The Hurricane Guide | Page 6

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