Mr D. Stevenson on Long's American Frame Bridge. 297 



dence which they do not deserve ; but they have appeared to 

 me sufficiently curious to be noticed. At all events, the in- 

 crease of the Spitzbergen glaciers cannot be unlimited, for it 

 will be always checked by the sea, which continually swal- 

 lows them up in proportion as they advance into it by de- 

 scending from the sides of the mountains. 



Notice relative to Long's American Frame-Bridge. By David 

 Stevenson, Civil Engineer, Edinburgh. With a Plate. 

 Communicated by the Society of Arts for Scotland.* 



The American bridges, as is generally known, are construct- 

 ed almost entirely of timber, a circumstance which is easily ac- 

 counted for by the abundance of wood and the scarcity of good 

 materials for building bridges of stone, both in the United States 

 and Canada. But even had good building materials been 

 abundant, the rivers, lakes, and arms of the sea, spanned by 

 the American bridges, are so extensive that the consumption 

 of time and money which would necessarily have attended the 

 building of stone-bridges, in such situations, would almost, in 

 every case, it is to be feared, have proved too great to warrant 

 their erection. Under these circumstances, as is natural to 

 suppose, the erection of timber-bridges has occupied much of 

 the attention of the American engineers ; and the finest spe- 

 cimens of timber-bridge architecture that I have met with are 

 to be found in that country. 



To give a single instance of the extent to which bridge- 

 building is carried in America, I may quote the dimensions of 

 the Columbia Bridge over the River Susquehanna in Pennsyl- 

 vania, which I visited in 1837. This magnificent work con- 

 sists of no less than 29 arches of 200 feet span, and is support- 

 ed on two abutments and 28 piers of rubble masonry, which 

 are founded on rock at an average depth of G feet below the 



* Read before the Society of Arts for Scotland, 15th May J 839 ; when it 

 was remarked by Mr Burn, the architect, that the principle on which the 

 framing of this bridge is constructed might be applied with great advantage 

 to many other purposes than bridges, whore great lightness, combined with 

 strength and rigidity, are required.— J. T. See. 



