Procesdings of the Polytechnic Association. ]029 

 March 26th, 1869. 



Professor S. D. Tillman in the chair ; Mr. C. E. Emert, Secretary. 

 SusPENSioif Beidges. 



The chairman exhibited a small section of the wire ropes forming 

 the cable of the new suspension bridge at Niagara Falls. The speci- 

 men had been presented to the Association by the Hon. William J. 

 McAl})ine. The bridge has the longest span of any suspension 

 bridge yet constructed. The cable consisted of forty-nine strands of 

 the rope, like the specimen shown. 



Dr. D. D. Parmelee remarked that the bridge has now been open 

 for three months, and answers, admirably the pur])ose for which it 

 was erected. A curious fact connected with this bridge is, that the 

 ordinary difference of temperature occurring during our seasons, 

 causes an elevation or depression of nearly three feet in the roadway 

 at its middle. That is to say, during cold weather, the roadway is 

 three feet higher than it is during very warm weather. 



Mr, J. K. T'isher said that iron wire is better than steel for such 

 purposes. Iron is more ductile than steel. The most important 

 property of the wire is its elasticity, whereby the wires expand, and 

 thus every one of the wires is made to bear its share of the strain. 

 Iron will stretcli and steel will break, in the contraction of a bridge, 

 from the effects of cold. If twisted too much, the cable will not be 

 so strong. 



It was stated that some cables have straight central wires, and the 

 outer layers of the wires are laid spirally around them. 



Dr. J. J. Edwards remarked that this sj'stem would not be so 

 good, the twist gives elasticity. 



Letj^ee and Envelop made of Sheet Ieon. 



The chairman exhibited a letter he had receiv'ed from Mr. Norman 

 Wiard, which was written on sheet-iron, and inclosed in a sheet-iron 

 envelop. It was made in Pittsburg. Similar letters have been 

 exhibited before the Institute on former occasions. Careful measure- 

 ment with a reliable micrometer, shows the thickness of the sheet- 

 iron composing this letter to be -^\s of an inch. 



Dr. Vanderweyde said a book, of which the leaves were sheet iron, 

 was exhibited at the exhibition in London. It contained 300 leaves, 

 and was two inches thick, showing that the English sheets were 

 much thicker than the American. It was printed, and the printing 



