《宋书·志·卷十八》
祖冲之制造的指南车机理非常精密灵巧，车子即使千百次拐弯千百次旋转，所指方向没有移动改变。
其制甚精，百屈千回，未尝移变。
The compass cart invented by Zu Chongzhi has a very precise and ingenious mechanism. Even if the cart makes a thousand turns or a hundred turns, the direction it points will not move or change.