The rotating Egyptian statue at Manchester Museum: An explanation by Steve Gosling, 24 Acoustics
24 Acoustics Ltd was asked by Shiver TV to participate in a forthcoming new TV programme “Mystery Map” to investigate the possible cause of why an Egyptian statue mysteriously rotates in a locked display cabinet at the Manchester Museum (The University of Manchester).
Only two key-holders had access to the cabinet and both had differing opinions on how the statue should be displayed – facing forward (as is normally the case) or facing backward, given that there is an inscription on the rear of importance “An offering which the king gives to Osiris, Lord of Life, that he may give a voice offering, consisting of bread, beer, oxen and fowl for the Ka-spirit of’ Nebsenu“. As a result of the spinning, each keyholder thought that that the other was repositioning the object until the matter was finally discussed and both realised that neither had moved the statue. The Museum consulted their media department who agreed to setup a time lapse camera within the cabinet and the results were stunning.
The story originally broke in late June 2013 with press attention from around the world. The recorded footage showed that the statue span in an anti-clockwise direction and would usually slow the rate of spin during the night-time period.
The statue has a partially convex base and is made from fired steatite – a very hard material. The statue is also located on a glass shelf, hence the level of friction between the two materials is very low.
Various explanations were proposed for the movement, including a proposition that subtle vibrations caused by differential friction between the two surfaces were responsible.
In developing the possible options for Mystery Map, Shiver TV contacted 24 Acoustics to assess whether external factors such as vibration could be a possible cause. Steve Gosling, principal consultant and expert in building vibration at 24 Acoustics, visited the museum in mid August 2013 to undertake a series of vibration measurements. Setup of the instrumentation was filmed and discussed with Mystery Map presenter Julia Bradbury.
The measurements were undertaken overnight and involved attaching a precision vibration sensor (accelerometer) to the structure of the display cabinet. The device is normally used to measure ground vibration or vibration in buildings, most commonly to assess significant vibration sources such as overground or underground trains, construction and demolition activities including piling.
The same equipment is also used to assess the vibration impact on people, buildings, heritage sites, gas pipes, railway embankments and so on.
The sensor was configured to measure and store vibration levels continuously throughout the evening, night-time and morning periods. Vibration from footfall and external road traffic were considered the most dominant sources of vibration that could enter the cantilevered display structure.
The results, shown below, showed a marked difference in measured vibration velocity during the daytime and night-time periods. Further, when a detailed comparison of vibration levels and extent of spin is undertaken, it was found that the vibration level correlated very strongly with the rate of spin exhibited by the statue.
From the solid correlation between vibration level and rotational speed of the statue, it was concluded that vibration was the primary cause.
The findings were discussed and filmed afterwards with Julia Bradbury where expert Steve Gosling confirmed that the vibration was the primary cause of rotation. It was also confirmed that the measured levels are lower than the threshold of human perception hence Manchester Museum staff would not have been able to perceive the vibration present. Typically, human limits of perception are from 0.14 to 0.30 mm/s and, as the graph demonstrates, even the strongest vibrations would have been barely noticeable to even the most perceptive human senses.
Following a surge in visitor numbers to the museum during the summer months, senior personnel at the museum decided to end the rotation by placing the statue on small patches of museum wax – hence the statue no longer rotates.