Gi*rufy PktlotojAioal Society t Proceedings, Vol. II. 1870.] 



LXXV. On Bow's method of drawing diagrams in graphical statics with 

 illustrations from Peaucellier's linkage. 



THE use of Diagrams is a particular instance of that method of symbols 

 which ia so powerful an aid in the advancement of science. 



A diagram differs from a picture in this respect, that in a diagram no 

 attempt is made to represent those features of the actual material system which 

 are not the special objects of our study. 



Thus when we are studying the internal equilibrium of a particular piece 

 of a structure or a machine, we require to know its shape and dimensions, and 

 the specification of these may often be made easier by means of a drawing of 

 the piece. 



But when we are studying the equilibrium of a framework composed of 

 such pieces jointed together, in which each piece acts only by tension or by 

 pressure between its extremities, it is not necessary to know whether a particular 

 piece is straight or curved or what may be the form of its section. In order, 

 therefore, to exhibit the structure of the frame in the most elementary manner 

 we may draw it as a skeleton in which the different joints are connected by 

 straight lines. The tension or pressure of each piece may be indicated on such 

 a diagram by numbers attached to the line which represents that piece in the 

 diagram. The stresses in the frame would thus be indicated in a way which 

 is geometrical as regards the position and direction of the forces, but arithmetical 

 as regards their magnitude. 



But a purely geometrical representation of a force has been made use of 

 from the earliest beginnings of mechanics as a science. The force is represented 

 by a straight line drawn from the point of application of the force, in the 

 direction of the force, and containing as many units of length as there are 



