(Fig. 40). The pier buttress, a, which is incorporated with that portion of the internal pier upon which the vault thrusts are gathered, is a plain, square-edged mass of masonry reinforced by a flying buttress, b, which springs from the great outer buttress, c, the one inert member of the structure. The flying buttress is a square-edged rampant arch loaded with masonry, whose upper surface is brought into a right line which slopes a little less steeply than the chord of the arc, and is covered by a plain flat coping. The very massive lower buttress, c, is adjusted to the flying buttress, b, by a simple set-off, d, which penetrates the roof of the aisle, and is carried on a substructure of masonry over the transverse arch of the aisle, abutting against the pier at the springing of the vaults. These vaults are thus effectually propped both at their haunches and at their springing, but the whole construction is somewhat needlessly heavy.
Flying buttresses of lighter construction occur in the apse of St. Leu d'Esserent (Fig. 41), which dates from the last quarter of the twelfth century, or perhaps a little earlier. The pier buttress does not, in this system, rise above the abutting arch—the semicircular wall of the apse presenting an unbroken surface above this level, while below this point it is of the same form as that of St. Martin of Laon. The lower or outer buttress has three set-offs and rises to a considerable height above the roof of the aisle before the arch, whose intrados is set even with its inner face, springs. The back of the flying buttress is no longer a continuous sloping right line extending to the outer face of the system, but it is met by a level surface in which the outer portion of the buttress terminates, and the whole is covered, as at St. Martin, by a flat coping. In this case no portion of a lower prop is visible above the roof, though there probably is one beneath it.
Some improvements upon these forms are shown in the buttresses (Fig. 42) of the nave of the same building, which date from the end of the twelfth, or the beginning of the thirteenth century. The buttresses of the apse, by the number and depth of their set-offs, indicate that the builder considered their efficacy to depend upon a far -projecting foundation from which, by an inclined outline, somewhat of an oblique action, from the ground upwards, should be
