of Edinburgh, Session 1875-76. 
161 
Monday , 5th June 1876. 
D. STEVENSON, Esq., C.E., Vice-President, in the Chair. 
The following Communications were read : — 
1. Physical Observations in Northern Asia. By 
Professor G. Forbes. 
2. On Parallel Motions. By the Rev. John Wilson, M.A., 
Bannockburn Academy. Communicated by Professor 
Kelland. 
It has been well said that the transmission of force is an ‘‘essen- 
tial condition in machinery.” It is no less true that directness in 
transference is important; that the fewer links in the chain bind- 
ing driver and follower together, the less likely is the machine to 
be put out of gear. There is no question here as to the compara- 
tive values of the different modes of conveying motion from a 
prime mover, — rolling contact, sliding contact, wrapping connec- 
tors, or linkwork. Each has its own excellencies ; each its special 
advantages ; and one is to be preferred to another only according 
to the nature of the work to be done. 
I. Watt's Parallel Motion . — The general problem is the “com- 
mutation of circular with rectilinear motion.” The importance of 
the question began to be felt soon after the introduction of the 
steam-engine; and Watt, in 1784, patented an invention which 
not only had the credit of being the earliest, but up to recent 
times, the most reliable and accurate parallel motion in existence. 
This system was a great advance on the huge chains and arches 
which were affixed to the working beam of the engine for the pur- 
pose of obtaining the desired motion; and it has proved to be suf- 
ficiently accurate for all practical purposes. The construction is 
simple, consisting of three bars : two, rotating round fixed centres, 
and connected at their other extremities to the third bar. A point 
in this bar, either within or without the points of junction, accord- 
