helio

noun

1. A heliotrope (surveying instrument).

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

   “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.”

   “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.”