Richmond dispatch. [volume] (Richmond, Va.) 1884-1903, 1898-06-12

ahead. This is the chief purpose or the
two great observatories Of the storm
swept Indian and Chinese seas, Hong
kong and Nautilus, and the value OF the
work which they have done in prevent
ing the loss of ships, and the consequent
loss of lives and property, has been be-
You'd all estimate.
The Royal Observatory, Greenwich. Is
a meteorological as well as an astrono-
mical observatory, but it does not itself
issue any weather forecasts. The Green
wich observations OF weather are sent
TO the meteorological office, there to be
combined with similar records from
every part of the British isles, to from
the basis OF the daily forecasts which
the latter office publishes. To each Of
these three Offices therefore, the Royal
Observatory, Greenwich, stands in the re-
iatiOn Of a puISeyor. IL supplies them
with the original observations more or
less I'm reduced and corrected FORM with
out which they could not carry on most
important portions of their work. &.
Let II be noted how closely the three
several departments-the nautical al
manac' Office, the time department, and
the meteorological office, are related TO
practical navigation Whatever question
Of pure science-of knowledge that Is
apart from its useful appications-may
arise out Of the following up Of these
several inquiries, yet the first thought,
the first principle of each. is TO render
navigation more sure.
The first of all meteorological instru-
ments is the barometer. which, under its
two chief forms of mercurial and ane-
roid, is simply a means OF measuring the
pressure eXerted by the atmosphere.
IMPORTANT CORRECTIONS.
There are two important corrections
to which its readings are subject The
first is for the height of the station
above the level of the sea; the second is
for the effect of temperature upon the
mercury in the barometer itself, length
ening the column. TO overcome these,
the height Of the standard barometer at
Greenwich above the sea-leVel has been
most carefully ascertained, and the
heights relative TO it of the other baro-
meters of the observatory, particularly
those i'm rooms occupied by fundamental
telescopes, have also been determined
while the self-recording barometer is
mounted in a basement. where it is al
most completely protected from changes
of temperature
Next in Importance to the barometer
as a meteorological instrument comes
the thermometer. The great diff
Guilty in the observatory use Of the Their
mometer is TO secure a perfectly un-
exceptional exposure so that the
thermometer may be in free and perfect
contact with the air, ana yet completely
sheltered from any dlreCs ray from the
sun. This is secured in the great Their
mometer shed at Greenwich by a double
series of louvre boards, on the east,
south, and west sides Of the she'd the
north side being open. The shed itself
Is made a very roomy one, in order TO
t,f.r fn a pro fol a 81
A most important use of the thermome-
| ter is in the measurement of the amount
Of moisture In the air. To obtain this,
| a pair of therOmeters are mounted close
together, the bulb of one being covered
by damp muslin, and consequently the
two thermometers will read the same.
But If the air be comparatively dry, the
wet bulb and its temperature will sink
| to that at which the air would be fully
saturated with the moisture il already
contained. For the higher the tempera-
ture, the greater it's power OF containing
moisture The difference Of the reading
| of the two thermometers is, therefore, an
| index Of humidity. The greater the diff
ference, the greater the power Of abb
| sorbing moisture, or, in other words, the
| dryness of the air. The great shed al
| ready alluded 10 is devoted TO these com
panion Thermometers.
1 Very Closely connected WIN dNihpS-
| pheric pressure. as shown us by the ba-
| rometer is the study Of the direction OF
| winds. II we take a map of the British
| Isles and the neighboring countries. ana
put down upon them the barometer read
| ings from a great number of observing
| stations. and then Join together the diff
ferent places which show the same ba-
| rometric pressure, We shall md that
| these lines of equal pressure-technically
| called 'isobars'-are apt TO run much
| nearer together I'm some places than In
l others Clearly where the isopars are
| close together IL means that in very
| short difference Of country We have a
| great difference of atmospheric pres
| sure. In this case we are likely TO get
| very strong wind blowing from the re-
| eIon of high pressure TO the region OF
| low pressure in order to restore the bal
| ance.
| II further, we had information from
| these various observing stations or lhe
| direction In which the wind was blow
| ing. we should soon perceive other rela-
| tionshps For instance, If we found mat
/ the barometer read about the same IN
| line across the country from east to west
| but that IL was higher in the north or
| the islands than In the south. We should
| then have a general set of winds 10 the
l east. and a similar relation would hold
| good IL the barometer were highest In
| some other auarter-that Is. the prevail
| ing wind will come from a quarter at
J, right angles TO the region of highest pa
| rometer; or, as IL is expressed In what rs
known as "Euys Ballots law s'anu
with your back TO the wind ana the
| barometer will be lower on the left hand
| than ON your right" This law holds good
\ for the northern hemisphere generally.
| except near to the equator; In the south
| ern hemisphere the left hand is the side
| OF low barometer.
THE instruments for wind Observation
| are OF two classes-vanes, to show iti
! direction and thermometers. 10 show III
| speed and its pressure. fhesc may y.
reearded as two different modes In whicl
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