110 



BODIES SUPPORTED IN TWO POINTS. 



two lines intersect. According as the point of intersectioi 

 is above, at, or below the centre of gravity, the equi- 

 librium of the body is stable, neutral, or unstable. 



Experiments on this mode of support may be mad< 

 with a hollow cube of pasteboard, which has its centi 

 of gravity in the middle, that is, at the point of inters< 

 tion of the three straight lines joining the centres of th< 

 three pairs of opposite faces. In fig. 78 it is s. At th< 

 points where the cube is to be supported holes are pierce< 

 with the bradawl and a knitting needle passed through 

 them, which is held at both ends by the hand or fixed 

 with one end in the fork of the retort-stand. In fig. 78, 

 the points of support are always marked b and c, and 

 the point of intersection of the line of support and Hne 

 of gravitation is marked a ; A, B, C are respectively the 

 positions of stable, unstable, and neutral equilibrium. 

 The line of support, represented by the knitting-needle, 

 need not be horizontal, but may also be placed in other 

 positions. 



A cube for these experiments may be made of stout drawing-paper 

 or thin, smooth cardboard. Fig. 79 shows the shape into which 

 the cardboard should be cut for forming the six faces of the 

 cube. Along the lines round the square a, and also along the line 

 between e and /, the cardboard is cut half through with a knife, 

 the whole is then bent into the shape of the cube, and thin strips 



of paper pasted along the edges. 

 In pasting together hollow bodies, 

 the operation is facilitated by leav- 

 ing such flaps as those indicated 

 in the figure ; but in this case the 

 flaps would interfere with the posi- 

 tion of the centre of "gravity, and 

 the cube should be constructed with- 

 out making use of the flaps. If glue 

 FIG. 79 (^ real size). i s used i ns t ea d of paste, it should be 



broken with a hammer or cut with a pair of scissors, covered with 



f 



