EDITOR'S TABLE. 



267 



to length of span and method of construc- 

 tion, and not to the possible suspension 

 bridge of the future. 



In regard to the remainder of Mr. Lin- 

 denthal'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 suffi- 

 ciently 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 ad- 

 mit that, if the steel arches of the St. Louis 

 Bridge were inverted and braced and coun- 

 terbraced in a manner similar to that made 

 use of at present, the bridge would be as 

 stable, etc., as the present bridge ; but cer- 

 tainly not- if the vertical and lateral brac- 

 ing 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 desi<nis 

 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 : 



Plate girders 50 feet. 



Eiveted lattice 350 " 



Pin -connected with parallel chords or 



arched top chord 550 " 



Cantilever 1,750 " 



Suspension, over 1,750 " 



When the suspension bridge reaches such 

 a size that the weight of any probable load 

 that may come upon it is nothing as com- 

 pared to its own weight — as would be the 

 case in the proposed suspension bridge ex- 

 tending from New York to Jersey City, de- 

 signed by Mr. Lindenthal, with a central 

 span of 2,850 feet— then much that has been 

 said here in regard to the instability of sus- 

 pension bridges will not apply, owing to the 

 fact that under no circumstances would it be 

 possible to so load a bridge of such dimen- 

 sions that the load would bear even an ap- 

 preciable value to its own weight. 



One word more in regard to " the popu- 

 lar 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. Respectfully, 



Charles Davis Jameson. 

 Iowa City, April 3, 1S90. 



EDITOR'S TABLE. 



DOES MR. SPENCER INCULCATE SELF- 

 ISHNESS? 



VE print in our present number a 

 letter from President David J. 

 Hill, of the Baptist University of Roch- 

 ester, in which an attempt is made to 

 justify the statement contained in his 

 book on the Social Influence of Chris- 

 tianity, and reproduced, with the sanc- 

 tion of Bishop Vincent, in The Chautau- 

 quan, 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 illus- 

 trative 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 



