A differential analyzer was built at Manchester University, England. It was built first from “Meccano” parts, at a total cost of about 20 pounds, and later refined for more exact work. Some articles dealing with this differential analyzer are: HARTREE, D. R., The Differential Analyzer, _Nature_, vol. 135, June 8, 1935, p. 940. HARTREE, D. R., The Mechanical Integration of Differential Equations, _The Mathematical Gazette_, vol. 22, 1938, pp. 342-364. HARTREE, D. R., and A. PORTER, The Construction of a Model Differential Analyser, _Memoirs and Proceedings of the Manchester Literary and Philosophical Society_, vol. 79, July 1935, pp. 51-72. Other small scale differential analyzers built in England are covered in: BEARD, R. E., The Construction of a Small Scale Differential Analyser and Its Application to the Calculation of Actuarial Functions, _Journal of the Institute of Actuaries_, vol. 71, 1942, pp. 193-227. MASSEY, H. S. W., J. WYLIE, and R. A. BUCKINGHAM, A Small Scale Differential Analyser: Its Construction and Operation, _Proceedings of the Royal Irish Academy_, vol. 45, 1938, pp. 1-21. A differential analyzer constructed in Germany is briefly described in the following: SAUER, R., and H. POESCH, Integrating Machine for Solving Ordinary Differential Equations, _Engineers Digest_ (American Edition), vol. 1, May 1944, pp. 326-328. From the historical point of view there are some interesting papers on a machine for solving differential equations by Sir William Thomson (Lord Kelvin), including one by his brother James Thomson. They are in the _Proceedings of the Royal Society_, vol. 24, Feb. 1876, pp. 262-275. The method of integration by a machine is described, but the state of machine tools at the time was such that no accurate mechanism was constructed. Another interesting paper foreshadowing the differential analyzer is: WAINWRIGHT, LAWRENCE L., _A Ballistic Engine_, Chicago: University of Chicago, thesis for Master’s Degree, 1923, 28 pp. Some of the applications and mathematical limitations of differential analyzers are covered in: BUSH, V., and S. H. CALDWELL, Thomas-Fermi Equation Solution by the Differential Analyzer, _Physical Review_, vol. 38, no. 10, 1931, pp. 1898-1902. HARTREE, D. R., A Great Calculating Machine: the Bush Differential Analyser and Its Applications in Science and Industry, _Proceedings of the Royal Institution of Great Britain_, vol. 31, 1940, pp. 151-170. HARTREE, D. R., and A. PORTER, The Application of the Differential Analyser to Transients on a Distortionless Transmission Line, _Journal of the Institute of Electrical Engineering_, vol. 83, no. 503, Nov. 1938, pp. 648-656.