28 The Tay Bridge. 



if standing 5 yards to the leeward of an ordinary street-lamp. 

 Indeed, if the direction of the wind varied by 5° from a 

 line at right angles to the structure all the diagonals of the 

 lee girder would be fully exposed, and if at the same time 

 it varied a few degrees from the horizontal the exposure of 

 the whole of it would be practically effected. It will, I 

 think, be admitted that the case is favourably stated if the 

 diagonals and vertical members of the lee girder are 

 regarded as fully exposed, and the horizontal members fully 

 sheltered. Taking this view, we have 1960 square feet for 

 the windward, and 800 feet for the leeward girder, giving a 

 total of 2760 square feet. The effect of the wind on the 

 pier itself will not be large, the columns being circular, and 

 the flat braces in no case being presented at right angles. 

 Neglecting it, we shall considerably simplify the calculation, 

 and the small error introduced will be in favour of the 

 stability of the structure. 



The centre of pressure will be 96 feet above the base of 

 the columns, and the moment of overturning will be p X 

 2760 x 96 = 264,960p. Equating this with the moment of 

 stability previously determined, we have — 



4180 

 p = 264960 = * 0157 ton = 35 ' 2 lbs * 



This, then, is the critical wind pressure at which the 

 structure would overturn, assuming that there was no con- 

 nection other than simple contact between the iron columns 

 and the brick foundation, and that the pier itself was strong 

 enough to maintain its shape during such overturning. 



But these assumptions are not strictly correct ; the iron 

 pillars were anchored down by bolts to the foundation, so 

 that the pier could not overturn without carrying a part of 

 the foundation with it. Had these holding-down bolts gone 

 25 or 30 feet down so as to embrace several hundreds of 

 tons of brickwork, sufficient stability could have been 

 obtained. However, it is evident from the drawing that 

 such a proceeding was not contemplated, the bolts going down 

 only from 2 to 3 feet, and being small and imperfectly 

 secured. Allowing that they were capable of lifting all the 

 upper courses of the pier, the critical pressure, as previously 

 calculated, will be increased by about one-sixth, and will still 

 remain well within the range of actually recorded observa- 

 tions. 



The effect of the train upon the bridge is, from this point 



