4 MEASUREMENT OF SOLIDS. 



is thus seen to be half the product of one of its sides, 

 a b, into the perpendicular distance, e c, of the opposite 



2x4 

 vertex. The area of a b c (fig. 3) is -^ = 4 square 



2 



centimetres. 



The area of a circle is found by multiplying its 

 radius, that is, the distance of the centre from the cir- 

 cumference, once by itself and by the number 3'1416. 

 This number, which expresses the length of the circum- 

 ference, if the diameter is equal to 1, is frequently 

 denoted by the symbol ?r; its value is more precisely 



22 

 3-1415926, or approximately _ . The area of a circle 



of 6 cm diameter, or 3 cm radius, is 3 x 3 x 3-1416 = 28-2744 

 square centimetres. 



The unit for the measurement of the solid contents 

 or volumes of bodies is a cube, that is, a space bounded 



Fio. 4 (real size}. 



by six equilateral rectangular faces, each equal to the 

 unit of surface, and each of whose edges is equal to the 

 unit of length. Thus we have cubic metres, cubic 

 decimetres, etc. The cubic centimetre especially is of 



