Visual memory is one of the most valuable attributes for the student of material culture. But for those who have maintained a life-long interest in scientific instruments visual memory can pose as many questions as it answers. Yes, I am sure that I have seen one of those before, but was it in a museum where I have worked or merely a collection visited? Did it pass through the auction rooms or did I see it in the trade? Is the memory of an illustration from modern literature, or an engraving in an old book? Have I actually seen one before, or does ageing visual memory deceive?
Three instruments in the Museum’s current special exhibition, The Geometry of War, stimulated just such questions: the two unsigned, seventeenth-century military architect’s rules (catalogue numbers 68 and 69) and the military protractor by John Marke, c.1670 (catalogue number 70).
As it happens, the juxtaposition of these instruments in the showcase and the catalogue has proved to be fortuitous since the location of an illustration of the protractor, prompted by a visual memory lodged long ago, has led to the identification of a printed source for the engravings on the two rules.
The illustration of the protractor occurs in a small pamphlet penned by Sir Samuel Morland (1625-1695) outlining the basis of the fortification design methods of the French military engineer, the Count de Pagan. The pamphlet was published in 1672 within a compilation of texts usually catalogued under the name of the author of the first work, Thomas Venn, and there the illustration is described as ‘The figure of a most useful Instrument, by the help whereof any Poligone, from a square to a Dodecagone, (which is as much as is required in any Fortification) may be described, not only with greater expedition, but likewise much more easily than by any Sector, Scale, or other Instrument or method whatsoever.’
Besides the illustration of the protractor, Morland’s pamphlet also contains engraved tables that supplement his discussion of Pagan. These tables, just like the scales on the rules, are annotated in Latin and French, but they do not match the instruments’ scales. However, the British Library holds a volume of English mathematical tracts amongst which is a single sheet on which are printed two plates of engravings of fortification tables and diagrams. These tables, unlike those in Morland’s pamphlet, match the scales on the rules almost exactly.
The tables and diagrams give inward (INTRORSVM) and outward (EXTRORSVM) proportions for constructing bastioned fortifications. On the rules, the inward information is all grouped together on one side, with the outward on the reverse. By contrast, the sheet juxtaposes inward and outward on each side. But once reassembled, the match between printed source and instrument is clear. What is more, on the otherwise untitled sheet has been added, in a late 17th-century hand, ‘Sir Samuel Morlands fortification’.
One historical conundrum, however, remains to be solved. Somewhere there ought to be a text associated with the plates – probably in a French publication. Morland’s penchant for plagiarism means that it does not necessarily have to be associated with his name. Perhaps the visual memory of one of Sphæra’s readers will now be prompted to recall just such a text.