84 ELEMENTARY GENERAL SCIENCE CHAP. 



of the chalk marks ; each will be found to set itself in a 

 horizontal position. If the plates are of cardboard, a better 

 plan is to make a pin-hole at the centre of gravity and pass a 

 thread knotted at one end through it. The plate can then be 

 held up and will be found to set horizontally. 



EXPT. 68. Determine experimentally the centre of gravity 

 of a plate in the form of a parallelogram. Mark the point 

 with a pencil ; then turn over the plate and draw the two 

 diagonals upon the opposite side. Make a pin-hole where the 

 diagonals intersect. The point where the diagonals intersect 

 will be found to be practically the same as the centre of gravity. 



Centre of Gravity of a Triangular Plate. EXPT. 69. 

 Repeat the preceding experiment, with a triangle cut out of 

 cardboard. After finding the centre of gravity, prick a pin- 

 hole through the cardboard, then turn over the triangle and 

 draw a line from each angle through the pin-hole to the 

 opposite edge. Now, taking each edge in turn as the base of 

 the triangle, determine () the lengths of the two parts into 

 which each base is divided by the lines drawn, (/>) the propor- 

 tion which the distance of the hole from each base bears to 

 the length of the line from that base to the opposite angle. 



It will be found as the result of this experiment that the line 

 from each angle to the base divides the base into two equal 



ABC 



FIG. 30. Geometrical illustration of Centre of Gravity of a Triangular Plate. 



parts, and also that the distance of the hole from the base is 

 one-third the whole length of the line. We may, in fact, con- 

 sider a triangular plate as made up of a number of narrow strips 

 of material which decreases in length from the base to the apex. 

 The centre of gravity of each strip is the middle of the strip ; 

 hence the line drawn from the apex to the middle of the base 

 passes through each centre of gravity. By taking another side 



