Astronomical Images : Blagrave's birdcage-like instrument to give the 'Theoricall reason and grounde of all Dialles'

John Blagrave

Astronomical Images

<p style='text-align: justify;'>This image comes from John Blagrave's <i>The Mathematical Jewel </i>(1585). In this work, Blagrave drew on a long tradition of guides to the construction and use of astrolabes, referencing works by Gemma Frisius and Johann Stoeffler amongst others. The new instrument that Blagrave presented as the mathematical jewel was an astrolabe of his own devising which had the benefit that it could be used anywhere in the World without the need to substitute different plates according to the latitude. The device seen here was included in the sixth book of the <i>Jewel</i>, in which Blagrave promised to give the 'Theoricall reason and grounde of all Dialles'. It presents a birdcage-like instrument made of twenty-four semicircular wires with an internal circular board, the angle of which may be adjusted, with a hole in the centre through which a straight wire may be passed. The examination of this instrument in a variety of orientations was intended to guide readers in the construction of 'all manner of Dyals'. Although Blagrave relates each part of the instrument to explanatory celestial features, such as the poles of the heavens and meridians, the imparting of causal knowledge was not the primary function of this instrument. Rather, it had a fundamentally practical purpose, with Blagrave encouraging the reader to manipulate the depicted instrument, whether in the imagination or the hands, to find out how to mark out the lines of a dial for different latitudes. In this sense, the instrument acted as a repository from which the reader could extract information for a specific practical purpose - that of making a sundial - without having to grasp the causal relevance of each part of the instrument.</p>