to length of span and method of construction, and not to the possible suspension bridge of the future.
In regard to the remainder of Mr. Lindenthal's letter: A bridge to be stable and rigid, in the engineering meaning of the words, must be so designed that under any probable form of loading no change of form can take place, either in the bridge as a whole or any of its parts, other than that due to the elasticity of the material used.
The suspension bridge, as we know it, consists of a flexible chain or cable from which the roadway is hung: given a sufficiently heavy moving load relative to the dead weight of the bridge, and the form of the curve assumed by this chain or cable will change with each change in the position of the load, and the bridge can not be called stable.
The mere fact that the inverted arch is in stable equilibrium while the upright arch is not, has absolutely no bearing upon this question, when we consider the form of the materials that are used in each case. I admit that, if the steel arches of the St. Louis Bridge were inverted and braced and counterbraced in a manner similar to that made use of at present, the bridge would be as stable, etc., as the present bridge; but certainly not if the vertical and lateral bracing were dispensed with, and simply a chain substituted for the present compression arch.
It is, however, impossible to state the relative merits of different bridge designs without taking into account the length of span; and to a great extent the question is decided by the relation that exists between the dead load, consisting of weight of the bridge, and the live load, consisting of the passing train, etc. The following may be taken as the maximum economic lengths for railway bridges of iron or steel:
|Pin-connected with parallel chords or arched top chord||550||"|
When the suspension bridge reaches such a size that the weight of any probable load that may come upon it is nothing as compared to its own weight — as would be the case in the proposed suspension bridge extending from New York to Jersey City, designed by Mr. Lindenthal, with a central span of 2,850 feet — then much that has been said here in regard to the instability of suspension bridges will not apply, owing to the fact that under no circumstances would it be possible to so load a bridge of such dimensions that the load would bear even an appreciable value to its own weight.
One word more in regard to "the popular and fashionable misconception as to the merits of the cantilever bridge." Its greater deflection is due simply to the fact that it is in the form of a girder fastened at one end and strained over a pier, and does not amount to a demerit in the principle.
If I understand Mr. Lindenthal's use of the expression "all other things being equal," etc., correctly to mean vertical and lateral bracing, etc., practically everything never could be equal; and the cantilever bridge, within the limits of span given, is in every way superior to the suspension bridge as a modern railway bridge.
|Charles Davis Jameson.|
|Iowa City, April 3, 1890.|
WE print in our present number a letter from President David J. Hill, of the Baptist University of Rochester, in which an attempt is made to justify the statement contained in his book on the Social Influence of Christianity, and reproduced, with the sanction of Bishop Vincent, in The Chautauquan, that Mr. Spencer "advises us to follow our own self-interest, without concern for others, with the assurance that all will thus be happier, because more independent." If such a statement could be justified, President Hill would doubtless be highly qualified to perform the task. He has a high respect, he tells us, for Mr. Spencer's "ability as a thinker and sincerity as a man"; and we may presume that this high opinion has not been formed without adequate study of Mr. Spencer's works — such a study as would give ample command of illustrative passages in such a discussion as the present. The question now is: Has President Hill proved his case? Has he justified the damaging remark made by him in regard to Mr. Spencer's ethical system? We venture to say that he has not done so, but, on the contrary, has signally failed in his attempt. The reason is, that the facts are against him. Whether he is fully aware how much