The daily Cairo bulletin. (Cairo, Ill.) 1878-1???, 1879-01-08
American states arc pledged to give as
much as VIC do, the amount required of M
will be comparatively small.
THE REASON WHY.

VIM 4HLhMoMETERs DisAGREH--A CON
hUNicATloN FROM THE SIGNAL SER
VICE OFFICER.
Editor Of the Bulletin
In your issue Of the 9Th instant we no.
tice the qucsiions-"Why is it that the sig
nal service thermometer marks the tenn
perature OF the weather at from G to 0 de-
grees above the figures marked by private
thermometersf Since surroundings and al
titude don't explain the matter, what will1,
We presume that the above questions are
indirectly put for the signal office to solve.
which we will kindly endeavor to do for
the benefit of your readers. We will state
that the thermometer used by the signal
office are Fahrenheit's thermometers
all OF which have been carefully tested and
compared with the standard thermometer
at Washington, D. c. before being sent for
station use. Notwithstanding this feet
they are tested on station every three
months. The variation Of IS to 0 degrees"
between the signal service thermometer and
private ones may be attributed 10 numer
GUS causes. 1t must be remembered that
there are thermometers Of different manu-
tacturc, great many Of which are not rc-
liable. In order that we may have on in
telIigible measure of the changes of TOM
perature, the tube of the thermometer must
be graduated according to some general
principles. We need two invariable ten.
peratures for the determination oftwo fixed
points upon the scale The temperatures
generally adopted for this purpose arc those
OF melting ice and boiling water; and tho
interval between these points is varously
divided in different countries. The thermom-
eter used by the signal Office is Fahrenheite
and melting ice is marked 80 degrees, and
boiling water 2l2 degrees, the interval
beng divided into 1870 equal parts. Upon
Centigrade's thermometer this interval *
divided into 100 parts, and Reaumur,s 50.
In the two last named the freezing point is
marked 0. The requisites of good thrr-
mometer are-that the degrees upon tho
thermometer scale should correspond TO
equal volumes of Mcrcury-if the tube OF
the thermometer were Throughout of uniform
bore then the divisions upon tho scale
should be throughout of equal length; but
if the tube be not of uniform boro these
equal volumes will correspond TO unequal
lengths upon different parts of the scale,
hence the unreliability Of many Of our
cheapIy gotten up thermometers. The "sur-
roundings and altitude have considerable
to do with the temperature For the pur-
pose Of measuring the temperature of the
air, thermometer should be exposed in
the open air, where the circulation is unob-
structcd. 1t should face tho north,
and should be always in the shade. 1t
should be removed at least foot from the
wall of the building, and should be elevated
about ten feet from the ground. It should
be protected against the heat reflected
by neighboring objects, and sheltered from
the rain, since drops of water, by their
evaporation, would lower the temperature
Of the mercury in the bulb, The difference
between two thermometers, one whose bulb
is kept dry and the other moistcncd, will
sometimes vary 20 degrees or moro in tic
summer season, All these little obstiuc-
tons are avoided by the signal service thcr.
mometcrs. BY suspending thermometers
at different elevations above the earth from
one or two inches up to 200 feet, it is found
that the loss of heat by nocturnal radiation,
is quite sensible at the elevation Of 80 feet,
and does not entirely disappear at the
height of $50 feet During the night.
therefore, the temperature Of the air in.
creases as we rise above the carths surface
while during the DAY it is the reverse, |
trust, Mr.. Editor, that your readers, after
having read this article, will be convinced
that the signal service thermometer is the
one to rely upon.
W, II. RAY, s. s., U. S. A.
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