"U.S.G.S. No. 25"
base 18 inches long; telescope 9 inches long
This alidade has a telescope with level mounted below, and a vertical arc that is silvered, graduated to degrees, and read by vernier to single minutes. The base has six round holes, two short level vials, and a beveled edge with a 12-inch scale that is divided to sixteenths of an inch. The United States Geological Survey transferred this instrument to the Smithsonian in 1907, describing it as a "preliminary type" of alidade devised for the Survey about 1890.