One of the most beautiful features of French vaulting, almost entirely unknown in this country, is the great polygonal vault of the semi-dome of the chevet, which as an architectural object few will be disinclined to admit is, with its walls of painted glass and its light constructive roof, a far more beautiful thing than the plain semi-dome of the basilican apse, notwithstanding its mosaics. Still, as the French used it, they never quite surmounted the difficulties of its construction; and in their excessive desire to do away with all solid wall, and to get the greatest possible surface for painted glass, they often distorted these vaults in a very unpleasing manner. The chevet of Pontigny (Woodcut No. 643) presents a good example of the early form of vault, which owing to the small size of the windows and general sobriety of the composition, avoids the defects above alluded to. Of the later examples there are few, except that of Souvigny, represented in Woodcut No. 661, where the difficulty has been entirely conquered by constructing the spandrils with pierced tracery, so that the vault virtually springs from nearly the same height as the arch of the windows, and a very slight improvement would have made this not only constructively, but artistically perfect. This is a solitary specimen, and one which, though among the most beautiful suggestions of Gothic art, has found no admirers, or at least no imitators. Notwithstanding this difficulty of construction, these pierced semi-domes are not only the best specimens of French vaulting, but are among the most beautiful inventions of the Middle Ages, and form a finer termination to the cathedral vista than either the great windows of the English, or the wonderful rose windows of the French cathedrals. BUTTRESSES.