colors, and the sky serves as a background for viewing the analysis thus
made.

HEAT.--It is now conclusively proven, that heat, like light, magnetism
and electricity, is merely a mode of motion.

The _mechanical_ theory of heat may be shown by rubbing together several
bodies. Heat expands all substances, except ice, and in expanding
develops an enormous force.

EXPANSION.--In like manner liquids expand with heat. The power of
mercury in expanding may be understood when it is stated that a pressure
of 10,000 pounds would be required to prevent the expansion of mercury,
when heated simply 10 degrees.

Gases also expand. While water, and the different solids, all have their
particular units of expansion, it is not so with gases, as all have the
same coefficient.

CHAPTER VIII

HOW DRAUGHTING BECOMES A VALUABLE AID

The ability to read drawings is a necessary part of the boy's education.
To know how to use the tools, is still more important. In conveying an
idea about a piece of mechanism, a sketch is given. Now, the sketch may
be readable in itself, requiring no explanation, or it may be of such a
nature that it will necessitate some written description.

[Illustration: _Fig. 95. Plain Circle_]

LINES IN DRAWING.--In drawing, lines have a definite meaning. A plain
circular line, like Fig. 95, when drawn in that way, conveys three
meanings: It may represent a rim, or a bent piece of wire; it may
illustrate a disk; or, it may convey the idea of a ball.

Suppose we develop them to express the three forms accurately. Fig. 96,
by merely adding an interior line, shows that it is a rim. There can be
no further doubt about that expression.

Fig. 97 shows a single line, but it will now be noticed that the line is
thickened at the lower right-hand side, and from this you can readily
infer that it is a disk.

SHADING.--Fig. 98, by having a few shaded lines on the right and lower
side, makes it have the appearance of a globe or a convex surface.

[Illustration: _Fig. 96. Ring_
_Fig. 97. Raised Surface_
_Fig. 98. Sphere_]

Shading or thickening the lines also gives another expression to the
same circular line.

In Fig. 99, if the upper and left-hand side of the circle is heavily
shaded, it shows that the area within the circle is depressed, instead
of being raised.

DIRECTION OF SHADE.--On the other hand, if the shading lines, as in Fig.
100, are at the upper left-hand side, then the mind at once grasps t