COMPOSITION AND RESOLUTION OF FORCE. 



209 



gram of forces," let a line, P O, be drawn upon the upright board to which the 

 wheels are attached, from the point P upward, in the direction of the thread 

 C P. Also, let lines be drawn upon the board immediately under the threads 



Fig. 2. 



P M and P N. From the point P, on the line P O, take as many inches as 

 there are ounces in the weight C. Let the part of P O thus measured be P c, 

 and from c draw c a parallel to P N, and c b parallel to P M. If the sides P a 

 and P b of the parallelogram thus formed be measured, it will be found that P a 

 will consist of as many inches as there are ounces in the weight A, and P b of 

 as many inches as there are ounces in the weight B. 



In this illustration, ounces and inches have been used as the subdi visions of 

 weight and length. It is scarcely necessary to state, that any other measures 

 of these quantities would serve as well, only observing that the same denomi- 

 nations must be preserved in all parts of the same investigation. 



Among the philosophical apparatus of the University of London, is a very 

 simple and convenient instrument which I have constructed for the experimen- 

 tal illustration of this important theorem. The wheels M N are attached to the 

 tops of two tall stands, the heights of which may be varied at pleasure by an 

 adjusting screw. A jointed parallelogram, A B C D, fig. 3, is formed, whose 



Fig. 3. 



sides are divided into inches, and the joints at A and B are moveable, so as to 

 vary the lengths of the sides at pleasure. The joint C is fixed at the extremity 

 of a ruler, also divided into inches, while the opposite joint A is attached to a 

 brass loop, which surrounds the diagonal ruler loosely, so as to slide freely 

 alon^ it. An adjusting screw is provided in this loop, so as to clamp it in any ' 

 required position. 



VOI,. II. 14 



