

>ruM OF A PYRAMID. 



this plane cuts every x to 



use in parts having the 8a of 3 to 1 : and i 



fore tin- trianpilar pyram: :hrir centres of gravity in 



this plane, and therefore the wli<>K' p\ ramid lias its c 

 of gravity in this plane. 



iin, join the with the centre of gravity of the 



base; very section parallel to the l>ase will be similar 



-e, and i: th- pyramid divided into an 



number of indefinitely thin slices i 

 base, the centre of gravity of each slice will lie 

 on tl joining the vertex with I :re of 



base, llrnrv the whole pyramid ntre 



..vity in this straight line. 



Therefore the centre of gravity is one-fourth of the way up 

 the straight line joining the centre of gravity of 



with the vertex. 



(6) To find tie centre of gravity of the frustum of a pyra- 

 tltrmca ly parallel planes. 



Let ABCalc be the frustum; 

 ntres of gravity of 

 the pyramids DABC, Dale] it 

 is clear that the centre of gra 



.c frustum must be ingG pro- 

 duced; suppose it at G'. 



Let Ff=c, AB=a, al=l. 



L6 whole pyramid ])ABC 

 aade up of the frustum and 

 the small pyramid, therefore, 



11 pyramid 

 urn 



v. 



pyr. vol. of small pyx. 



