from the central signal office at Washington.

We shall see, presently, how the weather interprets itself to "Old
Probabilities." Although it has proved such a fruitful subject of
discourse in all ages, yet I am afraid many people who pass remarks
upon it do not really think what the weather is made of. Let us examine
its different elements.

The atmosphere has weight, just as water or any other fluid, although
it seems to be perfectly bodiless. We must comprehend that the
transparent, invisible air is pressing inward toward the center of the
earth. This pressure varies according to the state of the weather, and
the changes are indicated by an instrument called a barometer.
Generally speaking, the falling of the mercury in the tube of the
barometer indicates rain, and its rise heralds clear weather. Sometimes
the rise is followed by cold winds, frost and ice. What these changes
really indicate, however, can be determined only by comparing the
barometric changes, at certain hours, in a number of places very far
apart. This is done by the Signal Service. Observations are made at
about one hundred and forty stations, in different portions of the
country, at given hours, and the results are telegraphed at once to
Washington, where our faithful "weather clerk" receives them, reasoning
out from them the "probabilities" which he publishes three times in
every twenty-four hours.

But the atmosphere varies not only in weight, but also in temperature.
The thermometer tells us of such changes.

Besides this, the air contains a great amount of moisture, and it shows
as much variation in this characteristic as in the others. For the
purpose of making known the changes in the moisture of the atmosphere,
an instrument has been invented called a "wet-bulb" thermometer.

We are thus enabled to ascertain the weight or pressure, the
temperature, and the wetness of the air, and now it only remains for us
to measure the force, and point out the direction, of the wind. This is
done by the familiar weather-vane and the anemometer. The vane shows
the direction, and the anemometer is an instrument which indicates the
velocity of the wind.

It is by a right understanding of all these instruments that the signal
service officer is enabled to tell what the weather says of itself; for
they are the pens with which the weather writes out the facts from
which the officer makes up his reports for the benefit of all
concerned. Thus, however wild