The Cambria freeman. [volume] (Ebensburg, Pa.) 1867-1938, 1891-02-27

But il evidently does not mattor
whether the air moves rapidly across
the thermometer or the latter moves
rapidly through the air. and hence ii
suflices to attach the instrument to
short cord and swing it rapidly around
in n circle.
This arrangement is called ''sling
thermometer" some form Of which Is
generally used for measuring air-tem-
peratures accurately.
4 few simple experiments with
cheap house-thermometer- say q
cheap one. because il is just possible
that IL NAV be smashed At the beginning
of the e1periments-will show the sur
priSing effect of rapid motion through
the air
On n hot summer day the thermon-
eter, hanging on Il sunny porch, may
stand at l2O degrees. OF course the air
has no such high temperature as this,
and If You take down the thermometer
and hang IL up in the shade IL VIII grad
ually come 20 nearly the true tempera-
ture of the air-perhaps so degrees or
90 degrees. This descent Of the mercury
win take a long time, but If in
stead of simply hanging the ther-
mOmeter up you tie a string TO il
and whirl it violently around your
head it viii reach its final temperature
in a very few minutes. Even if the
whirling is done in sunshine and with
great rapidity the direct effect Of the
suns rays viii be very small and the
instrument win soon record very near
lyl the temperature of the air at that
place, which may on n calm day be lit
tie lighter than that of the air in the
shade.
Even a thermometer which hangs
constantly in n shaded place, far from
any very hot or very cold bodies, does
not as a rule correctly indicate the tenn
perature of the air at the time Of obser-
vation. For the temperature of the air
is almost continually changing-at
times very rapidiy-and the thermome-
ter, as is evident from what has been
said, VIII not follow these changes at
all closely unless the wind is blowing
hard.
Within doors, too. and especially in
winter, the temperature of the air may
change rapidly, and the heat radiated
from human bodies. lamps and stoves
affects the thermometer.
In all cases, therefore, the 'sling"
method must be used, if we wish to
know the true temperature Of the air.
But. il may be asked, why confine our
attention 10 the temperature Of the arr
The suns rays, the radiation from
stoves, etc, affect as as well the
thermometer.
II the thermometer feels warm in the
sunshine, why not let it say so, rs we
do ourselves?
Simply beause the effect of radiated
heat on the thcrm omcter gives no reli-
able indication of its effect on our
selves. nor are all thermometers, even,
affected alike.
Two perfectly correct thermometers
which agree when ''slung' together in
the shade, or when immersed in a pail
OF water, may differ greatly when hung
up in the sunshine. For the amount Of
solar heat absorbed by the instruments
varies with their size and shape. the
quality and thickness Of the glass and
the character of the case. To say that
the thermometer stands at 100 degrees
in the sun gives no valuable informa-
ton 70 anybody who does not know all
about your thermometer and the place
where II hangs. The temperature Of
the air is all that the house thermom-
etcr can be expected 10 indicate, and il
viii not indicate this truly unless the
aforementioned precautions are taken.
Laurence L. Fletcher, NJ. d., in y. Y.
Ledger.