found in immense quantities in the limestone
caves in the south-western States.

[Footnote 11: See Potash, Chapter VII., article Glass.]

What do you mean by _Marly_?

Consisting of marl, a kind of earth composed of different proportions
of clay and carbonate of lime; it is much used for manure. There are
several different-colored marls, each possessing different qualities.
The most common are the red and the white, though there are grey,
brown, blue, and yellow colored marls.

What is a Telescope?

An optical instrument, which serves for discovering and viewing
distant objects, either directly by glasses, or by reflection. The
invention of the telescope is one of the noblest and most useful of
which modern ages can boast, since by means of this instrument the
wonderful motions of the planets and fixed stars, and all the heavenly
bodies, are revealed to us. The honor of the invention is much
disputed; it is certain, however, that the celebrated Galileo was the
first who improved the telescope so as to answer astronomical
purposes. The name is formed from two Greek words, one signifying
_far_, the other _to observe_.

_Optical_, relating to Optics, the science of vision.

_Astronomical_, relating to Astronomy.

Who was Galileo?

A most eminent astronomer and mathematician, born at Florence, in
Italy. His inventions and discoveries in Astronomy, Geometry, and
Mechanics, contributed much to the advancement of those sciences. He
died in 1642.

_Astronomer_, one versed in Astronomy.

_Mathematician_, one versed in Mathematics; a science which
treats of magnitude and number.

What is Astronomy?[12]

That science which teaches the knowledge of the heavenly bodies, with
the nature and causes of their various phenomena.

[Footnote 12: See Chapter xviii.]

What is Geometry?

An ancient, perfect, and beautiful science, which treats of the
relations and properties of lines, surfaces, and solids.

What is meant by Mechanics?

The science which investigates the laws of forces and powers, and
their action on bodies, either directly or by machinery. When the term
_mechanic_ is applied to a _person_, it means one skilled in
mechanics, accustomed to manual labor.

_Investigate_, to search, to inquire into.

_Manual_, performed by the hand.

What is a Microscope?

An optical instrument, by means of which very minute objects are
represented exceedingly large,