IDENTITY OF LINKS.
259
form-symbols and + are in the lower pairs absolutely interchange- able without alteration of the pairs, so that in the formula before us all the links might be indicated by the same symbols. Closer examination changes the defect into an advantage. The links appear alike in the formula because in their actual nature they are really alike. A clear indication of this is necessary
FIG. 180.
for the realization of the abstract form of the mechanism, for the perception of its essential nature under its material disguise. In the mechanism of Fig. 180 the ordinary mechanic sees a "beam" driving a " crank," -fg is the arm of the beam, de the connecting rod, I c the crank, and a h the frame of the machine, formed in the most various ways of columns or castings or timber, and supported
s 2
