Unpublished manuscript, 'Some differences in the behaviour of real fluids from that of the mathematical perfect fluid' by A [Henry Reginald Arnulph] Mallock Mallock writes: 'The perfect fluid known to mathematicians is defined as matter which has no power of resisting distortions and which fulfils the conditions of continuity in space [...] In this paper I propose to examine some of the differences which real fluid, such as air and water, show in their behaviour from that of a "perfect fluid" under the same conditions, and to suggest a possible cause for this difference.' Annotations in pencil and ink throughout. Includes 12 pages of diagrams. Subject: Mathematics / Fluid dynamics Received 17 May 1898. Communicated by Lord Rayleigh. 4 May 1898