Marine Barometer Adopted By Her Majesty's Government
On the recommendation of the Kew Observatory Committee of the British
Association for the Advancement of Science.
This instrument should be suspended in a good light for reading, but out
of the reach of sunshine or the heat of a fire or lamp. It should be as
nearly amidships, and exposed as little to sudden changes of
temperature, gusts of wind, or injuries, as possible. In a ship of war
it should be below the lowest battery or gun-deck. Light should have
access to the back of the tube, to admit of setting the index so as to
have its lower edge a tangent to the surface of the mercury--the eye
being on the same level, which is known by the back and front edges of
the index being in one line with the mercury surface. White paper or
card will reflect light for setting the vernier correctly. The height of
the cistern above or below the ship's water-line should be ascertained,
and entered on the register.
It is desirable to place the barometer in such a position as not to be
in danger of a side blow, and also sufficiently far from the deck above
to allow for the spring of the metal arm in cases of sudden movements of
the ship.
If there is risk of the instrument striking anywhere when the vessel is
much inclined, it will be desirable either to put some soft padding on
that place, or to check movement in that direction by a light elastic
cord; in fixing which, attention must be paid to have it acting only
where risk of a blow begins, not interfering otherwise with the free
swing of the instrument: a very light cord attached above, when
possible, will be least likely to interfere injuriously.
The vernier, as usual in standard barometers, reads to the two
thousandth (.002) part of an inch. Every long line cut on the vernier
corresponds to .01 part; each small division on the scale is .05; the
hundredth parts on the vernier being added to the five when its lower
edge is next above one of the short lines; or written down as shown by
the figures on the vernier only, when next above one of the divisions
marking tenths.
In placing this barometer, it is only necessary to fix the instrument
carefully, as indicated in the above directions, and give a few gentle
taps with the fingers on the bottom, to move the mercury. Without
further operation it will usually be ready for observation in less than
an hour.
When moving the barometer, or replacing it in its case, the mercury
should be allowed to run gently up to the top of the tube, by holding
the instrument for a few minutes inclined at an angle. The vernier
should be brought down to the bottom of the scale. No other adjustment
for portability is required. During carriage, it ought to be kept with
the cistern end uppermost, or lying flat, the former position being
preferable.
If the mercury should not descend at first by a few gentle taps, use
sharper (but of course without violence), by which, and two or three
taps, with the finger ends, on the tube--between the scale and the
tangent screw--the mercury will be made to begin to descend.
In reading off from a barometer, it should hang freely, not inclined by
holding, or even by touch.
Sometimes, though rarely, at sea the mercury seems stopped. If so,
take down the instrument (after sloping), reverse it, tap the tube
gently while the cistern end is upwards, and then replace as before.
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