Sang's Platometer, planimeter, 1851; in mahogany c
Sang's Platometer, planimeter, 1851; in mahogany case, 15 1/2"x8 3/4"x4 3/8" no key
This is a planimeter, an instrument which is used for measuring irregular figures drawn on paper. It was designed and made in 1851 by John Sang of Kirkcaldy and an example was exhibited by him at the 1851 exhibition.
In use, the planimeter is laid on the paper so hat the tracer can be carried round its outline. The handle is held like a pen and the tracer is brought on the outline and pressed very slightly into the paper so as to make a small mark. The index is then read before the tracer is carried along the outline until the mark is again reached. The index is read again, and the difference between the two readings gives the area of the figure in square inches.