One remarkable thing was the fussy interest that their projects excited. During the eighteenth century, for instance, a ridiculous ado was made about bridge-building. Voluntary guidance came from mathematicians, and chatter and hesitation implied that at last, for the first time in the history of the world, a reputable bridge would be erected. As for the results of all this flutter and fuss, they were usually out of joint with the public interests that bridges ought to have served efficiently. No attention was paid to military defence, and some famous men blundered like amateurs. Perronet was regarded as the most expert bridge-builder of his time; his knowledge was prodigious, and yet he made astounding mistakes, which would have shamed such mediæval masters as Bénézet and Isembert. As an example I will mention his Pont de Neuilly-sur-Seine, which was finished in 1772.[137] The delicate operation of striking the centres, by freeing the arches from their supports, was begun only eighteen days after the keystones were put in their places, when the mortar was not yet hard enough to resist new pressure. In one great arch the crown sank twenty-three inches--truly a historic mishap, and for several reasons. The upper part of this arch in Perronet’s plan was an arc of a circle 320 ft. in diameter; after the mishap it became an arc of a circle whose diameter would be 518 ft., hence a stone arch of this size--518 ft. on the chord line--might be constructed! No wonder that writers have been astounded by the Pont de Neuilly-sur-Seine, for it passed safely through a most dangerous experience. Perronet was saved, not by his good design, nor by his mathematical calculations, but by a rare stroke of good luck. Indeed, there are a good many technical faults in his work at Neuilly. The piers are only fourteen feet broad, too small to be in scale with the wide arches, and all lateral pressure travels along the bridge to the abutments. If one arch were cut the others would be endangered. In later years Perronet became wise, and told the French Government that two or three arches in every long bridge should have abutment piers, as a safeguard against mishaps in war.[138] Several famous engineers had to learn by experience, like Perronet, that a self-conscious desire to be “scientific” had dangers of its own in bridge-building. Smeaton’s bridge over the Tyne at Hexham was a tragic failure; Labelye produced a very perishable bridge on the Thames at Westminster; and learned engineering did not save the Tay Bridge from catastrophe, though science welcomed it with a din of confident approval.