194 VEGETABLE SUBSTANCES. 



a large span this is one of the happiest inventions for 

 a wooden bridge, since the descending catenary has 

 much less vibration than a rising arch, and since any 

 one piece, as it decayed, may be removed and re- 

 placed, without injury to the structure. 



Some wooden bridges of very superior construc- 

 tion have been erected in Scotland, by Mr. James 

 Burn, of Haddington: one of these is across the river 

 Don, about seven miles from the city of Aberdeen, 

 and on the road leading to Banff. This bridge con- 

 sists of a single arch of nearly one hundred and 

 ten feet span, and something- more than thirty feet 

 rise. The supporting part of it is formed of a 

 series of frames, each set being made double, or at 

 about four inches from each other, and of short 

 lengths of timber, so that any piece of a frame, or 

 even a whole frame, may be removed, when repairs 

 are necessary, without endangering the bridge or 

 interrupting the roadway. They are kept from lateral 

 vibration, by transverse struts and ties under the 

 planking of the road, and the sui-face is finisiied with 

 gravel. Another bridge by the same artist, and 

 across the same river, at a shorter distance from 

 Aberdeen, consists of two arches, each nearly seventy- 

 two feet in span, and having a rise of only ten feet 

 and a half. IVIr. Burn has also constructed a very 

 handsome wooden bridge across the South Esk, 

 in the park at Brechin castle ; it consists of three 

 arches, the middle one thirty-eight feet span, with a 

 rise of only ten feet. This bridge has the spandrils 

 boarded and drawn out, and sanded in imitation of 

 stone, so that, until closely examined, it has the ap- 

 pearance of a stone bridge of a ve y light and hand- 

 some construction. Those bridges by Mr. Burn 

 possess a peculiar advantage, in the shortness of the 

 timber of which the framing is composed. This 

 gi\es them greater stiffness with a less scantling; 



