect that
air could not creep down the tube and up the bore, as in ordinary
barometer tubes. This principle of adhesion or affinity of mercury for
platinum has been brought into play for the purpose of arresting the
mercury after it has reached the minimum temperature in a thermometer.
This thermometer is made as follows:--behind the bulb is placed a
supplementary chamber; in the space or neck between the bulb of the
thermometer and the chamber, is placed a small piece of platinum; this may
be of any shape or size, but the smaller the better. This is not to fit in
the neck; it must, on the contrary, be rather loose; it may be fastened in
position or not. The instrument is represented by fig. 59.
[Illustration: Fig. 59.]
_Directions for using._--Having suspended the thermometer in a horizontal
position, the mercury is made to stand in exact contact with the platinum
plug by slightly elevating the bulb end of the instrument. The thermometer
is now ready for observation. On a decrease of temperature, the mercury
will endeavour to contract first from the easier passage, viz. behind the
bulb; but in consequence of the adhesion of the mercury to the platinum,
it cannot recede from here, it is therefore forced to contract from the
indicating tube, and will continue to do so as long as the temperature
decreases; and as no indices are employed in this thermometer, the extreme
end of the mercurial column will show "how cold it has been." On an
increase of temperature the mercury will glide over the platinum plug and
expand by the easier passage into the supplementary chamber, and there
remain until a decrease of temperature again takes place, when the mercury
that had gone into the supplementary chamber will be the first to recede,
until it reaches the platinum plug, its further progress being arrested;
it will then fall in the indicating tube, and there remain until re-set.
=79. Casella's Mercurial Minimum Thermometer.=--The general form and
arrangement of this instrument is shown in fig. 60. A tube with large
bore, _a_, has at the end a _flat glass diaphragm_ formed by the abrupt
junction of a small chamber, _b c_, the inlet to which at _b_ is larger
than the bore of the indicating tube. The result of this is that on
setting the thermometer, as described below, the contracting force of the
mercury in cooling withdraws the fluid in the indicating stem only; whilst
on its expanding with heat, the long column does not move, the increased
bulk of mercury finding an easier passage into the small pear-shaped
chamber attached.
[Illustration: Fig. 60.]
We believe that a small speck of air must be confined in the chamber, _b
c_, to act as a spring to start the mercury from the chamber in the act of
setting the thermometer. Were this air not present, the mercury would so
adhere to the glass that no amount of shaking could induce it to flow from
the chamber.
_To set the Instrument_, place it in a horizontal position, with the back
plate, _d_, suspended on a nail, and the lower part supported on a hook,
_e_. The bulb end may now be gently raised or lowered, causing the mercury
to flow slowly until the bent part, _a_, _is full_ and the chamber, _b c_,
_quite empty_. At this point the flow of mercury in the long stem of the
tube is arrested, _and indicates the exact temperature_ of the bulb or air
at the time. On an increase of temperature the mercury will expand into
the small chamber, _b c_; and a return of cold will cause its recession
from this chamber only, until it reaches the diaphragm, _b_. Any further
diminution of heat withdraws the mercury down the bore to whatever degree
the cold may attain, where it remains until farther withdrawn by increased
cold, or till re-set for future observation.
MAXIMA AND MINIMA THERMOMETERS.
=80. Rutherford's= arrangement for obtaining a complete instrument for the
registration of heat and cold was simply mounting a maximum thermometer
and a minimum thermometer upon the same frame or slab. Thus constructed,
they are often called "day and night" thermometers, though somewhat
inappropriately; for in temperate climates the temperature of the night
sometimes exceeds that of the day, notwithstanding the reverse is the
general law of temperature. Fig. 61 will explain the arrangement of
Rutherford's day and night thermometer.
[Illustration: Fig. 61.]
=81. Sixe's Self-Registering Thermometer.=--The very ingenious and
certainly elegant instrument about to be described was invented by James
Sixe, of Colchester. It consists of a long cylindrical bulb, united to a
tube of more than twice its length, bent round each side of it in the form
of a syphon, and terminated in a smaller, oval-shaped bulb. Figure 62
gives a representation of this instrument. The lower portion of the syphon
is filled with mercury; the long bulb, the other parts of the tube, and
part of the small bulb, with highly rectified alcohol. A steel index moves
in the spirit in each limb of the syphon. The two indices are terminated
at top and bottom with a bead of glass, to enable them to move with the
least possible friction, and without causing separation of the spirit, or
allowing mercury to pass easily. They would, from their weight, always
rest upon the mercury; but each has a fine hair tied to its upper
extremity and bent against the interior of the tube, which acts as a
spring with sufficient elasticity to keep the index supported in the
spirit in opposition to gravity.
[Illustration: Fig. 62.]
The instrument acts as follows:--A rise of temperature causes the spirit
in the long bulb to expand and press some of the mercury into the other
limb of the syphon, into which it rises also from its own expansion, and
carries the index with it, until the greatest temperature is attained. The
lower end of this index then indicates upon the engraved scale the maximum
temperature. As the temperature falls the spirit and the mercury contract,
and in returning towards the bulb the second index is met and carried up
by the mercury until the lowest temperature occurs, when it is left to
indicate upon the scale the minimum temperature. The limb of the syphon
adjoining the bulb requires, therefore, a descending scale of
thermometric degrees; the other limb, an ascending scale. The graduations
must be obtained by comparisons with a standard thermometer under
artificial temperatures, which should be done in this way for every 5 deg., in
order to correct for the inequality in the bore of the tube, and the
irregular expansion of the spirit. The instrument is set for observation
by bringing the indices into contact with the mercury, by means of a small
magnet, which attracts the steel through the glass, so that it is readily
drawn up or down. They should be drawn nearly to the top of the limbs when
it is desired to remove the instrument, which should be carefully carried
in the vertical position; for should it be inverted, or laid flat, the
spirit may get among the mercury, and so break up the column as to require
the skill of a maker to put it in order again. For transmission by
ordinary conveyances, it requires that attention be given to keep it
vertical. The entanglement of a small portion of mercury with the indices
is sometimes a source of annoyance in this instrument, for the readings
are thereby rendered somewhat incorrect. Small breakages in the mercury,
either from intervening bubbles of spirit or adhesion to the indices, may
generally be rectified by cautiously tapping the frame of the instrument,
so as to cause the mercury to unite by the assistance thus given to its
superior gravity.
These thermometers, when carefully made and adjusted to a standard
thermometer, are strongly recommended for ordinary purposes, where strict
scientific accuracy is not required. This is also the only fluid
thermometer applicable for determining the temperature of the sea at
depths.