Description
The historian of science Derek De Solla Price is best known for his pioneering research into the Antikythera Mechanism. At about the same time that he began this research, while working as a member of the Cambridge Department of the History and Philosophy of Science, Price also studied this geared alidade as part of a survey of ‘The Early Observatory Instruments of Trinity College, Cambridge’. An extract from this paper, published in 1952, is reproduced on the label to the right.

An alidade is an instrument used to sight a distant object. Price identified this instrument as one for plane-table surveying – that is, for measuring angles between two lines of sight, most commonly undertaken to aid in the drawing of maps or site plans. What is rare and unusual about this object is that a gearing mechanism has been incorporated into the movable alidade mounted on top of the instrument. This was designed to increase the accuracy of measurement, as it allowed not only degrees of angle to be measured as the alidade is rotated around, but also minutes of angle (there are 60 minutes in one degree) and even seconds of angle (there are 60 seconds in one minute of angle) Price concluded that the instrument was likely an “experimental trial model”, as the last gear wheel and axle to the seconds dial was missing. Price suggested that this was almost certainly because this gear mechanism would not have worked due to “tremendous friction and rotary inertia” – the seconds dial would have had to spin round 60 times for every degree that the alidade was rotated, or 21,600 times for one full 360 degree rotation of the alidade!

Created by: Joshua Nall