Tuesday, 22 May 2012

The Mérode mousetrap

[Update 12.01.2018: Shawn Woods from Mousetrap Monday has a knack for engineering these old traps from the auld sources and they work very well as he shows at his youtube channel.]

The Mérode altarpiece is an annunciation tryptich dating from 1427-32. It was made in the workshop of Robert Campin and is now in the Cloisters of the Metropolitan Museum of Arts in New York. The MET also has a high resolution picture of it online with zoom function. The right panel shows Joseph who has made two mouse traps. One is on display on a window sill that goes out on the market place. It is a trap of the kind where a block of wood falls vertically.1 The other sits on his workbench and looks like this:
Mérode mousetrap
To understand the mechanism of this trap, it is best to look at a replica complete with the missing stick that holds the striker up, when it is set. 

Replica of Mérode trap (from Drummond 2005)
The stick on the string that is fixed to the overhead beam is clamped between the treadle and the striker board. It holds the striker up when the trap is set. If a mouse steps on the treadle, however, this stick will be released and the striker board will strike down. The other stick that is fixed between the twisted cords, which in turn are fixed to the upright posts at each side, works as an additional power source. The torsion power of the twisted cords presses this stick onto the striker board an makes it shut quicker when released. This mechanism was also used in cage traps, where the mouse would be trapped rather than killed.

Leonard Mascall (1590) recorded this trap as the 'following trappe', because he called the stick that presses on the striker board the 'following staffe'. Unfortunately, his depiction is a bird's eye view and therefore somewhat more difficult to understand. His verbal description, however, leaves no doubt that it is the same trap as on the Mérode altarpiece (Drummond 1992). Mascall's book is online at Early English Books Online or, if you have no access, click on the link in the reference list below.
Bird's eye view (from Mascall 1590)

A lot of intelligent things have been written on the Mérode trap concerning both the religious symbol and the artefact. In silly times, however, the Mérode trap might occur to creationists or ID-ologists as irreducibly complex - a piece of art that cannot have had working precursors. This argument from incredulity can be countered by an argument from credulity which, though equally inane, is at least based on some more historical evidence.

The following antique specimen, for example, are from Austria, but similar traps are known from Sweden and other places.
Tirolian traps (from Gasser 1988)
Add to this an even simpler trap made of an errand stone propped up by some sticks that form a set/release mechanism ...
'Figure-4' trap (from Gibson 1881)
... et voilà - an evolutionary series can be proposed, ...
Possible 'evolutionary' series of traps
... where a) is a 'Figure-4' trap, b) is like the left Tirolian, c) like the right Tirolian, and d) the Mérode trap. There are no unbridgeable gaps involved and taking the series backwards, of course, they get simpler but remain functional.

1. The love for detail is amazing. Zoom in on the market place that is visible through the window of Joseph's workshop and you'll see that it is snowing outside, but that is visible only against the background of the passersby with dark clothes. Furthermore you'll see another window sill with another trap on the far side of the market place. Er - not really in the online version of the MET. I had the great fortune to see the original, when it was part of an exhibition of the Städel Museum in Frankfurt am Main in 2009 or 10. The situation was strange, however, because an apparently very devout Christian had been sunken into deep prayer on the bench in front of the exhibit, while I was trying to get as close as the wards would permit with a different agenda in mind