Description and Notes:
An opisometer is used for measuring the length of curves of arbitrarily varying curvatures such as the boundaries of states or countries on a map. The one showing here is the simplest yet a very well made example of such devices. The handle is made of ivory and was delicate turned at the end. The body is made of nickel silver and steel. According to the maker's signature stamped underneath the top of the body frame, it was made by Elliott & Sons at 56 Strand, and therefore can be dated to circa 1850.
To use, the milled-edged wheel on the screw axis is first turned clockwisely all the way up to against the side with the pointer of the frame, and then placed on one end point of the curve to be measured with the pointer pointed to the end point. By firmly press the wheel on the paper and pushing the handle to run the wheel counterclockwisely along the contour of the curve, it will also be screwed toward the other side of the frame. When the wheel reached the other end point of the curve, the device is raised and then placed against a straight scale rule and draw backwardly along the edge until the wheel turned back to the home position. Consequently, the total length traveled against the rule would be equal to the distance between the two end points of the curve.
A so called Bennett's improved opisometer, which is basically a modified form of the device showing here, can be found in the book by W. F. Stanley, p. 258. Another version of the improved opisometer, made by the Army & Navy C.S.L., is contained in this collection.