III. PRINCIPLES OF MECHANISM. 145 



third, AE, movable around the peg A, always move in the diagonal of the 

 parallelogram BADE, produced. The greatest of the forces is always taken 

 in the direction of AB, and determined as equal to 100, and the value of 

 the two others is read on the graduations of the lines BG and AF. 



If, for example, the parallelogram is placed so that the lines are respec- 

 tively AB = 100, BF = 70, AF = 80, the ring in that case will poise freely 

 without coining into contact with the peg, if the weights in the pans amount 

 respectively to 100, 70, 80 grammes; the weights of the pans, of course, must 

 be adjusted previously by tare weights. 



528e. Centrifugal Apparatus, complete. 



Ferdinand Ernecke, Berlin. 



529. Drawing. Experimental demonstration of the theory 

 of the parallelogram of forces and velocities, used by the exhibitor 

 since 1835. Professor Daniel Colladon, Geneva. 



Two small pulleys are placed at some distance from each other on the edge 

 of a table. Ou the opposite edge, held by the hand, is a small ball of the 

 size of a musket ball, to which are attached, by one of their extremities, two 

 helical springs of fine brass wire ; the other extremity of these two springs 

 is drawn parallel to the plane of the table by cords passing over the pulleys, 

 and themselves stretched by the weights P and P'. The table being sprinkled 

 with sand or seeds, two lines are traced upon it, marking in direction and in- 

 tensity the tensions of the two springs which draw the ball, which are equal 

 to the weights P and P'. On discharging the ball it traces on the table a 

 straight line, which is in the direction of the diagonal of the parallelogram of 

 the forces P and P' of the two springs. 



111. PRINCIPLES OF MECHANISM. 



529a. Models (14) of the various Link-works for effecting 

 the exact rectilinear motion of a point, commonly known as 

 " parallel motions." A. B. Kempe, B.A. 



Selected from a number described by the exhibitor in a paper published in 

 the "Proceedings of the Royal Society," No. 163, 1875, and entitled, " On 

 " a General Method of producing exact Rectilinear Motion by Linkwork," 

 which points out the common principle on which the linkworks depend. 



The points which have rectilinear motion are denoted by stars. 



529b. The Sylvester-Kempe Parallel-Motion. Model of 

 a link work for effecting the exact rectilinear motion of a point. 



A. B. Kempe, B.A. 



Discovered simultaneously by Professor Sylvester and the exhibitor in 

 1875. The main portion of the apparatus consists of a linkwork of four bent 

 rods called a " Quadruplane," which is such that four points, one on each rod, 

 always lie at the angles of a parallelogram of constant area and angles. Two 

 of the points consequently are situate at such distances from a third that the 

 one distance is the inverse of the other. One of the points being fixed, 

 another is made by means of a link to move in a circle passing through the 

 fixed point, the other then describes a straight line. If the bent rods are 

 made straight, the four points lie in a straight line, and the parallel motion 

 becomes that of Mr. Hart. 



40075. K 



