To make it even structurally possible, they had to invent super-strong concrete, and to control weight, varied the density of the aggregate as they worked their way up the dome.
仅仅要让它的结构可行， 他们不得不创造超强的混凝土， 还得控制重量， 这是通过在搭建圆顶的过程中 不断改变混合物的密度而达到的。