Mr. Town's new mode of Bridge-building, 165 



tion : and for this purpose it must be observed that a wood 

 arch may be formed of a much greater length or span than 

 it is possible to erect one of stone ; hence wooden bridges 

 are applicable to many situations where accumulated waters 

 bearing down trees and fields of ice, would tear a bridge of 

 stone from its foundation. 



It therefore becomes of importance to render bridges 

 of wood as permanent as the nature of the material will 

 admit. 



Hitherto, in bridges not covered from the weather, the 

 immense quantity of mortices and tenons, which, however 

 well done, will admit air and wet, and consequently tend to 

 expedite the decay of the weak parts, has been a material 

 error in constructing bridges of wood. 



But to render wood bridges of much more importance 

 than they have hitherto been considered, first from their 

 extensive span ; secondly from their durability ; two things 

 must be considered, first that the wood works should stand 

 clear of the stream in every part, by which it never would 

 have any other weight to sustain than that of the usual car- 

 viages, secondly that it will be so combined as to exclude as 

 much as possible the air and rain. 



When the true principle of building bridges of wood is 

 discovered, their progressive extension is as reasonable as 

 the increased dimensions of shipping ; which, ui early ages, 

 was deemed a great work if they amounted to one hundred 

 ions burthen ; but time and experience have extended the 

 art of ship-building to two thousand tons, and in the com- 

 bination and arrangement of the various and complicated 

 parts, there certainly is more genius and labour required 

 than in erecting a bridge of five hundred or one thousand 

 feet span : but the great demand for shipping has rendered 

 their formation familiar, and their increased bulk has gradual- 

 ly grown upon our senses. But had a man, in the infancy of 

 naval architecture, hinted at a vessel of two thousand tons, 

 I am inclined to think his coteraporary artists would have 

 branded him as a madman. 



Kote. 



Those who wish to purchase rights, and to Obtain particular 

 directions for building bridges according to this improve- 



