one
position, so that the boat will either turn in a circle or go straight.
Fig. 116 illustrates such an arrangement.

CHAPTER X

THE DESIGN OF MODEL STEAM-ENGINES

INSTEAD of describing the construction of several model engines of
different design, the author thinks it advisable to put the reader in
possession of the fundamentals of model steam-engine design and
construction. In this way the model engineer will be able to design and
construct model steam-engines according to his own ideas and in
accordance with the raw materials and miscellaneous parts he may find in
his workshop. Unless the young mechanic is in possession of a very well
equipped workshop, it is quite impossible to construct a steam-engine
according to certain specifications. However, if he has in mind the
fundamental principles of steam-engine design, he can go ahead and
design his engine, for which he will have no trouble in machining or
producing the parts that enter into its construction. By this the
author means that the workman can design his engine to meet the
materials he has on hand.

Notice Fig. 117. This is a cylinder into which is fitted a piston. If
steam is forced into the cylinder the piston will be forced to the
opposite end of the cylinder. If some means is then provided so that the
steam can escape and the piston come back, another impulse can be given
it by admitting more steam, and thus a continuous motion may be
produced. This is how the steam-engine works.

[Illustration: FIG. 117]

Having learned how motion is imparted to the piston by the expansion of
steam under pressure, attention is directed to what is known as the "D"
slide-valve. This slide-valve permits steam to enter the cylinder and to
exhaust at proper intervals. See Fig. 118. Steam enters the steam-chest
through the pipe _A_. The slide-valve is shown at _D_. When the
slide-valve is in the position shown, steam enters the cylinder, and by
the time the cylinder has arrived in the position shown by the dotted
line _C_, the slide-valve moves over, closing the passage _B_. The steam
under pressure forces the piston to the opposite end of the cylinder.
When the piston reaches the opposite end of the cylinder, steam that has
entered through the passage _F_ again forces the piston back to its
original position. This is caused by the slide-valve shifting its
position, because of the impulse it received at the opposite end of the
cylinder. Thus it will be seen t