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 o 
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. 
to render wood bridges of much more importance 
than they have hitherto been considered, first from: their 
extensive span ; secondly from their durability ; two thin 
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- 
riages, 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 
naval architecture, hinted at a vessel of two thousand tons, 
branded him as a madman, 
uate Pipe. A : ; Te a A oe 
pe ES ee Rie mee 
- Those who wish to purchase rights, ny to obtain particular cular 
directions for building bridges according to this improve- 
