r to include steam, electricity, optics, hydraulics, thermics,
light, and a variety of detached mechanisms which cannot be classified
under any one of these heads, within the compass of about 450 pages, I
have to be content with a comparatively brief treatment of each subject.
This brevity has in turn compelled me to deal with principles rather
than with detailed descriptions of individual devices--though in several
cases recognized types are examined. The reader will look in vain for
accounts of the Yerkes telescope, of the latest thing in motor cars, and
of the largest locomotive. But he will be put in the way of
understanding the essential nature of _all_ telescopes, motors, and
steam-engines so far as they are at present developed, which I think may
be of greater ultimate profit to the uninitiated.

While careful to avoid puzzling the reader by the use of mysterious
phraseology I consider that the parts of a machine should be given their
technical names wherever possible. To prevent misconception, many of
the diagrams accompanying the letterpress have words as well as letters
written on them. This course also obviates the wearisome reference from
text to diagram necessitated by the use of solitary letters or figures.

I may add, with regard to the diagrams of this book, that they are
purposely somewhat unconventional, not being drawn to scale nor
conforming to the canons of professional draughtsmanship. Where
advisable, a part of a machine has been exaggerated to show its details.
As a rule solid black has been preferred to fine shading in sectional
drawings, and all unnecessary lines are omitted. I would here
acknowledge my indebtedness to my draughtsman, Mr. Frank Hodgson, for
his care and industry in preparing the two hundred or more diagrams for
which he was responsible.

Four organs of the body--the eye, the ear, the larynx, and the
heart--are noticed in appropriate places. The eye is compared with the
camera, the larynx with a reed pipe, the heart with a pump, while the
ear fitly opens the chapter on acoustics. The reader who is unacquainted
with physiology will thus be enabled to appreciate the better these
marvellous devices, far more marvellous, by reason of their absolutely
automatic action, than any creation of human hands.

A.W.

UPLANDS, STOKE POGES, BUCKS.

CONTENTS.

Chapter I.--THE STEAM-ENGINE.

What is steam?--The mechanical energy of steam--The boiler--The
circulation of water