But the new design had another merit, which probably weighed still more with the Bristol builders. The cardinal difficulty of the mediæval builders was how to keep up on the top of lofty clerestory walls a heavy stone vault which was always striving to push them asunder. They succeeded at length in keeping the clerestory walls from being thrust out by propping them up with flying-buttresses, perilously exposed, however, to all the vicissitudes of English weather. But there was another solution of the problem, which had been worked out in the thirteenth century in the Temple Church, and in the twelfth century in many a church in central and southern France, such as the old cathedral of Carcassonne. It was to stop the outward thrusts of the nave vault not by the inert resistance of buttress, pinnacle, and flying-buttress, but by bringing into play opposing thrusts—_i.e._, the inward thrusts of vaults built over the aisles. But to make these outward and inward thrusts balance and neutralise one another, the aisle vaults must be of pretty much the same height and span as those of the nave. The nave must be lowered or the aisles must be raised, or both. At Bristol the architect has preferred to raise the aisles. Then, the stability of the nave vault being secured, all the builder has to do is to stop the outward thrusts of the vaults of the aisles. This it is easy enough to do by a row of buttresses weighted with pinnacles. So Bristol Cathedral, to eyes accustomed to contemporary cathedrals, presents the strange solecism of having neither clerestory nor flying-buttresses. The whole design may well have been based on that of the English cathedral at Poitiers. [Illustration: VAULT OF AISLE.] The Bristol architect was an architectural Radical. When it came, however, to the question of vaulting the aisles he had not the courage of his convictions. Having constructed aisles unusually lofty and impressive, he immediately sets to work to make them look low and ordinary by a complicated, costly and unnecessary system of skeleton vaulting. He seems to have thought that without some method of reducing the apparent height of the aisles, the church would look like no other church; it would look not as if it had the ordinary aisled nave, but as if it had three naves side by side. He ought to have been thankful to have the means of making it look unlike other churches; and, as a matter of fact, a design of parallel naves has a most noble effect, as may well be seen in the nave of Warwick Church, built in the most debased Gothic of the seventeenth century, but one of the most impressive designs in the country, full of light and atmosphere.