nd-steel principle was
made by certain Esquimo tribes who obtained fire by striking a piece of
quartz against a piece of iron pyrites. The latter is a yellow sulphide
of iron, of crystalline form, best known as "fool's gold." Doubtless,
the more primitive beings used dried grass, leaves, and moss as
inflammable material upon which the sparks were showered. In later
centuries the tinder-box was filled with charred grass, linen, and
paper. There was a long interval between the development of fire-sticks
and that of the tinder-box as measured by the progress of civilization.
During recent centuries ordinary brown paper soaked in saltpeter and
dried was utilized satisfactorily as an inflammable material. Such
devices have been employed in past ages in widely separated regions of
the earth. Elaborate specimens of tinder-boxes from Jamaica, Japan,
China, Europe, and various other countries are now reposing in the
collections in the possession of museums and of individuals.

If the radiant energy from the sun is sufficiently concentrated upon
inflammable material, the latter will ignite. Such concentration may be
achieved by means of a convex lens or a concave mirror. This method of
producing fire does not antedate the more primitive methods such as
striking quartz or rubbing wooden sticks, because the materials required
are not readily found or prepared, but it is of very remote origin.
Aristophanes in his comedy "The Clouds," which is a satire aimed at the
science and philosophy of his period (488-385 B. C.), mentions
the "burning lens." Nearly every one is familiar with an achievement
attributed to Archimedes in which he destroyed the ships at Syracuse by
focusing the image of the sun upon them by means of a concave mirror.
The ancient Egyptians were proficient in the art of glass-making, so it
is likely that the "burning-glass" was employed by them. Even a crude
lens of glass will focus an image of the sun sufficiently well to cause
inflammable material to ignite.

The energy in sunlight varies enormously, even on clear days, because
the water-vapor in the atmosphere absorbs some of the radiant energy
emitted by the sun. This absorbed radiation is chiefly known as
infra-red energy, which does not arouse the sensation of light. When the
water-vapor content of the atmosphere is high, the sun, though it may
appear as bright to the eye, in reality is not as hot as it would be if
the water-vapor were not present. However, a fi