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 wheels used sometimes to crack between the spokes at the boss, but these are not made now, though a considerable number are still in service. Broken or flawed tyres are replaced immediately, no attempt being made to repair them by welding. The chief defect in tyres is the ordinary wear in service, through which the “tread” on the rail decreases in diameter. The flange therefore becomes deeper relatively to the tread as shown by the dotted line B, Fig. 50, and when it becomes too deep it is liable to catch in crossings on the line and so become a source of danger. The tyres are not removed from the wheels, but the whole set is put into a wheel lathe and re-turned. The re-turning has also to be done when flat places occur on the tyres. Very powerful wheel lathes are required, since a number of exceedingly hard spots are always found on the treads of tyres that have been some time in service. An unavoidable but considerable waste of metal occurs when tyres are re-turned. In the first place