vi PARALLEL FORCES AND CENTRE OF GRAVITY 85 



as base a similar line can be drawn from the middle to the 

 opposite angle. These lines intersect at one-third the distance 

 up, measured from the base, and the point of intersection is the 

 of gravity of the triangular plate (Fig. ?>(>). 



Centre of Gravity of a Quadrilateral. EXPT. 70. Cut 

 a four-sided figure out of cardboard, and draw a line connect- 

 ing two of the opposite angles. Find by bisecting this line and 

 taking one-third the distance from the middle to the opposite 

 angle, the centre of gravity of each of the triangles into 

 which the figure is divided. Connect the two points found. 

 Then draw the other diagonal of the quadrilateral, repeat the 

 measures, and connect the centres of gravity as before. 

 Make a hole where this short line cuts the other, and show, 

 by passing a piece of knotted thread through it, and so sus- 

 pending the cardboard, that the point determined in this 

 way is the centre of gravity of the whole figure. 



Other Centres of Gravity. EXPT. 71. Procure a skeleton 

 cube or tetrahedron, and suspend it as in the preceding experi- 

 ments. Mark the verticals through the point of suspension 

 by light wires attached by wax, ,and thus find the position of 

 the centre of gravity. 



The centre of gravity of a skeleton cube may be found in this 

 way to be the intersection of the diagonals. In a similar 

 manner, the centre of gravity of a right cylinder may be shown 

 to be the middle point of the axis. 



EXPT. 72. Find the centre of gravity of an open wicker- 

 work basket, such as a waste-paper basket. To do this, 

 suspend the basket, and hang a plumb-line from the point of 

 suspension. Tie a piece of thread across the basket in the 

 direction of the plumb-line ; then suspend the basket from 

 another point, and notice where the plumb-line crosses the 

 thread. The point of intersection is the centre of gravity. 



Equilibrium. When a body is at rest all the forces acting 

 upon it balance one another (or, what is the same thing, any 

 force is equal and opposite to the resultant of the remaining 

 forces) and it is said to be in equilibrium. It is in stable equi- 

 librium when any turning motion to which it is subjected raises 



