in Regard to Wind Pressures. 43 



withstand a pressure of 50 or 60 lbs. to the square foot, 

 although it would be so in the case of a large factory 

 chimney. The failure of the latter would not only be a 

 great loss of valuable property, but would, in all probability, 

 occasion stoppage to works, damage to adjacent property, 

 and possibly loss of life. 



The structure, however, in which every condition is 

 present, demanding the utmost degree of safety, is that of a 

 large railway viaduct. In this the full maximum for the 

 locality should be adopted without limitation of any kind. 



Having now considered the manner of deciding what 

 wind pressure to adopt as the maximum to be provided for 

 in any proposed structure, I will proceed briefly to examine 

 the means of making this provision. All structures in this 

 connection may be placed in three classes, of which the 

 following are types — viz. — 



1. A factory chimney. 



2. A roof 



8. A girder bridge. 



1. A factory chimney. The stresses here will be those 

 of a beam fixed at one end and loaded uniformly. The 

 resistance will be due to the weight of the material multi- 

 plied by half the diameter of the building. 



It is convenient in building structures of this kind to 

 carry up the walls in lengths of different thickness, 

 diminishing by half a brick at each change. The weakest 

 points will then be the joints at which these changes take 

 place. To these points only is it necessary to give our 

 attention. 



In a brickwork chimney, where the resistance is obtained 

 from the material itself, and not from a comparatively 

 trifling and independent system, as in the case of a girder 

 bridge, it will be undoubtedly of the utmost importance 

 that the material be disposed of with the greatest 

 economy, and duly proportioned to meet the stresses 

 it will have to bear. If the whole chimney, except 

 one joint, will stand a pressure of 60 lbs. per foot, but 

 that joint only 45 lbs., then every bit of the material 

 going to strengthen that chimney beyond 45 lbs. is 

 absolutely wasted. 



The largest chimney, and the fourth highest bailding in 

 the world, is Tennant's, in Glasgow. It rises 435 feet above 

 the ground, and it might have been expected that the 

 design would have been beyond criticism. The following 



