We are all too familiar with the surprises – and disappointments
– of Antiques Roadshow, so it always comes as a pleasant surprise
when, out of the blue, someone gifts a rare and significant object
to our collection. And it’s an odd feeling indeed to discover that
the object is not what it first appeared to be.
Quite recently an anonymous donor presented a 17th century
scientific instrument of which there are only three known examples
by the same maker – what was initially thought to be an ‘astrolabe’
turned out to be an even more rare ‘circles of proportion’ by Elias
Allen. Along with English mathematician William Oughtred’s
horizontal instrument, this earliest form of slide rule allows
problems of multiplication and division to be reduced to addition
and subtraction by the use of logarithms. The horizontal
instrument, when placed on the reverse of the circles of
proportion, demonstrates astronomical principles and was used for
laying out sundials.
This is one of only a handful of these important scientific
instruments in existence – two others by Elias Allen can be found
at Oxford and Cambridge Universities where they have been since the
day they were constructed – and the third has now found a home in
the National Museum of Scotland.