162 ./l^y. To'i07i's new mode of Bridge-huUding. 



sides of wood towers, steeples, &;c. &c. of public buildings, 

 as it requires nothing more than common planks instead of 

 long timber — being much cheaper, easier to raise, less sub- 

 ject to wet or dry-rot, and requiring no iron work. 



Some of the adv^antages of constructing bridges accord- 

 ing to this mode are the following : 



1. There is no pressure against abutments or piers, as 

 arched bridges have, and consequently perpendicular sup- 

 ports only, are necessary ; this saving in wide arches is very 

 great, sometimes equal to a third part of the whole expense 

 of the bridge. 



2. The shrinking of timber has little or no effect as the 

 strain upon each plank of the trusses, both of the braces 

 and string-pieces, is an end-grain strain or lengthwise of the 

 wood. 



3. Suitable timber can be easily procured and sawed at 

 common mills, as it requires no large or long timber — de- 

 fects in timber may be discovered, and wet and dry rot pre- 

 vented much more easily than could be in large timber. 



4. There is no iron work required, which at best is not 

 safe, especially in frosty weather. 



5. It has less motion than is common in bridges, and 

 which is so injurious and frequently fatal to bridges — and 

 being in a horizontal line, is much less operated upon by 

 winds. 



6. A level road-way is among the most important advan- 

 tages of this mode of construction. 



7. The side-trusses serve as a fi'ame to cover upon, and 

 thereby save any extra weight of timber, except the cover- 

 ing itself — and the importance and economy of covering 

 bridges from the weather, is too well understood to need re- 

 commendation after the experience which this country has 

 already bad. 



8. Draws for shipping to pass through, may with perfect 

 safety bo introduced in any part of the bridge, without 

 weakening it as in arched bridges, where the strength and 

 safetyof the arches depend so much on their pressure 

 against each other and abutments, that a draw, by destroy- 

 ing the connexion, weakens the whole superstructure. 



9. The great number of nearly equal parts or joints into 

 which the strain, occasioned by a great weight upon the 

 bridge, is divided, is a very important advantage over any 



