helio

plural helios

1. A heliotrope (surveying instrument).

   A helio or sun-reflecting signal was evidently the desideratum.

   1874, Verplanck Colvin, Annual Report on the Progress of the Topographical Survey of the Adirondack Wilderness of New York, for the year 1873

   2 more examples

   A light keeper's duties require him to be alone on a station for days or even weeks at a time, showing a helio to the observer in the afternoon for two or three hours and watching by the signal lamp at night.

   1925, U.S. Coast, Geodetic Survey, Special Publication - Coast and Geodetic Survey, page 62

   On a cloudy but bright day, it is possible to use a helio effectively over moderate distances of about 6 miles. Light keepers should be made aware of this, since they often think that brilliant sun is essential.

   1968, Arthur Laidlaw Allan, J. R. Hollwey, J. H. B. Maynes, Practical Field Surveying and Computations, page 258