[volvelle] Cox's Stadia Computer

Troy, N.Y.: W. & L. E. Gurley, 1899. Very Good. Roughly 5 7/8 inches square. Cellulite volvelle with brass grommet. Surface scuffing from use. Front with volvelle, rear with great illustration of surveying instrument and table below. A nice advertising piece for surveying instrument manufacturer Gurley, and useful besides. Inside ring is labeled Difference of Elevation. We have also seen this device offered as part of a pamphlet in a paper format. This format is far more useful for the surveyor's bag and no doubt was a response to customers interested in its use.

A wonderfully graphic example of a working surveyor's tool. Includes directions for use. While at first glance 1899 might seem early for the use of the term "computer" applied to an object instead of a person, the term was first used on a volvelle by Aaron Palmer in 1843. Gurley is recognized as one of the most successful surveying instrument makers in the United States. Their instruments and related material are avidly collected by those interested in the "pre-digital" age of surveying.

Stadia is a method of surveying in which distances are read by noting the interval on a graduated rod intercepted by two parallel cross hairs (stadia hairs or stadia wires) mounted in the telescope of a surveying instrument, the rod being placed at one end of the distance to be measured and the surveying instrument at the other. (dictionarycom).