Postcard showing a weather station on an unknown Royal Naval ship between 1913 and 1919, courtesy of Janet Lomas
The meteorological instruments usually supplied to a large ship are:
One mercurial barometer.
Two aneroid barometers.
Two maximum thermometers.
Two minimum thermometers.
Barometer is an instrument which is
governed entirely by the pressure of the atmosphere, and, in consequence, shows
at once when that pressure is changing. If the pressure at one place on the
earth's surface be greater than at another, the air has a tendency to move from
the place where the pressure is greater towards that where it is less, and
thus" wind" is caused. A change of weather comes almost always with a
change of wind, and on the extent of this change depends whether the new wind
is warmer or colder, damper or drier, than that which has been blowing.
if the level of the mercury in the barometer continues steady, settled weather
may be expected, but when it is unsteady a change must be looked for.
A sudden rise
of the barometer is very nearly as bad as a sudden fall, because it shows that
the atmospheric equilibrium is unsteady.
Mercurial Barometer in its simplest form
is merely a glass tube closed at one end and filled with mercury, which is then
boiled in order to extract any minute quantities of air which may be adhering
to the sides of the glass, after which it is inverted and its open end placed
below the level of the surface of the mercury in a small cistern. The mercury
now descends in the tube until the weight of the column is balanced by the
pressure of the atmosphere on the mercury in the cistern, thus affording a
correct measure of such pressure. By means of a scale of inches, whose zero is
level with the surface of the mercury in the cistern, the exact height of the
column can be recorded. Certain corrections have to be made to the readings as
shown by the barometer, in order to make comparisons with other barometrical
barometer is used as the standard on board, though it differs materially in
construction from the standard on shore, the two chief differences being:
(1) The bore
of the marine instrument is contracted for the greater part of its length in
order to prevent an up-and-down motion of the mercury (technically called
"pumping"), due to the rolling and pitching of the ship; this
contraction, however, increases the friction of the mercury in the tube, and
consequently makes the marine instrument somewhat slower in recording a change
second important difference is that the zero point in all standard barometers
from which the graduation commences is a fixed ivory point depending
from the top of the cistern in the shore instrument, and provision is made for
altering the capacity of the cistern so as to enable the level of the mercury
to be exactly set at this point just previous to an observation being made,
thus cutting out the connection for "capacity," which is allowed for
when reading a marine barometer, but the necessity
for applying an actual correction in the marine barometer is avoided by
manipulating the scale in such a manner as to give a correct difference between
the level of the mercury in the cistern and the height recorded. This is done
by the manufacturers. It should be remembered that when reading the barometer
the zero of the vernier should always be made to
coincide with the highest part of the curve of the mercury column.
should be exercised in handling this instrument, as a sudden lowering of the
top generally means breakage, owing to the vacuum at the top of the mercury
permitting it to fall practically as if it was a solid metal rod; barometers
should be inverted very slowly, and packed with their cisterns ends up.
should be chosen for it; suspension low down, and, if possible, near to the
centre of gravity of the ship, out of the way of traffic, and in as uniform a
temperature as possible.
to read the Barometer.
Turn the vernier wheel until the lower edge of the former is level
with the top of the mercury. (Great care should be taken to acquire the habit
of reading with the eye exactly on a level with the top of the mercury.) Then
(1) Note the
scale line next below the level of the top of the mercury column, as shown by
the lower edge of the vernier. (This in the figure is
showing Scale and Vernier on a Mercurial Barometer.
(2) Look at
the vernier, and note that line which is in one and
the same direction with a line on the scale. (This in the figure is the second
line from the bottom of the vernier, and represents
two hundredths of an inch, or '02 of an inch.)
(3) Add the
two together, which will give the correct reading. (This in the figure is 29.40
+ .02 = 29.42)
Aneroid Barometer depends for its indications
on the movements of the top of a metal drum which is made
very thin and corrugated, after which it is
partially exhausted of air, thus making it very susceptible to the slightest
changes in the external pressure. This top is connected to a pointer by means
of very delicate mechanism, which has the effect of greatly magnifying its
movements. This pointer can be set at any pressure by means of a screw at the
back of the instrument, and, as owing to its extreme delicacy, the mechanism is
always somewhat liable to derangement, the reading of the instrument should
frequently be compared with that of the mercurial barometer, and if any
difference is noted it should be set to correspond with the latter instrument.
advantage of the aneroid is its convenient size, and also the rapidity with
which it will show any change of pressure. Also it can be kept in the
chart-house immediately under the eyes of the officer of the watch.
Thermometer is an instrument for
measuring the temperature. It consists of a closed glass tube with a hair-like
bore, terminating in a bulb at one end. The bulb and part of the tube contains
mercury, the remainder of the tube being a vacuum. A graduated scale is attached.
thermometers used for sea temperature are enclosed in metal cases to prevent
breakage. The water should be drawn clear from any discharge from the ship, the
nearer the bow the better, and the thermometer should be immersed for several
minutes. If a canvas bag is used it should not be placed in a draught, as
evaporation will materially lower the temperature.
Maximum Thermometer differs only in
construction to an ordinary one in that the zero of graduation is at the end of
the tube farthest away from the bulb, and it also has a small contraction in
the bore just above the bulb; the effect of this contraction is to largely
increase the friction set up between the mercury and the sides of the bore, and
therefore to prevent any passage of mercury except under considerable pressure.
Its action depends on the difference of the forces caused by gravity, and the
expansion of mercury caused by an increase of temperature, the latter being
largely in excess of the former. Supposing the instrument to be suspended
vertically, bulb end up, and that the temperature remains uniform at freezing
point, the mercury in the bulb will remain there, as the force of gravity is
not sufficient to overcome the friction at the point of contraction. If the
temperature decreases, still nothing happens, except that a small space is
formed in the bulb due to the contraction of the mercury there. On the other
hand, if the temperature increases, the mercury expands and refills this small
space, after which the surplus portion is forced through the contraction, and
falls to the bottom of the bore, which it fills, by an amount depending on the
rise of temperature, and which amount can be read off on the scale
re-setting, all that is necessary is to swing the instrument bulb down; this
force, together with that of gravity, enabling the mercury at the bottom of the
bore to overcome the friction in the contraction, and thus re-enter the bulb.
After being re-set and suspended bulb up it should read the same as an ordinary
thermometer observed at the same time.
Minimum Thermometer differs from an
ordinary one in that alcohol is substituted for mercury on account of its
transparency and non-liability to freeze; also a small black glass index,
shaped like a dumb-bell, is inserted into the column of liquid in the bore of
the tube. Its action depends on the power of cohesion of the particles forming
the surface of any liquid which is illustrated by the familiar experiment of
floating a needle on the surface of water in a cup. To set the instrument it should
be held with the bulb inclined upwards, the index will then slide down until
one end comes to the surface of the liquid in the bore, through which it is unable
to break, and this end should show the then temperature of the air. Supposing
that the instrument is placed horizontal, and that the temperature rises, the
index will remain stationary, and the liquid flows past it up the bore, but if,
on the contrary, the temperature falls, the index is carried towards the bulb,
as soon as the surface of the liquid in the bore touches it, its movement being
continued until the temperature stops falling.
cause of all winds is a difference of temperature; which, in turn, owing to hot
air rising, causes a difference in pressure over two areas,
and the air flows from the regions of high pressure to the regions of low
The hot air
over the equator rising causes the air to flow towards the equator from the
If the air
remained at rest, relative to the earth's surface, wind would not be felt; but
it is felt at once if the air is driven to a latitude
having either a faster or slower revolutionary motion.
equator the earth's rotation is about 1,000 miles an hour, and at the poles
nil. The air from the higher latitudes, where the earth's velocity is
comparatively small, flows to the lower latitudes; where the velocity is great,
and is deflected from East to West, or in a direction opposite to the earth's
rotation, hence trade winds blow roughly from N.E. and S.E.
are either "cyclonic" or "anti-cyclonic."
Cyclonic Winds are those which blow round an area of
low pressure, and generally travel quickly, and are accompanied by strong winds.
They blow spirally inwards round the area of low pressure; against the hands of
a watch in the northern hemisphere, and with the hands of a watch in the
Anti-cyclonic Winds are those which blow round an area
of high pressure: they travel slowly, and are generally accompanied by fine and
settled weather. They blow spirally outwards from the area of high pressure;
with the hands of a watch in the northern hemisphere, and against the hands of
a watch in the southern hemisphere.
of finding out the direction of the area of low pressure or that of high
pressure, the following law should be remembered:
with your face to the wind and the area of low pressure will be on your right
hand in the northern hemisphere, and on your left hand
in the southern hemisphere."
Wind is said
to "veer" if it shifts in the direction of the hands of a watch, and
to "back" if it shifts in the opposite direction.
Force of the Wind.- An instrument called an anemometer (a
sketch of which is given) registers the velocity of the wind. The instrument is
placed in the air, with the fan-wheel facing the wind. The velocity can then be
read from the dial by noting the difference in the readings between times given
by a stopwatch. The pressure in pounds per square foot is given by P = .003V2
where V is miles per hour. There are several forms of anemometers, varying in
size from a pocket edition upwards.
TABLE SHOWING THE FORCE OF THE WIND AS
REGISTERED BY BEAUFORT'S
(Note.- These anemometers are obtainable
from Short and Mason, Macdonald Road, Walthamstow)
Cable.- As a measure used in marine surveying is one-tenth of a
nautical mile, which is estimated roundly at 6,000 feet. Thus, 1 cable = 600
feet = 100 fathoms.
Chain.- The unit employed in the actual measurement of lengths by
surveyors. The land surveyor's chain consists of 100 links = 4 poles = 22 yards
= 66 feet. The marine surveying chain is 100 feet.
Degree.- Is the angle subtended at the centre of a circle by an arc
equal to the 360th part of the circumference. A degree is subdivided into 60
minutes, and each minute into 60 seconds.
degree of longitude is equal to 4 minutes of time.
Earth.- The circumference of the earth at the equator is little
less than 25,000 miles.
Fathom.- Measure of length and equal to 6 feet. Soundings are
reckoned in fathoms.
Knot.- The common term for a nautical mile used at sea = 6,080
League.- Three nautical miles.
Mile.- The common and authorised land mile used by the English =
1,760 yards = 5,280 feet.
Sound.-At an average temperature of 62° (F) travels at the rate
of 1,125 feet per second.
Time.- A definite portion of duration. It is marked in a general
manner by the recurrence of striking natural phenomena, such as the alterations
of light and darkness, and the succession of the seasons. Thus the two natural
measures of time are the "day," or period of the earth's rotation on
her axis, and the "year," or period of the earth's revolution in her
Common time is reckoned from an "initial
instant" or "epoch," independent of local situation, which is
the same for all the inhabitants of the earth.
Local time is reckoned at each particular place
from an epoch determined by local convenience, such as the transit of the sun's
centre over the meridian of the place.
Greenwich time is the mean solar time of
Ship time is
the mean solar time at the place where the ship happens to be. In East
longitude it is evidently before Greenwich time, and
in West longitude behind, every 15 degrees of longitude making a difference of