[Illustration: Figure 13.--THE FOUR BOLLMAN SPANS at Harpers Ferry that survived the Civil War. The spans were completed in 1862-1863. (_Photo courtesy of Baltimore and Ohio Railroad._)] It apparently occurred to Bollman that by extending the number of rods in a longitudinal direction, this effect could be practically amplified to such an extent as to be capable of spanning considerable distances. He almost certainly did not at first contemplate an all-iron system, but rather a composite one such as described. It is entirely likely that such trussed beams, with multiple systems of tension rods, were used by Bollman as bridging in temporary trestlework along the line as early as 1845 (see fig. 4). It is impossible to say whether Bollman himself, or Latrobe, was struck with the logic of further elaborating upon the system and, simultaneously, translating the timber compression member into one of cast iron. Cast iron would naturally have been selected for a member that resisted a compressive stress, as it was considerably cheaper than wrought iron. But more important, at that time wrought iron was not available in shapes of sufficient sectional area to resist the appreciable buckling stresses induced in long compression members. The cost of building up members to sufficient size from the very limited selection of small shapes then rolled would have been prohibitive. The trussing rods, subjected to tension, were of wrought iron inasmuch as the sectional area had only to be sufficient to resist the primary axial stress. The first all-iron Bollman truss was constructed over the Little Patuxent River at Savage Factory, near Laurel, Maryland, in 1850. In the chief engineer's report for the year 1850, Latrobe was able to state that the truss had been completed and was giving "much satisfaction." He went on at some length to praise the "valuable mechanical features" embodied therein, and expressed great confidence that iron would become as important a material in the field of civil engineering as it was in mechanical engineering. [Illustration: Figure 14.--THE HARPERS FERRY BRIDGE as completed after the Civil War. It was used by the Baltimore and Ohio until 1894, and as a highway bridge until 1936. (Photo 690, Baltimore and Ohio Collection, Museum of History and Technology.)] The cost of this first major Bollman bridge was $23,825.00. Its span was 76 feet. Latrobe's confidence was well placed. The Savage span and another at Bladensburg may be considered successful pilot models, for, in spite of a certain undercurrent of mistrust of iron bridges within the engineering profession--due mainly to a number of failures of improperly designed spans--Latrobe felt there was sufficient justification for the unqualified adoption of iron in all subsequent major bridge structures on the B. & O.