194 Transaction's of the American Institute. 



was a girder, merely using suspension chains instead hi scaffolding. 

 Our own engineer, Koebling, the Pontifix Maximus of modern engi- 

 neering, has j)ractically demonstrated that these two principles may 

 be usefully, safely and economically combined, and has applied them 

 to spans of more than twice those of the Britannia bridge, and is also 

 prepared to undertake those of four times that span. Unprofessional 

 persons will better understand the difRcnlty of constructing bridges 

 of long span when it is stated that the strains increase with the length 

 of the spans. That is, that a bridge of 200 feet must be twice as 

 strong as one of 100 feet ; that the ^Niagara bridge is siibjected to 

 strains twice as gi*eat as those of the Britannia ; and that the proposed 

 bridge from ISTew York to Brooklyn must be twice as strong as that 

 at Niagara. Having determined upon the girder, Mr. Stephenson 

 next proceeded to consider its proportions. A green willow wand, if 

 laid upon two supports, not too far apart, will show the bark with a 

 smooth surface on all sides. J^ow, if a weight be suspended from the 

 middle, and the wand bent downward, it will be noticed that the 

 bark upon the upper side is wrinkled up, and on the lower side that 

 it is stretched out. The first is the result of compression, and the 

 second that of tension. That is, all of the fibers of the bark on the 

 upper side are forced together, and those on the lower side are drawn 

 out. It will also be observed that about midw^ay in the depth the 

 bark remains smooth, not having been aftected by either compression 

 or tension. This is called the neutral axis, and this part of the stick 

 is not subjected to any strain except to hold tlie upper and lower 

 parts together. If a hole is bored out of the middle of the stick, it 

 will be found that so far from weakening, it has given it the power 

 to sustain as much more weight as that removed. Keasoning in this 

 manner, the engineer determined to make experiments to ascertain 

 how much of the interior he could remove with advantage. His first 

 trial was with a cylindrical hollow beam of wa'ought iron, heavily 

 plated on the top and bottom ; and as these yielded first, he kept on 

 strengthening them. Meanwhile he had gradually changed the form 

 of his tube, from circular to elliptic, and finally to a rectangular 

 shape, which his experiments determined as the best fo9'm. He spent 

 a year or more in these experiments, aided by Fairbairn, one of the 

 best mechanicians of the day, and Hodgkinson, a distinguished sci- 

 entist, by means of which the best form and proportions of the 

 difierent parts of the girder were determined from a model of forty 

 feet length. Other experiments were made to determine the strength 



