ent isotherms. The direction of the wind at any point is
indicated by an arrow which flies with the wind; and the state of the
weather--clear, partly cloudy, cloudy, rain, snow, etc.--is indicated
by symbols.

84. Components of the Air. The best known constituent of the air is
oxygen, already familiar to us as the feeder of the fire without and
within the body. Almost one fifth of the air which envelops us is made
up of the life-giving oxygen. This supply of oxygen in the air is
constantly being used up by breathing animals and glowing fires, and
unless there were some constant source of additional supply, the
quantity of oxygen in the air would soon become insufficient to
support animal life. The unfailing constant source of atmospheric
oxygen is plant life (Section 48). The leaves of plants absorb carbon
dioxide from the air, and break it up into oxygen and carbon. The
plant makes use of the carbon but it rejects the oxygen, which passes
back into the atmosphere through the pores of the leaves.

Although oxygen constitutes only one fifth of the atmosphere, it is
one of the most abundant and widely scattered of all substances.
Almost the whole earth, whether it be rich loam, barren clay, or
granite boulder, contains oxygen in some form or other; that is, in
combination with other substances. But nowhere, except in the air
around us, do we find oxygen free and uncombined with other
substances.

A less familiar but more abundant constituent of the atmosphere is the
nitrogen. Almost four fifths of the air around us is made up of
nitrogen. If the atmosphere were composed of oxygen alone, the merest
flicker of a match would set the whole world ablaze. The fact that the
oxygen of the air is diluted as it were with so large a proportion of
nitrogen, prevents fires from sweeping over the world and destroying
everything in their path. Nitrogen does not support combustion, and a
burning match placed in a corked bottle goes out as soon as it has
used up the oxygen in the bottle. The nitrogen in the bottle, not only
does not assist the burning of the match, but it acts as a damper to
the burning.

Free nitrogen, like oxygen, is a colorless, odorless gas. It is not
poisonous; but one would die if surrounded by nitrogen alone, just as
one would die if surrounded by water. The vast supply of nitrogen in
the atmosphere would be useless if the smaller amount of oxygen were
not present to keep the body alive. Nitrogen is so importa