INSTRUMENTS FOR ASCERTAINING TEMPERATURE.
=53. Temperature= is the energy with which heat affects our sensation of
feeling.
Bodies are said to possess the same temperature, when the amounts of heat
which they respectively contain act outwardly with the same intensity of
transfer or absorption, producing in the one case the sensation of warmth,
in the other that of coldness. Instruments used for the determination and
estimation of temperatures are called _Thermometers_.
Experience proves that the same body always occupies the same space at the
same temperature; and that for every increase or decrease of its
temperature, it undergoes a definite dilatation or contraction of its
volume. Provided, then, a body suffers no loss of substance or peculiar
change of its constituent elements or atoms, while manifesting changes of
temperature it will likewise exhibit alterations in volume; the latter
may, therefore, be taken as exponents of the former. The expansion and
contraction of bodies are adopted as arbitrary measures of changes of
temperature; and any substance will serve for a thermometer in which these
changes of volume are sensible, and can be rendered measureable.
=54. Thermometric Substances.=--Thermometers for meteorological and
domestic purposes are constructed with liquids, and generally either
mercury or alcohol, because their alterations of volume for the same
change of temperature are greater than those of solids; while being more
manageable, they are preferred to gases. Mercury is of all substances the
best adapted for thermometric purposes, as it maintains the liquid state
through a great alteration of heat, has a more equable co-efficient of
expansion than any other fluid, and is peculiarly sensitive to changes of
temperature. The temperature of solidification of mercury, according to
Fahrenheit's scale of temperature, is -40 deg.; and its temperature of
ebullition is about 600 deg. Sulphuric ether, nitric acid, oil of sassafras,
and other limpid fluids, have been employed for thermometers.
=55. Description of the Thermometer.=--The ordinary thermometer consists
of a glass tube of very fine bore, having a bulb of thin glass at one
extremity, and closed at the other. The bulb and part of the tube contains
mercury; the rest of the tube is a vacuum, and affords space for the
expansion of the liquid. This arrangement renders very perceptible the
alterations in volume of the mercury due to changes of temperature. It is
true, the glass expands and contracts also; but only by about
one-twentieth of the extent of the mercury. Regarding the bulb, then, as
unalterable in size, all the changes in the bulk of the fluid must take
place in the tube, and be exhibited by the expansion and contraction of
the column, which variations are made to measure changes of temperature.
56. STANDARD THERMOMETER.
The peculiarities in the construction of thermometers will be best
understood by describing the manufacture of a _Standard Thermometer_,
which is one of the most accurate make, and the scale of which is divided
independently of any comparison with another thermometer. Fig. 36 is an
illustration of such an instrument, on a silvered brass scale.
[Illustration: Fig. 36]
_Selection of Tube._--In selecting the glass tube, much care is requisite
to ascertain that its bore is perfectly uniform throughout. As received
from the glass-house, the tubes are generally, in their interior, portions
of very elongated cones, so that the bore is wider at one end than at the
other. With due care, however, a proper length of tube can be selected, in
which there is no appreciable difference of bore. This is ascertained by
introducing into the tube a length of mercury of about a half or a third
of an inch, and accurately measuring it in various positions in the tube.
To accomplish this, the workman blows a bulb at one end of the tube, and
heats the bulb a little to drive out some of the air. Then, placing the
open end in mercury, upon cooling the elasticity of the enclosed air
diminishes, and the superior pressure of the atmosphere drives in some
mercury. The workman stops the process so soon as he judges sufficient
mercury has entered. By cooling or heating the bulb, as necessary, the
mercury is made to pass from one end of the tube to the other. Should the
length of this portion of mercury alter in various parts of the bore, the
tube must be rejected. If it is, as nearly as possible, one uniform
length, the tube is set aside for filling.
The _bulb_ is never blown by the breath, but by an elastic caoutchouc ball
containing air, so that the introduction of moisture is avoided. The
spherical form is to be preferred; for it is best adapted to resist the
varying pressure of the atmosphere. The bulbs should not be too large, or
the mercury will take some time to indicate sudden changes of temperature.
Cylindrical bulbs are sometimes desirable, as they offer larger surfaces
to the mercury, and enable thermometers to be made more sensitive.
The _mercury_, with which the bulb is to be filled, should be quite pure,
and freed from moisture and air by recent boiling.
_Filling the Tube._--The filling is effected by heating the bulb with the
flame of a spirit-lamp, while the open end is embedded in mercury. Upon
allowing the bulb to cool, the atmospheric pressure drives some mercury
into it; and the process of heating and cooling is thus continued until
sufficient mercury is introduced. The mercury is next boiled in the tube,
to expel any air or moisture that may be present. In order to close the
tube and exclude all air, the artist ascertains that the tube contains
the requisite quantity of mercury; then, by holding the bulb over the
spirit flame, he causes the mercury to fill the whole of the tube, and
dexterously removing it from the source of heat, he, at the same instant,
closes it with the flame of a blow-pipe. If any air remain in the tube, it
is easily detected; for if the instrument be inverted, the mercury will
fall to the extremity of the tube, if there is a perfect vacuum, unless
the tube be so finely capillary that its attraction for the mercury is
sufficient to overcome the force of gravity, in which case the mercury
will retain its position in every situation of the instrument. If,
however, the mercury fall and does not reach quite to the extremity of the
bore, some air is present, which must be removed.
_The Graduation._--The thermometer is now prepared for graduation, the
first part of which process is the determination of two fixed points.
These are given by the temperatures of melting ice and of the vapour of
boiling water. Melting ice has always the same temperature in every place
and under all circumstances; provided only that the water from which the
ice is congealed is free from salts. The temperature of the vapour of
boiling water depends upon the pressure of the atmosphere, but is always
constant for the same pressure.
The fixed point corresponding to the temperature of melting ice is called
the _freezing point_. It is obtained by keeping the bulb and the part of
the tube occupied by mercury immersed in melting ice, until the mercury
contracts to a certain point, where it remains stationary. This position
of the end of the mercury is then marked upon the tube.
The _boiling point_ is not so easily determined, for the barometer must be
consulted about the same time. The boiling apparatus is generally
constructed of copper. It consists of a cylindrical boiler, heated from
the base by a spirit lamp or charcoal fire. An open tube two or three
inches in diameter and of suitable length enters the top of the boiler.
This tube is enveloped by another fixed to the top of the boiler but not
opening into it, and so that the two tubes are about an inch apart. The
object of the outer tube is to protect the inner tube from the cold
temperature of the air. The outer tube has an opening at the top for the
admission of the thermometer, and a hole near the bottom for the escape of
steam through a spout. When the water is made to boil, the steam rises in
the inner tube, fills the space between the tubes, and escapes at the
spout. The thermometer is then passed down into the inner cylinder, and
held securely from the top by means of a piece of caoutchouc. The tubes or
cylinders should be of sufficient length to prevent the thermometer
entering the water. This is necessary because the temperature of boiling
water is influenced by any substance which it holds in chemical solution;
and, moreover, its temperature increases with the depth, owing to the
pressure of the upper stratum. The thermometer being thus surrounded with
steam, the mercury rises in the tube. As it does so, the tube should be
depressed so as always to keep the top of the mercury just perceptible.
When the temperature of the vapour is attained, the mercury ceases to
rise, and remains stationary. The position of the end of the mercury is
now marked upon the tube, and the "_boiling-point_" is obtained.
=57. Methods of ascertaining the exact Boiling Temperature.=--The normal
boiling temperature of water all nations have tacitly agreed to fix under
a normal barometric pressure of 29.922 inches of mercury, having the
temperature of melting ice, in the latitude of 45 deg., and at the sea-level.
If the atmospheric pressure at the time or place of graduating a
thermometer does not equal this, the boiling temperature will be higher or
lower according as the pressure is greater or less. Hence a reading must
be taken from a reliable barometer, which must also be corrected for
errors and temperature, and reduced for latitude, in order to compare the
actual atmospheric pressure at the time with the assumed normal pressure.
Tables of vapour tension, as they are termed, have been computed from
accurate experimental investigations and theory,--giving the temperatures
of the vapour of water for all probable pressures; Regnault's, the most
recent, is considered the most accurate; and his investigations are based
upon the standard pressure given above, and are for the same latitude. His
Table, therefore, will give the temperature on the thermometric scale
corresponding to the pressure.
The Commissioners appointed by the British Government to construct
standard weights and measures, decided that the normal boiling-point,
212 deg., on the thermometer should represent the temperature of steam
generated under an atmospheric pressure equal in inches of mercury, at the
temperature of freezing water, to 29.922 + (cos. 2 latitude x .0766) +
(.00000179 x height in feet above the sea-level). Hence, at London, lat.
51 deg.30' N., we deduce 29.905 as the barometric pressure representing the
normal boiling point of water,--the trifling correction due to height
being neglected. If then, in the latitude of London, the barometric
pressure, at the time of fixing the boiling point, be not 29.905 inches,
that point will be higher or lower, according to the difference of the
pressure from the normal. Near the sea-level about 0.59 inch of such
difference is equivalent to 1 deg. Fahrenheit in the boiling point.
Suppose, then, the atmospheric pressure at London to be 30.785 inches, the
following calculation gives the corresponding boiling temperature for
Fahrenheit's scale:--
Observed pressure 30.785
Normal " 29.905
------
Difference .880
=======
As 0.59 is to 0.88, so is 1 deg. to 1 deg.5.
That is, the water boils at 1 deg.5 above its normal temperature; so that, in
this case, the normal temperature to be placed on the scale, viz. 212 deg.,
must be 1 deg.5 lower than the mark made on the tube at the height at which
the mercury stood under the influence of the boiling water.
The temperature of the vapour of boiling water may be found, at any time
and place, as follows:--Multiply the atmospheric pressure by the factor
due to the latitude, given in the annexed Table V., and with the result
seek the temperature in Table VI.
TABLE V. TABLE VI.
+----------------------------------------------------------------+
|Latitude.| Factor. |||Temperature|Tension.||Temperature|Tension.|
| | ||| of Vapour.| || of Vapour.| |
|---------+---------+++-----------+--------++-----------+--------|
|Degrees. | ||| Degrees. |Inches. || Degrees. |Inches. |
| 0 | 0.99735 ||| 179 | 14.934 || 197 | 22.036 |
| 5 | 0.99739 ||| 180 | 15.271 || 198 | 22.501 |
| 10 | 0.99751 ||| 181 | 15.614 || 199 | 22.974 |
| 15 | 0.99770 ||| 182 | 15.963 || 200 | 23.456 |
| 20 | 0.99797 ||| 183 | 16.318 || 201 | 23.946 |
| 25 | 0.99830 ||| 184 | 16.680 || 202 | 24.445 |
| 30 | 0.99868 ||| 185 | 17.049 || 203 | 24.952 |
| 35 | 0.99910 ||| 186 | 17.425 || 204 | 25.468 |
| 40 | 0.99954 ||| 187 | 17.808 || 205 | 25.993 |
| 45 | 1.00000 ||| 188 | 18.197 || 206 | 26.527 |
| 50 | 1.00046 ||| 189 | 18.594 || 207 | 27.070 |
| 55 | 1.00090 ||| 190 | 18.998 || 208 | 27.623 |
| 60 | 1.00132 ||| 191 | 19.409 || 209 | 28.185 |
| 65 | 1.00170 ||| 192 | 19.828 || 210 | 28.756 |
| 70 | 1.00203 ||| 193 | 20.254 || 211 | 29.335 |
| 75 | 1.00230 ||| 194 | 20.688 || 212 | 29.922 |
| 80 | 1.00249 ||| 195 | 21.129 || 213 | 30.515 |
| | ||| 196 | 21.578 || 214 | 31.115 |
+----------------------------------------------------------------+
_How to use the Tables._--When the _temperature_ is known to decimals of a
degree, take out the tension for the degree, and multiply the difference
between it and the next tension by the decimals of the temperature, and
add the product to the tension, for the degree.
Required the tension corresponding to 197 deg.84.
deg.
197 = 22.036 .465 x .84 = .391
198 = 22.501 197 deg. = 22.036
------ ------
Difference .465 197.84 = 22.427
====== ======
When the _tension_ is given, take the difference between it and the next
less tension in the Table, and divide this difference by the difference
between the next less and next greater tensions. The quotient will be the
decimals to add to the degree opposite the next less tension.
Thus, for 23.214 inches, required the temperature.
Given 23.214 Next greater 23.456
22.974 Next less 22.974
------ ------
.240 Difference .482
.240
And ---- = .5
.482
Temperature opposite next less 199.0
-----
Temperature required 199.5
=====
A similar method of interpolation in taking out numerical quantities is
applicable to almost all tables; and should be practised with all those
given in this work.
_Example._--Thus, in Liverpool, lat. 53 deg. 30' N., the barometer reading
29.876 inches, its attached thermometer 55 deg., and the correction of the
instrument being + .015 (including index error, capillarity and capacity),
what temperature should be assigned for the boiling point marked on the
thermometer?
Observed barometer 29.876
Correction + .015
------
29.891
Correction for temperature - .074
------
Reduced reading 29.817
Factor from Table V. 1.00077
-------
208719
208719
29817
-----------
Equivalent for lat. 45 deg. 29.83995909
===========
In Table VI., 29.84 gives temperature 211 deg.86.
=58. Displacement of the Freezing Point.=--Either the prolonged effect of
the atmospheric pressure upon the thin glass of the bulbs of thermometers,
or the gradual restoration of the equilibrium of the particles of the
glass after having been greatly disturbed by the operation of boiling the
mercury, seems to be the cause of the freezing points of standard
thermometers reading from a few tenths to a degree higher in the course of
some years, as has been repeatedly observed. To obviate this small error,
it is our practice to place the tubes aside for about six months before
fixing the freezing point, in order to give time for the glass to regain
its former state of aggregation. The making of accurate thermometers is a
task attended with many difficulties, the principal one being the
liability of the zero or freezing point varying constantly, so much so,
that a thermometer that is perfectly correct to-day, if immersed in
boiling water, will be no longer accurate; at least, it will take some
time before it again settles into its normal state. Then, again, if a
thermometer is recently blown, filled, and graduated immediately, or, at
least, before some months have elapsed, though every care may have been
taken with the production of the instrument, it will require some
correction; so that the instrument, however carefully made, should from
time to time be plunged into finely-pounded ice, in order to verify the
freezing point.
=59. The Scale.=--The two fixed points having been determined, it is
necessary to apply the scale. The thermometers in general use in the
United Kingdom, the British Colonies, and North America are constructed
with Fahrenheit's scale. Fahrenheit was a philosophical instrument maker
of Amsterdam, who, about the year 1724, invented the scale which has given
his name to the thermometer. The freezing point is marked 32 deg., the boiling
point 212 deg., so that the intermediate space is divided into 180 equal
parts, called degrees. "The principle which dictated this _peculiar
division_ of the scale is as follows:--When the instrument stood at the
greatest cold of Iceland, or 0 degree, it was computed to contain 11124
equal parts of quicksilver, which, when plunged in melting snow, expanded
to 11156 parts; hence the intermediate space was divided into 32 equal
portions, and 32 was taken as the freezing point of water: when the
thermometer was plunged in boiling water, the quicksilver was expanded to
11336; and therefore 212 deg. was marked as the boiling point of that fluid.
In _practice_, Fahrenheit determined the divisions of his scale from two
fixed points, the freezing and boiling of water. _The theory_ of the
division, if we may so speak, was derived from the lowest cold observed in
Iceland, and the expansions of a given portion of mercury" (_Professor
Trail_).
The divisions of the scale can be carried beyond the fixed points, if
requisite, by equal graduations. Fahrenheit's scale is very convenient in
some respects. The meteorological observer is seldom troubled with
negative signs, as the zero of the scale is much below freezing. Again,
the divisions are more numerous, and consequently smaller, than on other
scales in use; and the further subdivision into tenths of degrees, seems
to give all the minuteness usually required.
_Celcius_, a Swede, in 1742, proposed zero for the freezing point, and 100
for the boiling point, all temperatures below zero being distinguishable
by the sign (--) minus. This scale is known as the _centigrade_, and is in
use in France, Sweden, and the southern part of Europe. It has the
advantage of the decimal notation, with the embarrassment of the negative
sign.
_Reaumur_, a Frenchman, proposed zero for the freezing point, and 80 deg. for
the boiling point, an arrangement inferior to the centigrade. It is,
however, in use in Spain, Switzerland, and Germany.
It is merely a simple arithmetical operation to change the indications of
any one of these scales into the equivalents on the others. To facilitate
such conversions, tables are convenient, when a large number of
observations are under discussion; and they can be easily formed or
obtained.
In the absence of such tables, the following formulae will insure accuracy
of method, and save thinking, when occasional conversions are wanted to be
made:--F. stands for Fahrenheit, C. for Centigrade, and R. for Reaumur.
Given. Required. Solution.
F. C. = (F.-32) 5/9
F. R. = (F.-32) 4/9
C. F. = 9/5 C. + 32
C. R. = 4/5 C.
R. F. = 9/5 R. + 32
R. C. = 5/4 R.
_Example._--Convert 25 deg. of Fahrenheit's scale into the corresponding
temperature on the Centigrade scale.
Here C. = (25 - 32) 5/9
C. = -35/9 = -3.9
or nearly 4 deg. _below_ zero of the Centigrade scale. The algebraical sign
must be carefully attended-to in the calculations.
=60. The method of testing Thermometers= for meteorological purposes is
very simple. Such thermometers are seldom required to read above 120 deg. In
these the freezing point having been determined, the divisions of the
scale are ascertained by careful comparisons, with a standard thermometer,
in water of the requisite temperature. "For the freezing point, the bulbs,
and a considerable portion of the tubes of the thermometers, are immersed
in pounded ice. For the higher temperatures, the thermometers are placed
in a cylindrical glass vessel containing water of the required heat: the
scales of the thermometers intended to be tested, together with the
Standard with which they are to be compared, are read through the glass.
In this way the scale readings may be tested at any required degree of
temperature, and the usual practice is to test them at every ten degrees
from 32 deg. to 92 deg. of Fahrenheit."--_FitzRoy._
=61. Porcelain Scale Plates.=--Thermometer scales of brass, wood, or
ivory, either by atmospheric influence or dipping in sea-water, are very
liable to become soiled and discoloured, so much so that after a very
little time the divisions are rendered nearly invisible. To obviate this
inconvenience, Messrs. Negretti and Zambra were the first to introduce
into extensive use thermometer and barometer scale-plates made of
porcelain, having the divisions and figures engraved thereon by means of
fluoric acid, and permanently burnt-in and blackened, so as always to
present a clear legible scale. That these scales have been found superior
to all others, may be inferred from the fact that all the thermometers now
supplied to the various government departments are provided with such
scales.
They can be adapted to replace any of the old forms of brass or zinc
scales, the divisions and figures of which have become obliterated or
indistinct.
=62. Enamelled Tubes.=--Nearly all thermometer tubes are now made with
enamelled backs. This contrivance of enamelling the backs of the tubes
enables the makers to use finer threads of mercury than had before been
found practicable; for were it not for the great contrast between the dark
thread of mercury and the white enamel on the glass, many of the
thermometers now in use would be positively illegible. The enamelling of
thermometers is an invention of Messrs. Negretti and Zambra. It is
necessary to state this, as many persons, from interested motives, are
anxious to ignore to whom the credit of the invention is due.
=63. Thermometers of extreme Sensitiveness.=--Thermometers for delicate
experiments are no novelty. Thermometers have been made with very delicate
bulbs to contain a very small quantity of mercury. Such instruments have
also been made with spiral or coiled tubular bulbs, but the thickness of
glass required to keep these coils or spirals in shape, and in fact to
prevent their falling to pieces, served to nullify the effect sought to be
produced, viz. instantaneous action; and where a small thin bulb was
employed, the indicating column was generally so fine that it was
positively invisible except by the aid of a powerful lens. Messrs.
Negretti and Zambra have now introduced a new form of thermometer, which
combines sensitiveness and quickness of action, together with a good
visible column. The bulb of this thermometer is of the gridiron form. Care
has been taken in constructing the bulb, so that the objections attending
spirals and other forms have been overcome; for whilst the reservoir or
bulb is made of glass so thin that it is only by a spirit lamp and not a
glass blower's blowpipe that it can be formed, yet it is still so rigid
(owing to its peculiar configuration) that no variations in its
indications can be detected, whether it be held in a horizontal, vertical,
or oblique position, nor will any error be detected if it be stood on its
own bulb. They have made thermometers with bulbs or reservoirs formed of
about nine inches of excessively thin cylindrical glass, whose outer
diameter is not more than a twentieth of an inch; so that, owing to the
large surface presented, the indications are positively instantaneous.
This form of thermometer was constructed expressly to meet the
requirements of scientific balloon ascents, to enable thermometrical
readings to be taken at the precise elevation. It was contemplated to
procure a metallic thermometer, but on the production of this perfect
instrument the idea was abandoned.
64. VARIETIES OF THERMOMETERS.
Fig. 37 is an illustration of boxwood scale thermometers for general use
and common purposes.
Fig. 38, Negretti and Zambra's Travelling Thermometer; it is fixed in a
plated metal (silver or otherwise) case, similar to a pencil-case, and has
the scale divided upon its stem.
Fig. 39, Thermometer mounted on a slab of glass, upon which the scale is
etched, the back being either oak, mahogany, or ebony.
Fig. 40, Portable Thermometer, in a bronzed brass or German silver
revolving case.
Fig. 41, Pocket Thermometer, on ivory or metallic scale, in morocco or
papier-mache case.
[Illustration: Fig. 37.]
[Illustration: Fig. 38.]
[Illustration: Fig. 39.]
[Illustration: Fig. 40.]
[Illustration: Fig. 41.]
Fig. 42, an Ornamental Drawing-room Thermometer, on ebony or ivory stand,
with glass shade.
Fig. 43, representation of highly carved or engine-turned design for
thermometer mounts, in ivory or wood, for the drawing-room. Some have the
addition of a sundial or compass at the top; they may also be formed for a
watch-stand.
Fig. 44, =Bath Thermometer=, having a float to admit of its being kept in
the water.
[Illustration: Fig. 42.]
[Illustration: Fig. 43.]
[Illustration: Fig. 44.]
Fig. 45, Thermometer with ivory scale in glass cylinder, mounted on oak
bracket with metal top, for out-door use; as at a window.
Fig. 46, Thermometer for the window, on patent porcelain or glass scale,
with oak bracket and convenient brass supports, for placing the instrument
at any angle.
Fig. 47, =Chemical Thermometer=, on boxwood scale, jointed near the bulb
on a brass hinge, ranging from 300 deg. to 600 deg.
Fig. 48, =Chemical Thermometer=, for acids, graduated on its own stem,
suitable for insertion in the tubulure of retorts; they are also made
insulated in glass cylinder to protect the graduated stem; ranging from 0 deg.
to 600 deg.
[Illustration: Fig. 45.]
[Illustration: Fig. 46.]
[Illustration: Fig. 47.]
[Illustration: Fig. 48.]
[Illustration: Fig. 49.]
[Illustration: Fig. 50.]
=65. Superheated Steam Thermometer.=--The great advantage gained by the
use of superheated steam in marine and other steam-engines being now
generally admitted by engineers, reliable thermometers, reading to 600 deg. at
least, are of the utmost importance. To meet this want, Messrs. Negretti
and Zambra have constructed for the purpose a substantial form of
thermometer, on their patent porcelain scales, in strong and convenient
metal mountings, with perforated protection to the bulb. The scales cannot
be deteriorated by steam, heat, oil, or dirt; and an occasional wiping
will be all that is necessary to keep the divisions and figures clean and
visible for any length of time; while careful calibration of the
thermometer tubes ensures the most accurate indications attainable. These
thermometers are illustrated by figs. 49 & 50. A similar, but cheaper,
construction is given to thermometers to be used with hot air, or hot
water, apparatus.
=66. Thermometer for Sugar Boiling= is protected by a metallic frame; and
is usually from three to four feet long, the graduations being confined to
a space of about twelve inches at the upper part of the instrument,
allowing the bulb and greater part of the tube to be immersed in the
boiling sugar. The graduations extend to 270 deg. or further. An index is
sometimes attached to the scale, which may be set to any degree of heat
required to be maintained.
67. EARTH THERMOMETER.
The Earth Thermometer is for ascertaining the temperature of the soil at
various depths. It is protected by a brass frame, pointed and strengthened
at the end to facilitate insertion into the ground, as in fig. 51.
[Illustration: Fig. 51.]
_Utility of a Knowledge of the Temperature of the Soil._--The temperature
of the soil is an important element in the consideration of climate, as it
concerns the vegetable kingdom.
Dr. Daubeny, in his _Lectures on Climate_, gives the following statement
with respect to some temperatures which have been observed just beneath
the earth's surface, in different parts of the globe:--
--------------------------------------------------------------------
Country. | Temperature. | Authority.
-----------------+--------------------------------------------------
Tropics, often |162-184 deg. | Humboldt.
| |
Egypt |133-144 | Edwards & Colin.
| |
Orinoco |In white sand, 140 | Humboldt.
| |
Chili |113-118, among dry grass | Boussingault.
| |
Cape of Good Hope|150, under the soil of a bulb | Herschell.
garden |
| |
Bermuda |142, thermometer barely covered | Emmet.
| in earth |
| |
China |Water of the fields, 113; | Meyer.
| adjacent sand, much higher; |
| blackened sides of the boat |
| at midday, 142-150 |
| |
France |118-122, and in one instance 127| Arago.
--------------------------------------------------------------------
"The importance of this to vegetation may be estimated by the following
considerations:--
"It is known that every plant requires a certain amount of heat, varying
in the case of each species, for the renewal of its growth, at the
commencement of the season.
"Now when this degree of heat has spurred into activity those parts that
are above ground, and caused them to elaborate the sap, it is necessary
that the subterranean portions should at the same time be excited by the
heat of the ground to absorb the materials which are to supply the plant
with nourishment. Unless the latter function is provided for, the aerial
portions of the plant will languish from want of food to assimilate.
Indeed, it is even advisable that the roots should take the start of the
leaves, in order to have in readiness a store of food for the latter to
draw upon." In another place the professor remarks:--"It has been
calculated by Mr. Raikes, from experiments made at Chat Moss, that the
temperature of the soil when drained averages 10 deg. more than it does when
undrained; and this is not surprising, when we find that 1 lb. of water
evaporated from 1,000 lbs. of soil will depress the whole by 10 deg., owing to
the latent heat which it absorbs in its conversion into vapour."
68. MARINE THERMOMETER.
This instrument is a special construction to meet the requirements of
navigation. It consists of a carefully constructed thermometer divided on
its stem to degrees, which are sufficiently large to admit of subdivision
into tenths of degrees by estimation, and ranging from 0 deg. to 130 deg. The
scale is porcelain, having the degrees etched upon it, and burnt-in a
permanent black. The instrument is made to slide into a japanned metallic
case, for handy use and protection. It is therefore adapted for almost any
ordinary purpose; and cannot be injuriously affected by any chemical
action arising from air or sea-water. A set of these thermometers consists
of six, carefully packed in a neat box; two having japanned metallic cases
(fig. 52), the others being designed for use without the case, or to
replace a breakage.
[Illustration: Fig. 52.]
This thermometer is employed in the Royal Navy, and for the observations
made at sea for the Board of Trade.
The thermometer is now considered a necessary instrument on board ship.
Not only is it of invaluable utility in connection with the barometer as a
guide to the weather, but its indications are of service in showing the
presence of a warm or cold current in the sea; many of the great oceanic
currents being characterised by the warmth or coldness of their waters. In
seas visited by icebergs, the habitual use of the thermometer would
indicate their proximity, as the water is rendered colder for some
distance around by the thawing of huge masses of ice. The water over a
shoal in the sea is generally colder than the surface-water of the
surrounding ocean; which may result from the cold water being brought to
the surface by the current of water encountering the shoal. With this fact
navigators are well acquainted; and therefore a fall in the sea-water
thermometer may forebode that shallow water is at hand. It has been
ascertained that fish inhabit regions of the oceans and seas having the
peculiar temperature suitable to their habits. The better and firmer sort
of fish are found where cold waters exist. Those taken in warmer belts or
streams of water, even in the same latitude, are far inferior in
condition, and less approved by the palate. The fish of the Mediterranean,
a warm sea, are generally poor and scarce. Fish taken in the cold waters
between the American shore and the Gulf Stream are much esteemed; while in
and on the other side of the stream they are said to be tasteless, and of
no flavour. Between the coasts of China and the warm waters of the
Japanese current, the seas abound with excellent fish; but in the warm
waters of the current and beyond, they are never seen in such shoals.
In fact, it is clearly ascertained that fishes are adapted to climates,
like birds and beasts. It has been even affirmed, after careful
investigation, that herrings, which abound in the British Seas, and form a
most important branch of our fisheries, can only be found in a temperature
varying from 54 deg. to 58 deg. Hence the thermometer, if brought into use by the
fishermen, would guide them to the spots where they may with the best
chance cast their nets on dark nights, when other indications are not
perceptible.
This thermometer in its metallic case is perfectly suited for dipping
overboard, or placing in a bucket of water just taken from the sea, to
ascertain its temperature.