442 REPORT— 1891. 



which may be expected froui similar axles under the conditions 

 existing in ordinary railway practice. The French make use 

 of the drop-test not only for axles, but for small samples of the 

 materials used in the construction of guns. 



In general it may be stated that the conditions under which 

 a given material is tested should conform as nearly as possible 

 with those existing in the structure, machine, rail, tire, axle, 

 or gun of which it forms a part. 



It is rarely necessary to test wrought-iron and steel in com- 

 pression or torsion in order to determine its suitability for 

 constructive purposes, as the tensile tests generally show all 

 that is necessary with regard to the quality of the materials. 



The tensile strength of cast-iron varies from 7 tons to 10 

 tons per square inch, its modulus of tensile elasticity from 

 10,000,0001b. to 15,000,0001b. per square inch, while the com- 

 pressive strength varies from 30 tons to 40 tons per square inch. 

 It is usual to test sample-bars of cast-iron as beams loaded in 

 the centre and supported at each end. The span is usually 

 36in., and the cross-section of the test-bar lin. wide by 2in. 

 deep. The breaking central vv-eight varies from 24cwt. to SOcwt. 



Working -STRESS and Factor of Safety. 



Working-stress signifies the intensity of stress — generally 

 expressed in tons per square inch — to which a piece of material 

 may be subjected without ceasing to fulfil its purpose efficiently 

 under the conditions on which the stresses are applied. 



The factor of safety is the ratio of the ultimate strength to 

 the working-load. Sir William Fairbairn proved that a riveted 

 girder loaded to one-third of the load which would have broken it 

 if applied gradually, failed after 313,000 applications of tliis load. 



The experiments of Herr Wohler and Professor Spangen- 

 berg demonstrate the following law, known as " Wohler's law:" 

 " Eupture may be caused not only by a steady load which ex- 

 ceeds the carrying-strength, but also by repeated applications 

 of stresses none of which are equal to this carrying-strength. 

 The differences of these stresses are measures of the disturbance 

 of the continuity, in so far as by their increase the maximum 

 stress which is still necessary for rupture diminishes." Pro- 

 fessor Bauschinger has made a long series of experiments which 

 confirm those made by Herr Wohler. Professor Bauschinger's 

 experiments are the most valuable on this subject which have 

 ever been made, and the results are summarised in Table VIII. 



In England, Sir B. Baker has made various experiments 

 which also confirm Wohler's original experiments. For ex- 

 ample, he found that when a shaft is loaded with one-half its 

 gradually-applied breaking-weight, and set rotating, about five 

 thousand reversals of stress produced fracture. He mentions 

 an experiment with a bar of cast-iron loaded with a weight 

 which, according to Fairbairn 's experiments, it should have 



