167 
of Edinburgh, Session 1875 - 76 . 
In this system the connectors are joined not at the opposite 
angles of the rhombus but at such points in the adjacent sides 
produced that at every moment they are parallel to the remaining 
sides (fig. 6). APC 
is a straight line. 
AM = CMsince AM||LP. 
Let LP be produced to 
F, makingLF = LM,and 
complete the rhombus 
LFKM* The bars AN, 
ON, and IP can be re- 
moved without interfer- 
ing with the motion of 
C and P. Thus AM, Fig. 6. 
MC, with LF, FK form a pantagraph, and for every position of 
C, P takes another, equiangular and proportional. 
III. The Gorgon Linkage . — The parallel motion of Mr Scott 
Bussell is exact, and constitutes a two-bar motion. From the fact 
that it was fitted by Mr Seaward to the engines of the “ Gorgon,” 
it may conveniently be called a Gorgon Linkage. A link AB is 
bisected in C, and at this point another link CD, equal to CA or 
CB is attached. D is the centre of revolution. If one end A of 
the link AB be guided along the axis of x, the motion of B is at 
every moment in the axis of y from D. 
For every position C is the centre of a circle ADB, and ADB is 
the angle in a semi-circle, that is, a right angle. Hence B de- 
scribes a straight line. 
This system of links derives additional interest from the dis- 
covery of the Peaucellier cell, as by it the motion of the parallel 
point can be thrown in a direction at right angles to itself ; that 
is, parallel to the line joining fulcrum and pivot ; and this can be 
transferred to the line of centres by means of the pantagraph. 
IV. Hypocycloidal Parallel Motion . — Another very interesting 
case of the problem of Parallel Motion is that produced by a hypo- 
cycloidal movement. When one circle is made to revolve on the 
concave circumference of another circle, any point in it describes 
a curve, which is called the hypotrochoid or hypocycloid. If the 
diameter of the revolving circle be equal to the radius of the circle 
VOL. IX. 
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