390 



We may, therefore, conceive that much greater advantage 

 would arise from such trussing than is pointed out by the 

 formula. 



There is much analogy between such considerations of 

 trussed beams, and the results of some experiments by M. 

 Duhamel on beams of soft wood, into the upper side of 

 which a cut with a saw having been made, a wedge of hard 

 wood was driven. 



But it would be vain to attempt to reduce these v#gue 

 ideas, or the effects derived from such varying causes, to any 

 formulae, without much previous investigation of the laws of 

 the stiffness of beams, and many experiments, conducted 

 with the greatest care, and of a very expensive nature, as it 

 would be necessary that they should be on a considerable 

 scale. The necessity of such experiments is now apparent 

 from the frequent use of cast iron girders, trussed with rods 

 of wrought iron. In regard to these, the results of the 

 inquiries consequent on the falling of the Dee Bridge shew 

 that substantial data are wanting, particularly as regards the 

 effects of temperature on such structures as are compounded 

 of cast and wrought iron. 



If I may throw out an opinion on this subject, I think that 

 trussing should not be resorted to for the purpose of giving 

 absolute strength in a weak structure, but that the girder 

 being made strong enough, trussing may be advantageously 

 used so as to confer stiffness. 



Perhaps the most important part of our subject is the 

 consideration, What is the proportionate load which may be 

 , borne with safety, in comparison with the weight which, 

 according to calculation, would break the structure ? It 

 would evidently be very unsafe to load a beam or a 

 pillar with a weight just, or nearly, sufficient to break 

 it, for then the slightest additional weight or load incau- 

 tiously added, the vibration produced by wind, or tremor 



