by Dale Beeks
The word "Heliotrope", when examined in detail, means "the sun" and "to turn". It is a fitting name for an instrument which does just that, reflects the sun toward a given point.
The heliotrope was utilized by surveyors as a specialized form of target; it was employed during large triangulation surveys where, because of the great distance between stations (usually twenty miles or more), a regular target would appear indistinct.
The heliotrope was limited to use on sunny days and was further limited (in regions of high temperatures) to mornings and afternoons when atmospheric aberration did not affect the instrument-man's line of sight. The heliotrope operator was called a "heliotroper" or "flasher" and would sometimes employ a second mirror for communicating with the instrument station through heliography, a form of language using impulsed reflecting surfaces.
The firm of Fauth & Company produced precise instruments for the U. S. Coast Survey. A Fauth catalogue dated 1883 offered two different forms of heliotropes: Wurdemann's heliotrope was a telescopic form which incorporated two sights, tow signal mirrors and was offered with or without graduated horizontal and vertical axis. A non-telescopic form known as "Steinheil's Pocket Heliotrope" incorporated a flat mirror with a non-silvered opening at the center of the plate. During the use of the mirror, the angle of the cone of reflected sun offered a base diameter of about 50 feet at one mile and 1000 feet at 20 miles. Therefore, high accuracy was not required when directing the mirror at the instrument station.
It is interesting to note that in the year 1879, a Mr. C. O. Boutelle with the Coast & Geodetic Survey, experimented with night signals using argand and coal oil lamps as targets, which were slightly more accurate than day observations and the cost of the apparatus was less than that of a good heliotrope. He further noted that night observations might be employed during periods of cloudy weather when heliotropes could not be used. It is not certain whether this nocturnal procedure was adopted by the Coast & Geodetic Survey.