The steam engine familiarly explained and illustrated; with an historical sketch of its invention and progressive improvement; its applications to navigation and railways; with plain maxims for railway speculators. By the Rev. Dionysius Lardner ... With additions and notes by James Renwick ...

DOUBLE-ACTING STEAM ENGINE. 97
move neither to the right nor to the left, but straight downward.'*
Now Watt conceived that his object would be attained if
he could contrive to make the beam perform the part of A B
in fig. 14, and to connect with it other two rods, c D and
D B, attaching the end of the piston to the middle of the rod
D B. The practical application of this principle required
some modification, but is as elegant as the notion itself is
ingenious.
The apparatus adopted for carrying it into effect is repre
sented on the arm which works the piston in fig. 15. The
beam, moving on its axis c, every point in its arm moves in
the arc of a circle of which c is the centre. Let B be the
point which divides the arm A c into equal parts, A B and
B c; and let D E be a straight rod equal in length to c B, and
playing on the fixed centre or pivot D. The end E of this
rod is connected by a straight bar E B with the point B, by
pivots at B and E on which the rod B E plays freely. If the
beam be supposed to move alternately on its axis c, the point
B will move up and down in a circular are, of which c is the
centre, and at the same time the point E will move in an
equal circular are round the point D as a centre. According
to what we have just explained, the middle point F of the
rod B E will move up and down in a straight line.
Also, let a rod A G, equal in length to B E, be attached
to the end A of the beam by a pivot on which it moves
freely, and let its extremity G be connected with E by a
rod G E, equal in length to A B, and playing on pivots at o
and E.
By this arrangement the joint A G being always parallel
* In a strict mathematical sense, the path of the point P is a curve of a high
order, but in the play which is given to it in the application used in the steam
engine, it describes only a part of its entire locus; and this part extending
equally on each side of a point of inflection, its radius of curvature is infinite,
so that, in practice, the deviation from a straight line, when proper proportions
are observed in the rods, and too great a play not given to them, is insignificant.
I 13