PLATOMETER (πλατὸν, broad, μέτρον, a measure) is the name of an instrument invented by Mr John Sang, Kirkcaldy, 24th December 1851, the principal feature of which is the employment of a cone acting on an index-wheel, from which is read off the area of any figure whose outline has been followed by the tracer of the instrument. It has the peculiar advantage of giving the areas of the most irregular figures with the same accuracy that it gives the areas of the most regular, and with the same facility. It will be found described and figured in the Transactions of the Royal Scottish Society of Arts, vol. iv.