New-York tribune., 1900-08-26

sweltering humanity obtains relief from sum
mer warmth principally through evaporation
from the skin The relief is proportional 10 the
rapidity with which that process goes on And
this in turn depends on the extent 10 which
the air is already saturated with moisture The
atmosphere is capable OF holding only a given
amount OF water vapor in suspension. although
the amount varies with the temperature. The
higher the latter the greater the former. But
for practical purposes the precise number OF
grains OF water in a cubic foot OF air Tor the
absolute humidityO is Of less account than the
percentage OF saturatiOn, Or relative humidity"
The latter represents the appetite OF the air
so TO speak With a relative humidity OF from
80 10 60 per cent. the air is eagerly disposed 10
seize upon and absorb moisture wherever It can
find it. But when the percentage rises &c so or
80 evaporation from q wet surface proceeds
slowly and the cooling effect becomes almost
lmperceptibIe.
The share which a fan OF the wind has IN
aIleviatng discomfort on hot day Is a result OF
humidity When evaporation takes place only
a thin stratuTc Of air next the skin becomes
charged with vapor. Inasmuch this Is
quickly saturated, there is no chance for further
evaporation unless the stratum referred 70 can be
removed and new supplies OF dryer air be brought
IN contact with the moist surface The latter
object Is attained when a natural OF artificial
breeze sweeps the face and hands. However. II
the air fcr miles around be already pretty nearly
saturated neither fan Tor q brisk wind will
prove Of much value. You can lead the air Qike
q horse) 20 water. but you can't make II drink
unless it IS thirsty.
Two we'll known styles or instrument are used
for measuring humidity. One. devised by De
Saussure, employs a hair whose tension varies
with the moisture and II thus can be made 10
actuate an index q graduated scale DR..
Daniel Draper. OF the Central Park Observatory
regards this type OF hygrometer more accurate
than the one commonly used by the Govern
ment. The latter consists OF two carefully
selected Thermometers placed side by side. The
bulb OF one is covered with thin muslin and
means are provided for keeping the cloth con
stantly vet. Evaporation chills the mercury.
and hence the wet bulb Thermometer reads lower
than the dry bulb, except when the air is COM
pletely saturated. The rapidity OF the evapora-
ton is indicated by the difference in the read
irgs OF the two instruments; and this difference
enables the observer with the aid Of certain
tables OF figures. 10 tell the humidity OF the air
at a glance.
It has been remarked that the higher the tenn
perature the more moisture given volume Of
air is capable OF absorbing Hence If the abso-
lutte humidity we're 10 remain unchanged the
relative humidity would fall of as the day ad-
vanced from the coolest hour Cbefore dawn 10
the waTmest (about p. M There is a more
less regular fuctuation IN temperature daily,
and also in humidity. The latter moves in the
opposite direction from the former. and though
the readings OF both the wet and dry bulb ther-
mometers should rise and fall simultaneously
the difference between them would ordinarily
be least during the night and early morning and
greatest IN the early afternoon
A highly instructive comparison is that az
forded by the wet and dry bulb Thermometer
records on August II and - this year. On the
former day. the hottest OF the summer, a maxi
mum temperature OF 96 degrees was observed.
But the air was remarkably dry AT p. m.
when the mercury in the dry bulb thermometer
stood at the height just indicated On Central
Park. the wet bulb instrument registered only
- degrees. Here was a difference OF 81 de-
grees. According 20 the system OF computation
used by Uncle sam the relative humidity az that
hour was 81 per cent DR.. Diapers hair
hygrometer made it SO On August - the high
est temperature observed was So. but the
weather was as vigorously denounced by the
public as that OF four days before The relative
humidity August - ranged from IS 70 81
per cent and the wet bulb reading kept up
within from > 10 degrees OF the dry bulb Al
the hottest time OF the day II stood at 80. or
only , degrees lower than the wet bulb reading
OF August IL. Thus II will be seen that the wet
bulb Thermometer is far better index OF gen
eraI discomfort than the dry bulb instrument
with which 1t is usually found in partnership
And this IS because the skin kept continually
moist by perspiration at this season OF the year
Is really more nearly in the condition OF the
wet bulb thermometer than OF the dry
Another point OF difference exists between the
two types o, day here described When it is in
tensely not and dry such wind as there is comes
from the west OF southwest. In other words il
is a land breeze But in typical dog day weather
it comes from the south or southeast, that is /
10 say. from the ocean Here. then is the secret
OF such abundant humidity as prevailed on Au-
gust -. But in the chain OF causes one may go /
a little further The direction OF the wind is |
dependent upon the relative positions OF areas |
OF high and low bsroTnetric pressure. The air ]
always blows out OF highs into --lows"-nOt /
directly but al angle. This Is general law /
Now generally IN summer there is a banking |
up or air over the ocean. especially IN the lati-
tude OF the Carolinas. and reduction OF press 1
ures inland. The normal situation 81 such times