In many problems arising from scientific investigations their theoretical formulation leads to differential equations, the numerical solution of which is frequently an extremely laborious process. To handle such problems various mechanical devices have been developed from time to time and the Differential Analyser is one such device, possessing great flexibility in the range of differential equations which can be dealt with by it. Lord Kelvin conceived of such a machine, and the first practical model was designed by Dr V. Bush of the Massachusetts Institute of Technology. Subsequently another full-scale machine was designed by Prof. D. R. Hartree and is in operation at Manchester University. Various small-scale machines have been constructed in this country and mention may be made of those at London University and Cambridge University, designed by Profs. H. S. W. Massey and J. E. Lennard-Jones respectively.