The great confidence of the Tower's builder in his own engineering ability can be fully appreciated, however, only when notice is taken of one exceptional way in which the project differed from works of earlier periods as well as from contemporary ones. In almost every case, these other works had evolved, in a natural and progressive way, from a fundamental concept firmly based upon precedent. This was true of such notable structures of the time as the Brooklyn Bridge and, to a lesser extent, the Forth Bridge. For the design of his tower, there was virtually no experience in structural history from which Eiffel could draw other than a series of high piers that his own firm had designed earlier for railway bridges. It was these designs that led Eiffel to consider the practicality of iron structures of extreme height. [Illustration: Figure 1.--The Eiffel Tower at the time of the Universal Exposition of 1889 at Paris. (From _La Nature_, June 29, 1889, vol. 17, p. 73.)] [Illustration: Figure 2.--Gustave Eiffel (1832-1923). (From Gustave Eiffel, _La Tour de Trois Cents Mètres_, Paris, 1900, frontispiece.)] There was, it is true, some inspiration to be found in the paper projects of several earlier designers--themselves inspired by that compulsion which throughout history seems to have driven men to attempt the erection of magnificently high structures. One such inspiration was a proposal made in 1832 by the celebrated but eccentric Welsh engineer Richard Trevithick to erect a 1,000-foot, conical, cast-iron tower (fig. 3) to celebrate the passing of the Reform Bill. Of particular interest in light of the present discussion was Trevithick's plan to raise visitors to the summit on a piston, driven upward within the structure's hollow central tube by compressed air. It probably is fortunate for Trevithick's reputation that his plan died shortly after this and the project was forgotten. One project of genuine promise was a tower proposed by the eminent American engineering firm of Clarke, Reeves & Company to be erected at the Centennial Exhibition at Philadelphia in 1876. At the time, this firm was perhaps the leading designer and erector of iron structures in the United States, having executed such works as the Girard Avenue Bridge over the Schuylkill at Fairmount Park, and most of New York's early elevated railway system. The company's proposal (fig. 4) for a 1,000-foot shaft of wrought-iron columns braced by a continuous web of diagonals was based upon sound theoretical knowledge and practical experience. Nevertheless, the natural hesitation that the fair's sponsors apparently felt in the face of so heroic a scheme could not be overcome, and this project also remained a vision. Preparatory Work for the Tower