Take a hollow cylinder, the bottom closed while the top remains open, and pour in water to the height of a few inches.  Next cover the water with a flat plate or piston, which fits the interior of the cylinder perfectly; then apply heat to the water, and we shall witness the following phenomena.  After the lapse of some minutes the water will begin to boil, and the steam accumulating at the upper surface will make room for itself by raising the piston slightly.  As the boiling continues, more and more steam will be formed, and raise the piston higher and higher, till all the water is boiled away, and nothing but steam is left in the cylinder.  Now this machine, consisting of cylinder, piston, water, and fire, is the steam-engine in its most elementary form.  For a steam-engine may be defined as an apparatus for doing work by means of heat applied to water; and since raising such a weight as the piston is a form of doing work, this apparatus, clumsy and inconvenient though it may be, answers the definition precisely.[17]

=Reference to Experience= is one of the most vital principles in exposition—­as in every other form of discourse.

“Reference to experience, as here used, means reference to the known.  The known is that which the listener has seen, heard, read, felt, believed or done, and which still exists in his consciousness—­his stock of knowledge.  It embraces all those thoughts, feelings and happenings which are to him real.  Reference to Experience, then, means coming into the listener’s life.[18]

The vast results obtained by science are won by no mystical faculties, by no mental processes, other than those which are practised by every one of us in the humblest and meanest affairs of life.  A detective policeman discovers a burglar from the marks made by his shoe, by a mental process identical with that by which Cuvier restored the extinct animals of Montmartre from fragments of their bones.  Nor does that process of induction and deduction by which a lady, finding a stain of a particular kind upon her dress, concludes that somebody has upset the inkstand thereon, differ in any way from that by which Adams and Leverrier discovered a new planet.  The man of science, in fact, simply uses with scrupulous exactness the methods which we all habitually, and at every moment, use carelessly.

    —­THOMAS HENRY HUXLEY, Lay Sermons.

Do you set down your name in the scroll of youth, that are written down old with all the characters of age?  Have you not a moist eye? a dry hand? a yellow cheek? a white beard? a decreasing leg? an increasing belly? is not your voice broken? your wind short? your chin double? your wit single? and every part about you blasted with antiquity? and will you yet call yourself young?  Fie, fie, fie, Sir John!

    —­SHAKESPEARE, The Merry Wives of Windsor.

Finally, in preparing expository material ask yourself these questions regarding your subject: