1464. A stratum of warm air indicates the presence of the devil. Boston, Mass. (Irish). 1465. If, when a newly-married couple go to housekeeping, she slyly takes her mother's dish-cloth or dish-wiper, she will never be homesick. Old... Read more of Various at Superstitions.ca

Privacy

Home - Storms - Weather Folklore - Barometer Guide

Most Viewed

Testing Barometers Hydrometers And Thermometers
Other Types Of Barmeters Fortelling Weather Part 4
How To Foretell Weather With A Barometer Part 2
Marine Barometer Adopted By Her Majesty's Government
Reading Barometers
How To Foretell Weather
Foretelling Weather Important Observations Final Part
Expert Details Foretelling Weather Part 3

Least Viewed

Expert Details Foretelling Weather Part 3
Foretelling Weather Important Observations Final Part
How To Foretell Weather
Reading Barometers
Marine Barometer Adopted By Her Majesty's Government
How To Foretell Weather With A Barometer Part 2
Other Types Of Barmeters Fortelling Weather Part 4
Testing Barometers Hydrometers And Thermometers

Testing Barometers Hydrometers And Thermometers

In the year 1853 a conference of maritime nations was held at Brussels,
on the subject of meteorology at sea. The report of this conference was
laid before Parliament, and the result was a vote of money for the
purchase of instruments and the discussion of observations, under the
superintendence of the Board of Trade. Arrangements were then made, in
accordance with the views of the Royal Society and the British
Association for the Advancement of Science, for the supply of
instruments properly tested.

In the barometers now in general use by meteorologists on land, the
diameters of the tubes are nearly equal throughout their whole length,
and a provision is made for adjusting the mercury in the cistern to the
zero point, previous to reading the height of the top of the column. The
object of the latter arrangement, it is well known, is to avoid the
necessity of applying a correction to the readings for the difference of
capacity between the cistern and the tube. At sea, barometers of this
construction cannot be used. Part of the tube of the marine barometer
must be very much contracted to prevent "pumping," and the motion of the
ship would render it impracticable to adjust the mercury in the cistern
to the zero point. In the barometer usually employed on shore, the index
error is the same throughout the whole range of scale readings, if the
instrument be properly made; but in nearly all the barometers which have
till recently been employed at sea, the index correction varies through
the range of scale readings, in proportion to the difference of capacity
between the cistern and the tube. To find the index correction for a
land barometer, comparison with a Standard at any part of the scale at
which the mercury may happen to be, is generally considered sufficient.
To test the marine barometer is a work of much more time, since it is
necessary to find the correction for scale readings at about each half
inch throughout the range of atmospheric pressure to which it may be
exposed; and it becomes necessary to have recourse to artificial means
of changing the pressure of the atmosphere on the surface of the mercury
in the cistern.

The barometers intended to be tested are placed, together with a
Standard, in an air-tight chamber, to which an air pump is applied, so
that, by partially exhausting the air, the Standard can be made to read
much lower than the lowest pressure to which marine barometers are
likely to be exposed; and by compressing the air it can be made to read
higher than the mercury ever stands at the level of the sea. The tube of
the Standard is contracted similarly to that of the marine barometer,
but a provision is made for adjusting the mercury in its cistern to the
zero point. Glass windows are inserted in the upper part of the iron
air-chamber, through which the scales of the barometers may be seen; but
as the verniers cannot be moved in the usual way from outside the
chamber, a provision is made for reading the height of the mercury
independent of the verniers attached to the scales of the respective
barometers. At a distance of some five or six feet from the air-tight
chamber a vertical scale is fixed. The divisions on this scale
correspond exactly with those on the tube of the Standard barometer. A
vernier and telescope are made to slide on the scale by means of a rack
and pinion. The telescope has two horizontal wires, one fixed, and the
other moveable by a micrometer, screw so that the difference between
the height of the column of mercury and the nearest division on the
scale of the Standard, and also of all the other barometers placed by
the side of it for comparison, can be measured either with the vertical
scale and vernier or the micrometer wire. The means are thus possessed
of testing barometers for index error in any part of the scale, through
the whole range of atmospheric pressure to which they are likely to be
exposed, and the usual practice is to test them at every half inch from
27.5 to 31 inches.

In this way barometers of various other descriptions have been tested,
and their errors found to be so large that some barometers read half an
inch and upwards too high, while others read as much too low. In some
cases those which were correct in one part of the scale were found to be
from half an inch to an inch wrong in other parts. These barometers were
of the old and ordinary construction. In some the mercury would not
descend lower than about 29 inches, owing to a fault very common in the
construction of the marine barometer till lately in general use, that
the cistern was not large enough to hold the mercury which descended
from the tube in a low atmospheric pressure.

The practice which has long prevailed of mounting the marine barometer
in wood is objectionable. The instrument recently introduced agreeably
to the recommendation of the Kew Committee, is greatly superior to any
other description of marine barometer which has yet been tested, as
regards the accuracy with which it indicates the pressure of the
atmosphere. The diameter of the cistern is about an inch and a quarter,
and that of the tube about a quarter of an inch. The scale, instead of
being divided into inches in the usual way, is shortened in the
proportion of about 0.04 of an inch for every inch. The object of
shortening the scale is to avoid the necessity of applying a correction
for difference of capacity between the cistern and the tube. The
perfection with which this is done may be judged of from the fact, that
of the first twelve barometers tested at the Liverpool Observatory with
an apparatus exactly similar to that used at Kew (whence these
instruments were sent by railway, after being tested and certified), the
index corrections in the two pressures of 28 and 31 inches in three of
them were the same; two differed 0.001 of an inch; and for the remainder
the differences ranged from 0.002 to 0.006 of an inch. The corrections
for capacity were therefore considered perfect, and, with one
unimportant exception, agreed with those given at Kew.

In order to check the pumping of the mercury at sea, the tubes of these
barometers are so contracted, through a few inches, that, when first
suspended, the mercury is perhaps twenty minutes in falling from the top
of the tube to its proper level. When used on shore, this contraction of
the tube causes the marine barometer to be always a little behind an
ordinary barometer, the tube of which is not contracted. The amount
varies according to the rate at which the mercury is rising or falling,
and ranges from 0.00 to 0.02 of an inch. As the motion of the ship at
sea causes the mercury to pass more rapidly through the contracted tube,
the readings are almost the same there as they would be if the tube were
not contracted, and in no case do they differ enough to be of importance
in maritime use.

The method of testing thermometers is so simple as scarcely to require
explanation. 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; and 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 maybe 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. For this range of 60 deg. the makers who
supply Government are limited to 0.6 of a degree as a maximum error of
scale reading; but so accurately are these thermometers made, that it
has not been found necessary to reject more than a very few of them.

* * * * *

Hydrometers are tested by careful immersion in pure distilled water; of
which the specific gravity is taken as unity.

In water less pure, more salt, dense, and buoyant, the instrument floats
higher, carrying more of the graduated scale out of the fluid.

The zero of the scale should be level with the surface of distilled
water, and rise above it in proportion as increase of density causes
less displacement.

The scale is graduated to thousandths--as far as .040 only--because the
sea water usually ranges between 1.014 and about 1.036. Only the last
two figures need be marked.

Previous: Marine Barometer Adopted By Her Majesty's Government

Report
Privacy

SHARE

BOOKMARK

Viewed 355