[Illustration: Figure 4.--The proposed 1,000-foot iron tower designed by Clarke, Reeves & Co. for the Centennial Exhibition of 1876 at Philadelphia. (From _Scientific American_, Jan. 24, 1874, vol. 30, p. 47.)] That this theory was successful not only practically, but visually, is evident from the resulting work. The curve of the legs and the openings beneath the two lower platforms are primarily responsible for the Tower's graceful beauty as well as for its structural soundness. The design of the Tower was not actually the work of Eiffel himself but of two of his chief engineers, Emile Nouguier (1840-?) and Maurice Koechlin (1856-1946)--the men who had conducted the high pier studies--and the architect Stéphen Sauvestre (1847-?). In the planning of the foundations, extreme care was used to ensure adequate footing, but in spite of the Tower's light weight in proportion to its bulk, and the low earth pressure it exerted, uneven pier settlement with resultant leaning of the Tower was considered a dangerous possibility.[2] To compensate for this eventuality, a device was used whose ingenious directness justifies a brief description. In the base of each of the 16 columns forming the four main legs was incorporated an opening into which an 800-ton hydraulic press could be placed, capable of raising the member slightly. A thin steel shim could then be inserted to make the necessary correction (fig. 5). The system was used only during construction to overcome minor erection discrepancies. In order to appreciate fully the problem which confronted the Tower's designers and sponsors when they turned to the problem of making its observation areas accessible to the fair's visitors, it is first necessary to investigate briefly the contemporary state of elevator art. Elevator Development before the Tower While power-driven hoists and elevators in many forms had been used since the early years of the 19th century, the ever-present possibility of breakage of the hoisting rope restricted their use almost entirely to the handling of goods in mills and warehouses.[3] Not until the invention of a device which would positively prevent this was there much basis for work on other elements of the system. The first workable mechanism to prevent the car from dropping to the bottom of the hoistway in event of rope failure was the product of Elisha G. Otis (1811-1861), a mechanic of Yonkers, New York. The invention was made more or less as a matter of course along with the other machinery for a new mattress factory of which Otis was master mechanic. [Illustration: Figure 5.--Correcting erection discrepancies by raising pier member--with hydraulic press and hand pump--and inserting shims. (From _La Nature_, Feb. 18, 1888, vol. 16, p. 184.)] [Illustration: Figure 6.--The promenade beneath the Eiffel Tower, 1889. (From _La Nature_, Nov. 30, 1889, vol. 17, p. 425.)]