But a buttress acting by its strength is not of mass sufficient to resist the weight by mere inertia; but it conveys the weight through its body to something else which is so capable; as, for instance, a man leaning against a door with his hands, and propping himself against the ground, conveys the force which would open or close the door against him through his body to the ground. A buttress acting in this way must be of perfectly coherent materials, and so strong that though the weight to be borne could easily move it, it cannot break it: this kind of buttress may be called a conducting buttress. Practically, however, the two modes of action are always in some sort united. Again, the weight to be borne may either act generally on the whole wall surface, or with excessive energy on particular points: when it acts on the whole wall surface, the whole wall is generally supported; and the arrangement becomes a continuous rampart, as a dyke, or bank of reservoir. § VI. It is, however, very seldom that lateral force in architecture is equally distributed. In most cases the weight of the roof, or the force of any lateral thrust, are more or less confined to certain points and directions. In an early state of architectural science this definiteness of direction is not yet clear, and it is met by uncertain application of mass or strength in the buttress, sometimes by mere thickening of the wall into square piers, which are partly piers, partly buttresses, as in Norman keeps and towers. But as science advances, the weight to be borne is designedly and decisively thrown upon certain points; the direction and degree of the forces which are then received are exactly calculated, and met by conducting buttresses of the smallest possible dimensions; themselves, in their turn, supported by vertical buttresses acting by weight, and these perhaps, in their turn, by another set of conducting buttresses: so that, in the best examples of such arrangements, the weight to be borne may be considered as the shock of an electric fluid, which, by a hundred different rods and channels, is divided and carried away into the ground.