My recollections of W. J. Greenstreet date back half a century when we were both undergraduates at Cambridge, but we did not become personally acquainted till many years later. He had a slightly stooping figure and kind protective face which dissolved any feeling of awe produced by his great strength and height, penetrating eyes, and full-grown beard. He had two inseparable friends, E. F. J. Love and G. F. Stout. When in company together they drew the attention of every one who saw them. They looked more like three generations than contemporaries, Greenstreet being plainly the responsible head and Stout the cheerful but inscrutable infant, while Love appeared to be more normal and rather embarrassed by the strangeness of his companions.

For more than twenty years W. J. Greenstreet was scientific correspondent of the evening Westminster Gazette, and he was one of the first to bring scientific developments regularly under the purview of a newspaper. His work consisted in writing articles on any discovery or research of special importance, contributing a regular fortnightly article on current science, and reviewing scientific books and periodicals. He read widely, English, American and foreign books and scientific records of all sorts, to equip himself for this work, and, whenever possible, gave it a practical application.

Nobody who has paid attention to the acknowledgments of gifts to the Library during the past seven years will have failed to observe that Mr. Greenstreet was the most lavish and the most persistent of benefactors. The records of the books of which he, as he put it, unloaded his own shelves as soon as the Library was removed to Reading, occupy several pages of vols, xi and xii of the Gazette; his name is in many of the volumes which were in the Library before that time, and few months have passed since without evidence of his goodwill.

The total of the fund which was opened on the occasion of the 200th number of the Gazette was £270. Of this amount, £200 was given to Mr. Greenstreet in June of last year, and the balance, with a list of subscribers, in May of this year, when it became all but certain that the last contribution had been made. From the first it was impressed on Mr. Greenstreet that the desire of his friends was to add to his personal comforts. He arranged at once to take a much needed and much enjoyed holiday in the West country, a holiday of which he had previously been afraid to incur the expense.

The title of my lecture is rather misleading. When I was asked to deliver it I thought I could profitably indicate how very definitely mathematics is used in a particular activity, such as gunnery, the application being neither complicated nor varied. If one was asked to write a paper on the utility of mathematics in engineering or in wireless electricity, the problem would be much more diffuse. The man who makes or fires a gun is only, after all, making one particular machine or doing one particular job. Hence it is possible briefly to state what mathematical difficulties he encounters.

The present communication forms, in essentials, part of a paper read before the Liverpool Mathematical Society in 1900 by Rev. S. Sircom, S.J., M.A. Formulae for the stream lines in the two-dimensional irrotational motion of a liquid were given in certain particular cases and illustrated graphically by means of a series of remarkable diagrams. These diagrams are unique; it was felt that their reproduction in permanent form was desirable and would at the same time prove of interest to readers of the Mathematical Gazette.

1. The applications of spherical harmonic functions in mathematical physics are closely connected with the various potentials—gravitational, electrostatic, magnetic, or the velocity potential in hydrodynamics. It is of interest to recall, briefly, the development of the potential theory. The actual name “potential function” or potential was first introduced by Green in the memoir of 1828 in which he published his famous theorem on the transformation of surface and volume integrals. But the use of the potential function goes back to Euler (1707–1783), who formulated the fundamental differential equations of hydrodynamics, and integrated them in the case where a velocity potential exists, which he recognized to be a special case.