The existence of various forms of energy had been known, of course, for
ages; there was the energy of a falling stone, the energy produced by
burning wood or coal or any other substance, but the essential
_identity_ of all these forms of energy had not been suspected. The
conception of energy as something which, like matter, was constant in
amount, which could not be created nor destroyed, was one of the great
scientific acquisitions of the past century.
[Illustration: WAVE SHAPES
Wave-motions are often complex. The above illustration shows some fairly
complicated wave shapes. All such wave-motions can be produced by
superposing a number of simple wave forms.]
[Illustration: THE POWER OF A MAGNET
The illustration is that of a "Phoenix" electric magnet lifting scrap
from railway trucks. The magnet is 52 inches in diameter and lifts a
weight of 26 tons. The same type of magnet, 62 inches in diameter, lifts
a weight of 40 tons.]
[Illustration: _Photo: The Locomotive Publishing Co., Ltd._