Observations
In sketching out a system of observation having especial reference to
atmospheric waves and rotatory storms, regard has been had--_first_, to
the instruments that should be used, the observations to be made with
them, the corrections to be applied to such observations, and the form
of registry most suitable for recording the results: _second_, to the
times of observation: _third_, to the more important localities that
phænomena requiring extraordinary observations for their elucidation:
and _fifth_, to particular seasons, when the instruments should be
watched with more than ordinary care.
The more important objects of observation having especial reference to
atmospheric waves are those points which have been termed _crests_ and
_troughs_. These are simply the _highest_ and _lowest_ readings of the
barometer, usually designated _maxima_ and _minima_, and should for the
object in view receive particular attention. Whenever there is reason to
believe that the barometer is approaching either a _maximum_ or
_minimum_, additional observations should be resorted to, so as to
secure as nearly as possible _the precise time_ as reckoned at the ship,
with her position, of its occurrence, as well as the altitude of the
mercurial column at that time and place. By means of such observations
as these on board several ships scattered over the surfaces of our great
oceans, much valuable information may be accumulated of a character
capable of throwing considerable light on the _direction_ in which the
lines of barometric maxima and minima stretch, and also a tolerably
accurate notion may be formed of their progress, both as regards
direction and rate. In immediate connexion with such observations
particular attention should be paid to the direction of the wind
according to the season.
HURRICANE OBSERVATIONS INSTRUMENTS.
_Description and Position of Instruments._--The principal instrument
requisite in these observations is the barometer, which should be of the
marine construction, and as nearly alike as possible to those furnished
to the Antarctic expedition which sailed under the command of Sir James
Clark Ross. These instruments were similar to the ordinary portable
barometers, and differed from them only in the mode of their suspension
and the necessary contraction of the tubes to prevent oscillation from
the motion of the ship. The barometer on shipboard should be suspended
on a gimbal frame, which ought not to swing too freely, but rather so as
to deaden oscillations by some degree of friction. To the upper portion
of the tube in this construction of instrument light is alike accessible
either in front or behind, and the vernier is furnished with a back and
front edge, both being in precisely the same plane, nearly embracing the
tube, and sliding up and down it by the motion of rack-work; by the
graduation of the scale and vernier the altitude of the mercury can be
read off to ·002 inch.
When the barometer is placed in the ship, its position should be as near
midships as possible, out of the reach of sunshine, but in a good light
for reading, and in a situation in which it will be but little liable to
sudden gusts of wind and changes of temperature. Great care should be
taken to ascertain the exact height of its cistern above the water-line,
and in order to facilitate night observations every possible arrangement
should be made for placing behind it a light screened by white paper.
_Observations._--The first thing to be done is the reading off and
recording the temperature indicated by the thermometer that in this
construction of instrument dips into the mercury in the cistern. Sir
John Herschel has suggested that "the bulb of the thermometer should be
so situated as to afford the best chance of its indicating the exact
mean of the whole barometric column, that is to say, fifteen inches
above the cistern enclosed within the case of the barometer, nearly in
contact with its tube, and with a stem so long as to be read off at the
upper level."
Previous to making an observation with the barometer the instrument
should be slightly tapped to free the mercury from any adhesion to the
glass; any violent oscillation should, however, be carefully avoided.
The vernier should then be adjusted to the upper surface of the mercury
in the tube; for this purpose its back and front edges should be made to
coincide, that is, the eye should be placed in exactly the same plane
which passes through the edges; they should then be brought carefully
down until they form a tangent with the curve produced by the convex
surface of the mercury and the light is _just_ excluded from between
them and the point of contact. It is desirable in making this adjustment
that the eye should be assisted by a magnifying-glass. The reading of
the scale should then be taken and entered in the column appropriated to
it in the proper form. If the instrument have no tubular or double-edged
index, the eye should be placed carefully at the level of the upper
surface of the mercury and the index of the vernier brought gently down
to the same level so as apparently just to touch the surface, great care
being taken that the eye index and surface of the mercury are all in the
same plane.
Each observation of the barometer should be accompanied by an
observation of the direction of the wind, which should be noted in the
usual manner in which it is observed at sea. In connexion with the
_direction_ the _force_ of the wind should be recorded in accordance
with the following scale, contrived by Admiral Sir Francis Beaufort:--
0. Calm
1. Light air or just sufficient to give steerage way.
2. Light breeze { or that in which a well- } 1 to 2 knots.
3. Gentle breeze { conditioned man of war, } 3 to 4 knots.
4. Moderate breeze { with all sail set, and } 5 to 6 knots.
{ clean full, would go in }
{ smooth water, from }
5. Fresh breeze } { Royals, &c.
6. Strong breeze } { Single-reefed top-sails
} { and top-gallant
} or that in which such a { sails.
7. Moderate gale } ship could just carry in { Double-reefed
} chase full and by { topsails, jib, &c.
8. Fresh gale } { Triple-reefed
} { topsails, &c.
9. Strong gale } { Close-reefed top-sails
} { and courses.
10. Whole gale or that with which she could scarcely bear
close-reefed main topsail and reefed foresail.
11. Storm or that which reduces her to storm staysails.
12. Hurricane or that which no canvas could withstand.
_Corrections._--As soon after the observations have been made as
circumstances will permit, the reading of the barometer should be
_corrected_ for the relation existing between the capacities of the tube
and cistern (if its construction be such as to require that correction),
and for the capillary action of the tube; and then _reduced_ to the
standard temperature of 32° Fahr., and to the sea-level, if on
shipboard. For the first correction the _neutral point_ should be marked
upon each instrument. It is that particular height which, in its
construction, has been actually measured from the surface of the mercury
in the cistern, and indicated by the scale. In general the mercury will
stand either above or below the neutral point; if _above_, a portion of
the mercury must have left the cistern, and consequently must have
_lowered_ the surface in the cistern: in this case the altitude as
measured by the scale will be _too short--vice versâ_, if below. The
relation of the capacities of the tube and cistern should be
experimentally ascertained, and marked upon the instrument by the maker.
Suppose the capacity to be 1/50, marked thus on the instrument,
"_Capacity 1/50:_" this indicates that for every inch of variation of
the mercury in the tube, that in the cistern will vary contrariwise
1/50th of an inch. When the mercury in the tube is _above_ the neutral
point, the difference between it and the neutral point is to be reduced
in the proportion expressed by the "capacity" (in the case supposed,
divided by 50), and the quotient _added_ to the observed height; if
_below, subtracted_ from it. In barometers furnished with a fiducial
point for adjusting the lower level, this correction is superfluous, and
must not be applied.
The second correction required is for the capillary action of the tube,
the effect of which is always to depress the mercury in the tube by a
certain quantity inversely proportioned to the diameter of the tube.
This quantity should be experimentally determined during the
construction of the instrument, and its amount marked upon it by the
maker, and is always to be _added_ to the height of the mercurial
column, previously corrected as before. For the convenience of those who
may have barometers, the capillary action of which has not been
determined, a table of corrections for tubes of different diameters is
placed in the Appendix, Table I.
The next correction, and in some respects the most important of all, is
that due to the temperature of the mercury in the barometer tube at the
time of observation, and to the expansion of the scale. Table II. of the
Appendix gives for every degree of the thermometer and every half-inch
of the barometer, the proper quantity to be added or subtracted for the
reduction of the observed height to the standard temperature of the
mercury at 32° Fahr.
After these the index correction should be applied. This is the amount
of difference between the particular instrument and the readings of the
Royal Society's flint-glass barometer when properly corrected, and is
generally known as the _zero_. It is impossible to pay too much
attention to the determination of this point. For this purpose, when
practicable, the instrument should be immediately compared with the
Royal Society's standard, and the difference of the readings of both
instruments, when corrected as above, carefully noted and preserved.
Where, however, this is impracticable, the comparison should be effected
by means either of some other standard previously so compared, or of an
intermediate portable barometer, the zero point of which has been _well
determined_. Suspend the portable barometer as near as convenient to the
ship's barometer, and after at least an hour's quiet exposure, take as
many readings of both instruments as may be necessary to reduce the
probable error of the mean of the differences below 0.001 inch. Under
these circumstances the mean difference of all the readings will be the
_relative_ zero or index error, whence, if that of the intermediate
barometer be known, that of the other may be found. As such comparisons
will always be made when the vessel is in port, sufficient time can be
allowed for making the requisite number of observations: hourly readings
would perhaps be best, and they would have the advantage of forming part
of the system when in operation, and might be accordingly used as such.
It is not only desirable that the zero point of the barometer should be
well determined in the first instance; it should also be carefully
verified on every opportunity which presents itself; and in every
instance, previous to sailing, it should be re-compared with the
standard on shore by the intervention of a portable barometer, and no
opportunity should be lost of comparing it on the voyage by means of
such an intermediate instrument with the standard barometers at St.
Helena, the Cape of Good Hope, Bombay, Madras, Paramatta, Van Diemen's
Island, and with any other instruments likely to be referred to as
standards, or employed in research elsewhere. Any vessel having a
portable barometer on board, the zero of which has been well determined,
would do well, on touching at any of the ports above named, to take
comparative readings with the standards at those ports, and record the
differences between the standard, the portable, and the ship barometers.
By such means the zero of one standard may be transported over the whole
world, and those of others compared with it ascertained. To do so,
however, with perfect effect, will require that the utmost care should
be taken of the portable barometer; it should be guarded as much as
possible from all accident, and should be kept safely in the "portable"
state when not immediately used for comparison. To transport a
well-authenticated zero from place to place is by no means a point of
trifling importance. Neither should it be executed hurriedly nor
negligently. Some of the greatest questions in meteorology depend on its
due execution, and the objects for which these instructions have been
prepared will be greatly advanced by the zero points of all barometers
being referred to one common standard. Upon the arrival of the vessel in
England, at the termination of the voyage, the ship's barometer should
be again compared with the same standard with which it was compared
previous to sailing; and should any difference be found, it should be
most carefully recorded.
The correction for the height of the cistern _above_ or _below_ the
water-line is _additive_ in the former case, _subtractive_ in the
latter. Its amount may be taken, nearly enough, by allowing 0·001 in. of
the barometer for each foot of difference of level.
An example of the application of these several corrections is
subjoined:--
_Attached Therm_. 54°·3. _Data for the correction of
the Instrument_.
+---------------------------------------+-------------------------------+
Barometer reading. 29·409 Neutral point 30·123
Corr. for capacity - ·017 Capacity 1/42
Capillary action + ·032
+---------------------------------------
29·392 Zero to Royal Society + ·036
Corr. for capillarity + ·032 Corr. for altitude above
water-line + ·004
+---------------------------------------
29·424
Corr. for temperature - ·068
+---------------------------------------
29·356
Corr. for zero and water-line + ·040
+---------------------------------------
Aggregate = pressure at
sea-level 29·396
+---------------------------------------+-------------------------------+
It would greatly facilitate the comparison of the barometric
observations by projecting them in curves when all the proper
corrections have been applied. This may be accomplished by a much
smaller expenditure of time than may at first be supposed. A paper of
engraved squares on which the observations of twelve days may be laid
down on double the natural scale, would be very suitable for the
purpose.[4] The projection of each day's observations would occupy but a
short time; and should circumstances on any occasion prevent the
execution of it, when the ship was becalmed or leisure otherwise
afforded, it would form an interesting and useful occupation, and serve
to beguile some of the tedium often experienced at such intervals.
_Registers._--For the particular object in view the register need not be
very extensive. One kept in the annexed form will be amply sufficient.
It should, however, be borne in mind that none but _uncorrected_
observations should find admission; in point of fact it should be
strictly a register of phænomena as _observed_, and on no account
whatever should any entry be made from recollection, or any attempt made
to fill up a blank by the apparent course of the numbers before and
after. The headings of the columns will, it is hoped, be sufficiently
explicit. It is desirable in practice that the column for remarks should
embrace an entire page opposite the other entries, in order that
occasional observations, as well as several other circumstances
continually coming under review in the course of keeping a journal, may
find entry.
METEOROLOGICAL REGISTER kept on board ______ during her voyage from
______ to ______ by ______.
+---------+----+------+-------+------+------------------+--------+----------+
Wind.
Att. -----------+------
Date. Lat. Long. Barom. Ther. Direction.Force. Remarks Observer.
-------------------------------------------------------------------
h. m.
+---+-----+----+------+-------+------+-----------+------+--------+----------+
The only difference between the above form and one for the reception of
_corrected_ readings will be the dispensing with the column for the
attached thermometer, and placing under the word Barom. "corrected."
More
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Appendix
TABLE I.--Correction to be added to Barometers for Capillary Action. +--------------------+---------------------------------+ Correction for Diameter of Tube. -----------------+--------------- ... Avoiding The Centres Of Rotating Storms
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