148 SECTIONAL ADDRESSES. 



such splendid success in connection with the work of the Bridge Stress 

 Committee. It is also of interest to note that history has repeated itself 

 in that the great contribution which Fairbairn made to structural engineer- 

 ing was only possible, as he himself admitted, by the co-operation of the 

 practical engineer and the mathematician. 



Aeroplanes and Airships. 



The development of the forms of aeroplanes and rigid airships and the 

 determination of the forces acting upon them — lift and drag — and the 

 distribution of these forces, is another illustration of the splendid use 

 that can be made of the combination of experiment and mathematical 

 analysis. Following the fundamental principles of Newtonian Mechanics, 

 dynamical laws of similarity have been developed which in combination 

 with the wind chambers and the precise measuring devices used for the 

 determination of forces and moments acting upon models have made it 

 possible to anticipate certain of the forces acting upon machines. Other 

 forces could only be estimated from meteorological data obtained from an 

 examination of the velocity of upward and downward currents of air 

 which a ship or aeroplane may suddenly encounter. Much could be said 

 of the many other contributions of ' pure science ' to the successful 

 development of the aeroplane. Many of the details and instrumental 

 equipment owe much to science, but success could not have been achieved 

 if these gifts had not been supplemented by the application of the experi- 

 mental method, in which precise measurement has played an important 

 part, to the solution of the specific engineering problems associated with 

 the engine and the structure. 



Materials. 



Allied to the subject of structures is that of the strength of materials 

 and their behaviour under various types of stress. For nearly seventy 

 years experiments have been carried out to determine the effect of repeated 

 stresses on metals, but the development of high-speed machinery during 

 the last twenty years has focussed attention upon this important subject, 

 and it is now realised how imperfect is our knowledge. A vast amount 

 of experimental data has been accumulated and it has been clearly shown 

 that the safe range of stress to which a piece of material can be subjected 

 for, let us say, an indefinite number of times, depends in some way, not 

 yet clearly defined, upon other and more easily determined physical 

 properties. But it is not yet always possible to foretell that some element 

 of a locomotive or of an aeroplane engine, or structure, or other machine 

 will not fail under the conditions it has to meet in practice. Small dis- 

 continuities in the structure of wheel tyres, a surface defect in an axle, a 

 wheel gripping very tightly on an axle at one section, surface cracks which 

 cannot even be seen by the microscope on a wire used for a rope, or in the 

 disc of a steam turbine rotor ; the surface condition of a quenched and 

 tempered spring used for a locomotive, an aeroplane engine, or a motor- 

 car may lead to failure under the repeated stresses due to the normal 

 forces acting upon the element, or due to repeated blows, such as, for 

 example, a locomotive experiences whenever it passes over a rail joint, or 



