Computing scale used to measure areas on plans at scale of 3 chains to an inch.
The computing scale comprises a wooden stock graduated in acres and a sliding frame in metal with a central vertical wire. This scale was used to calculate areas in acres of pieces of ground drawn to a scale of 2 chains to an inch. To do so, a plan containing the piece of land was overlaid with a transparent sheet of paper with horizontal lines ruled at intervals to correspond with the scale of the plan and the computing scale was laid on top of this. At the start of a measurement the vertical wire in the sliding frame would be set to zero on the left side of the scale. Normally the person measuring would start with the lowest ruled line. The wire in the frame would be lined up to the left edge of the area to be measured and then moved towards the right. The scale then lifted to the next line above (without moving the frame) and the wire set against the left hand edge. The frame was then moved right as before and repeated until the entire area of ground had been measured. The area in acres was then read off the graduated scale on the stock.
A similar computing scale at 2 chains to an inch is also in the collection click here to go to it.