B chapter.indd - Charles Babbage Institute - University of Minnesota

Erwin Tomash Library
Blagrave, John Blagrave, John
servants selected by town parishes who were to cast lots
for their prize money each Good Friday.
The instrument described is a planispheric astrolabe that
had a universal projection modified from the Catholicon
of Gemma Frisius—a description of which can be
found in the second booke. Blagrave added a movable
rete (often found on standard astrolabes but not on the
Catholicon), which simplified its use for astronomical
calculations. This astrolabe was universal in the sense
that it did not require a number of different plates or
maters to be used at different latitudes. The instrument
is illustrated in a number of full-page engravings serving
as frontispieces to the work—engraved by the author
according to the title page. This was an expensive
instrument to build and consequently was not much
used. While this is the only edition of this work, the
Jewel was described ten years later in a work by Thomas
Blundeville (Exercises, 1622), and instruction in its use
was also offered by Robert Hartwell, a London teacher
of mathematics, in 1623 (see Waters, David Watkin; Art
of navigation, 1958, p. 570).
B 174
Arc drawing instrument, B 174
Mathematical jewel, B 174
The work is divided into six bookes. The first deals with
elementary concepts of astronomy; the second with
the design and manufacturing of the jewel; the third
with the use of the instrument for both navigation and
astronomical calculations; the fourth considers the same
material as the third, but the examples and methods of
working come from Blagrave’s own research; the fifth
is a treatise on spherical triangles; and the last is a work
on the use of the jewel in creating sundials of all types.
For such a small volume, it is remarkably complete and
would have made a very useful reference work even
if one did not have a jewel to use. In the fourth book,
Blagrave mentions that he had made a jewel two feet in
diameter and that he had problems drawing all the arcs
on it. He then illustrates a drawing instrument that would
suffice in such a situation.
Blagrave is known to have made other instruments,
in particular a familiar staff, which may have been an
instrument for artillerymen.
The work contains a handwritten note which reads:
Here stands Mr. Gray master of this house
And his poor catt playing with a mouse.
John Balgrave marred this Grayes widdow (She was a
Hungerford) this John was symple had yssue by the widdowe
1 Anthony who marryed Jane Borlafs. 2 John the author of
the booke. 3 Alexander the excellent chess player in England.
Anthony had Sir John Blagrave knight who caused his teeth to
be all drawn out and after had a sett of ivory teeth in agayne.
157