er the continent and connected at headquarters
by telegraph, so that we can have the readings from a whole series of
barometers at once, then it becomes a very useful instrument. A
barometer may read low at one station by the scale, but may be high with
reference to some other barometer that reads very low.

What is a barometer? If we should take a glass tube closed at one end,
the area of the cross section of which is one inch square, and fill it
with mercury, and while thus filled plunge the open end into a vessel of
mercury, it will be found that the amount of mercury remaining in the
tube above the level of the mercury in the vessel will weigh about
fifteen pounds, if the experiment has been performed at sea-level. This
will vary, however, according to the temperature of the air. Of course
barometers are tested when the air is at a certain temperature. If the
weight of mercury in the tube is fifteen pounds, since it is sustained
by the air pressing down on the mercury in the open vessel, it shows
that the air-pressure on that open vessel is equal to fifteen pounds to
the square inch. In practice, of course, the tubes are made very much
smaller. If the air changes so that it is lighter than normal the
mercury will fall in the tube, because the pressure on the mercury in
the open vessel is less than fifteen pounds to the square inch. And,
again, conditions may arise that will condense the air and make it for
the time being weigh more than fifteen pounds to the square inch, in
which case the mercury will rise in the tube. Thus it will be seen that
the barometer will register the slightest change in air pressure.

Let us dwell for a moment on the causes of what are commonly called
"changes of weather," when we will again revert to the use of the
barometer.

The use of the telegraph in connection with the establishment of a
weather bureau having stations for observation at convenient points
throughout the country has contributed much to the science of
meteorology. It is found that there are areas of high and low pressure
existing at the same time in different parts of the country. These
usually have their origin in the far northwest, and follow each other,
sweeping down the eastern side of the Rocky Mountains and gradually
bending easterly and from that to northeasterly by the time they reach
the Atlantic coast. The areas of low pressure are called cyclones, while
the areas of high pressure are called anti-cyclones. (B