The Surveyors’Heliotrope:

TThe name “heliotrope” in the English language ismost frequently associated with a sweet scentedflowering plant of the genus heliotropium [N.O.Ehretiaceae or Boraginaceae], in the sunflowerand marigold family, the flowers and leaves ofwhich turn and follow the sun. It is the name

given also to a green semi-precious stone streaked with redknown as bloodstone. According to Webster’s Dictionary, it alsois the name of “an instrument used in geodetic surveying formaking long distance observations by means of the sun’s raysthrown from a mirror.” The Oxford English Dictionary providedadditional detail, describing it as “an instrument used in sur-veying, an apparatus with a movable mirror for reflecting therays of the sun, used for signaling and other purposes, especial-ly in geodesic operations.”

The heliotrope instrument, invented early in the nineteenthcentury, achieved importance in its time as a solution to a per-sistent annoying problem in surveying—dissatisfaction withexisting methods for observing distant points. As a recourse,lamps or powder flares at night could be used. For example,Andrew Ellicott (1754-1820) in the course of surveying severalof the national boundaries of the United States, utilized twocopper lanterns for tracing meridians and obtaining the direc-tion of lines when determined by celestial observations in thenight. He had designed the lanterns and had them made to his

Figure 1 The first heliotrope instrument made forGauss by Breithaupt of Kassel, Germany.Photo courtesy Hans-Helmutt Breithaupt.

>> By Silvio A. Bedini

Its Rise and Demise

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specifications, and described them in hispublished account of his boundary surveybetween the United States and theSpanish possessions. The lanterns werefour-sided and made of copper, each sideabout 4-1/2 inches wide and 8 incheshigh, and made to accommodate a can-dle. The front side of each lantern had avertical slit or aperture about 5 inches inlength, and 3/10ths of an inch in width,through which the light from a lightedcandle would be visible at night.Sometimes a slip of white paper wasplaced behind the lantern to render theaperture more distinct when the door onthe opposite side was opened.

As late as 1879 C. O. Boutelle, whenworking with the U. S. Coast andGeodetic Survey, used Argand and coaloil lamps as targets for night signals.Although heliotrope instruments werereadily available at that time, the lampswere less costly than a heliotrope of goodquality and proved to be a bit more accu-rate than observations made in daylight.

Even with nighttime methods in place,it had been universally realized for along time that some means was neededthat would substitute the sun’s reflectedrays for a surveyor’s target. It had to becapable of reflecting the sun’s raysbetween surveying stations when thedistance between the stations was sogreat that a conventional target wouldnot be sufficiently large to be visible ona hazy day.

Carl Friedrich GaussThe solution proved to be the heliotropeinstrument, an invention that was con-ceived early in the nineteenth century bythe celebrated German mathematicianand astronomer Carl Friedrich Gauss(1777-1855). A native of the Duchy ofBraunschweig (Brunswick), now part ofGermany, he was born of a poor familywith limited means. He proved to be aprecocious child having remarkable math-ematical abilities that became apparentfrom the age of three. His achievementsas he grew attracted the attention of

Ferdinand, Duke of Brunswick, whofinanced his education and became hispermanent patron and friend. Gaussattended the University of Göttingen andreceived his doctorate in mathematicsfrom the University of Helmstedt. In1807 he was appointed the director of theastronomical observatory in Göttingen, aposition he retained until his death, and itwas about ten years later, in 1817, that hebegan his studies in geodesy.

In the period of the lifetime ofGauss, the kingdom of Hannover innorthern Germany, situated betweenthe Elbe River and the Netherlands,belonged to England. In 1817 KingGeorge IV of England commissionedGauss to triangulate the entire king-dom of Hannover. The surveyrequired thirty years before it finallywas completed in 1847. Although themap of the region that resulted wasnot very accurate, Gauss compensatedby the use of curvilinear coordinateswhich he developed.

An Annoying ProblemIt was in 1820, while he was engaged inmaking trigonometrical observations atLüneberg for the purpose of extending theDanish triangles into Hannover, that Gaussfirst became aware of an unusual phenom-ena that ultimately inspired his invention.Every time that he directed his telescopetoward the steeple of St. Michael’s Churchat Hamburg, a distance of seven Germanmiles (thirty-two English miles), the littleround window in the upper part of thechurch reflected the image of the suntoward him. This was interfering with hisobservations and consequently repeatedlyimpeded the progress of his operations, tohis increasing annoyance.

This distracting problem led Gauss topause and meditate upon the need for abeacon that was sufficiently bright sothat it could be observed even in thedaytime. It occurred to him that it mightbe possible to utilize the sun’s rays forsignals, and he developed the concept ofcapturing the rays with a mirror andreflecting them to the place to which thesignal was to be given. He estimated thestrength of the sun’s light as well as theamount of diminution it suffered in theatmosphere. From these calculations heconcluded that a small mirror of notmore than two or three inches in diame-ter would be adequate to reflect the sun’simage to the distance of ten or moreGerman miles.

Figure 2a (Top) Front of German 10 DM bank note honoring Gauss. Note Gaussbell curve.

Figure 2b (Bottom) Reverse of bank note shows a navigation sextant and a Gauss triangulation diagram in northern Germany.

Heliotrope continued from page 38

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Thus was born the first heliotropeinstrument for surveying, appropriatelynamed after the sun-loving plant. In itstime the new instrument was considered tobe of great importance in the measurementof large triangles. It was believed likely thatit would supersede nighttime methods.

After having resolved the optical theo-ry, Gauss designed an instrument tofulfill his purpose, and in 1821 he com-missioned the firm of Breithaupt inKassel to construct the first model forhim (Figure 1). It exceeded his expecta-tions, proving to be extremely useful inpractice, having the brightness of a firstmagnitude star at a distance of fifteenmiles. Gauss gained full proof of theadvantage to be derived from theheliotrope when he used it on the sum-mit of Brocken Mountain, to determinethe three corners of the triangle formeasuring the meridian of the North ofGermany. On this occasion he sent sig-nals with his heliotrope to his assistantswho were stationed upon the Inselbergh,in the forest of Thuringia, some fourteenGerman miles from his position.

Among the earliest published descrip-tions of the instrument was a brief accountof Professor Gauss and his invention thatappeared in The Gentleman’s Magazine: andHistorical Chronicle by Sylvanus Urban,published in London in 1822. After describ-ing the circumstances that had led Gauss todevelop the instrument with a small mirror,it went on to describe how it would replacemethods hitherto employed:

These consisted in placing or fas-tening by night several Argandlamps, with reflectors, at those placeswhich it was intended to observefrom a great distance. This measur-ing by night is very inconvenient,and by day the light of the lamps ismuch too faint to be always seen atthe distance of several miles througha telescope. The inventor of theHeliotrope, on the other hand, hadfull proof of the great advantage tobe derived from it, when he was lastyear on the summit of the BrockenMountain, to determine the threecorners of the triangle for measuringthe meridian of the North ofGermany; on which occasionProfessor Gauss gave signals withthis instrument to his assistants, sta-tioned at 14 German miles fromhim, upon the Inselbergh, in the for-est of Thuringia. But the great use ofthe Heliotrope is not confined to suchoperations. It will be found greatlyto excel the telegraph for giving sig-nals, and in time probably willsupersede it [provided the Professorcould insure the perpetual appear-ance of the sun]. As the reflectedimage of the sun is visible at so greata distance, the signal stations may bemuch fewer. The mode of using it islikewise more simple, it being merelynecessary alternately to shew and tohide the mirror; the intervals, meas-ured by a stop watch, are the signals.

In recent years, shortly before itsadoption of the European euro currency,the Federal Government of the GermanRepublic issued a 10-mark banknotecommemorating Gauss. (Figures 2aand 2b)

According to the Oxford EnglishDictionary, the one who operated aheliotrope was designated as a“heliotroper” or “flasher.” Heliotropesalso figured in the work of FerdinandRudolph Hassler (1770-1843), the Swissborn engineer who helped establishedthe U. S. Coast Survey and became itsfirst superintendent in 1816. Apparentlyafter having learned about the inventionof the heliotrope, Hassler contactedGauss directly and with the inventor’sassistance, obtained from Breithaupt anumber of the instruments made toGauss’s design. In a report to theSecretary of the Treasury, datedNovember 17, 1837, Hassler wrote:

“Heliotropes, of which I had begun theuse of last fall, have this year been usedfor most of the station points, and forthe base points exclusively. I caused oneto be constructed in our shop last win-ter, after the two received fromGottingen, by the kind assistance ofProfessor Gauss, the inventor of thisinstrument, and during my work thissummer, I received four more; all sevenare now in activity. The aim of theinstrument is: to reflect the sun’s imagefrom the station point, at which theyare placed, to the observer at his sta-tion. The new instruments require aman of some intelligence to attendthem, and to replace them about everyfour minutes, according to the motionof the sun. . . . They will show a pre-cise luminous point, even though haze,so frequent on the eastern seashore,when the outline of the hill itself, uponwhich they stand, cannot be traced.”

In the course of the ensuing years, theheliotrope instrument went through anumber of modifications, and variousforms were devised to improve its con-venience in use. Some included the useof clockwork to keep their flat mirrors atthe correct angle as the sun crossed thesky during the day. Even greater preci-sion was achieved by combining mirrorswith a small telescope. One form of theheliotrope consisted of a telescope mount-ed with a vertical and horizontal motion.This was turned upon the station that theobserver occupied. Mounted on the tele-

Figure 3 “The Heliotrope” from Report of the Superintendent of the U. S. CoastSurvey . . . During the Year 1866, plate 27.

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scope was a mirror and two disks havinga circular opening. The mirror had twomotions so that it could be put in anyposition. Its center was coincident withthe axis of the disks, in all positions. Themirror could be turned so as to throw abeam of light symmetrically through theforward disk, in which position thereflected rays would be parallel to theaxis of the telescope, and thus fall uponthe distant point.

The heliotrope in its various formsbecame standard equipment for largescale triangulation until about 1840,when more sophisticated models becameavailable. Other types featured flat mir-rors attached atop small refractingtelescopes. An attendant would aim thetelescope directly toward the surveyorson the neighboring mountaintop. Themirror attached above the telescopewould then be positioned to reflect thesunlight through two circular ringsattached in front of one another at thefront of the telescope. When these werealigned with the optical axis of the tele-scope, the attendant was then assuredthat the sun’s rays were correctly posi-tioned and directed toward theinstruments on the neighboring moun-tain. The disadvantage of both systems,however, was that an attendant wasrequired to set up the device each day.

In 1866 the U.S. Coast Survey issueda report on the subject, proposing to dis-pense with the use of clockwork inheliotropes because it required too muchcareful handling, and furthermore, theuse of clockwork would not dispensewith the need for the services of anattendant, who could equally well beemployed in directing the heliotrope. Onthat account the use of the heliotropewas considered to be expensive, and inremote localities, such as on the tops ofhigh mountains, it was often difficult tosubsist the attendant, and the employ-ment of an additional hand if necessary.On the mountains of California, forexample, this difficulty was often a veryserious obstacle to the use of an other-wise desirable point (Figure 3).

In the catalogue issued in 1883 byFauth and Company, makers of survey-ing instruments in Washington, D.C.,two different types of heliotropes wereoffered. One was a heliotrope producedby the instrument maker WilliamWurdemann which had a telescope withtwo sights and two signal mirrors. It wasoffered with or without graduated hori-zontal and vertical axes. In the same

catalogue Fauth introduced a “PocketHeliotrope . . . a beautiful instrumentthat requires no adjustment” that hadbeen introduced in 1844 by Carl AugustSteinheil (1801-1870), a Swiss physicistbest known for his inventions rangingfrom an electric clock, telegraphic deviceand optical instruments. This instrumentwas sold also by the Washington-basedinstrument maker and dealer George N.Sagemuller for $20, and examples ofwhich were purchased by the U. S.Coast and Geological Survey.

Mirrored GlobesEventually a solution seemed to havebeen found that would eliminate theneed for an attendant. The problem wassolved by means of heliotropes in theform of mirrored glass globes silvered onthe inside; these were substituted in situ-ations in which an attendant could notbe spared for this task. The heliotropewas also found to excel the telegraph forgiving signals, and at one time it wasestimated that it might supersede it.

The brilliant reflection from the glassglobes silvered on the inside by amethod invented by the Germanchemist, Baron Justus von Leibig (1803-1873) favored their employment, thereport continued, since such a hemi-sphere would, for all positions of thesun, throw a reflection to nearly everypoint in the horizon. There was a diffi-culty, however, because subsequently itwas found that the image, although verybrilliant, was too small in diameter to be

readily seen in some situations, and infact in some it was lost, just as the lightof the stars was lost in the daytime.Owing to the perfect figure of the globe,the reflected image of the sun occupiednot much more than half a degree of itssurface, and it was estimated that itwould require a hemisphere of six oreight feet diameter to provide a suffi-ciently large image. Nonetheless, therewere many opportunities for the success-ful use of silvered globes.

Two heliotropes in the form of silveredglobes in fact played a significant role inthe surveying for the five-mile longHoosac Railroad Tunnel through the tal-cose slate rock of the Hoosac Mountainsin Massachusetts. The tunnel, which wasto connect Greenfield, Massachusetts andTroy, New York, was under constructionfor twenty-five years, from 1851 until itscompletion in 1876.

In charge of the project from 1863 untilits completion was Thomas Doane (1821-1897), civil engineer of the BurlingtonMissouri River Railroad, known for hisassociation with railroads. Doane waslargely responsible for the development inthe United States of tunneling withmachinery and high explosives, and healso was a pioneer in the use of com-pressed air machinery. In 1863, uponbeing appointed chief engineer of theHoosac Tunnel, he immediately intro-duced new engineering methods, relocatedthe line of the tunnel, and achieved greataccuracy in the meeting of the borings.

Figure 4 Hilgard’s Reflector. From Report of the Department of the United StatesCoast Survey Showing the Progress of the Survey For the Year 1867.

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Figure 5 Silvered globe heliotrope, patented by E. Varnish Cox, London. Thisheliotrope is housed in Boswell Observatory, Doane College, Crete, Nebraska. Photo by Janet Jeffries, Doane College, September 2001.

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When he undertook the project,Doane ordered a pair of heliotropes inthe form of two matching mirrored glassglobes especially designed to his specifica-tions. The globes were in the form ofspheres twelve inches in diameter madeof very thin glass and silvered from with-in for reflection as a mirror. These globeshad been patented and manufactured bythe London firm of E. Varnish Cox, fromwhich he purchased them (Figure 5).

Doane used the two heliotropes foralignment and to ensure precision indrilling in conjunction with two meridiantransits made by John Hapgood Templeof Boston. During the course of thetunnel excavation from 1863 to 1874,the transits were mounted upon andoperated from brick piers situated ateach end of thetunnel site. Usingthe two meridiantransits with thetwin heliotropes,Doane and hiscrew were able tobore from eachend of the tunnelto its center, withan alignmenterror at the meet-ing of theheadings of just9/16 of an inch! Both of the silveredglobes have been preserved, one atDoane College in Crete, Nebraskawith some of his other instruments,and the other globe was owned by anamesake descendant.

During the same period, the heliotropewas used to a great extent in India forthe great trigonometrical survey. ColonelH. Thuillier stated from experience, “Aheliotrope of 9 inches diameter answersfor 90 to 100 miles. For nearer distancesit is much too bright to be observedthrough a telescope, and the light mustbe diminished in the following propor-tion . . .” Also during the same period,the geodesist Julius Erasmus Hilgard(1825-1891), then Assistant of the Coastand Geodetic Survey and later itsSuperintendent, recommended the use ofthe heliotrope for making stations visibleat long distances because it penetratedthe haze whenever the outlines of thedistant hills have disappeared. He didnot approve of the clockwork-motivatedheliotropes or heliostats, however,because they required the services of anattendant, which could be betteremployed in directing the heliotrope.

Silvered Glass TubesInstead of the use of a silvered globe,Hilgard proposed the use of a series ofglass tubes silvered on the inside,arranged in parallel rows, bent in an arcof a circle of large diameter, to produce acurvature of about 24 degrees. Thesewere placed with the upper part in a ver-tical position so that each tube sent tothe horizon in every direction within arange of 120 degrees a line of light aboutone inch in length which appears nearthe top when the sun is in the horizon,and gradually travels downward as thesun rises, and at the same time movingacross the tube as it changed its azimuth,until the reflection was lost at the bottomwhen the sun reached the altitude of twicethe curvature of the tube, or about 44

degrees, at which time the heat of the daygenerally renders observing impracticable.The joint effect of nine of these brilliantlines, distributed over a breadth of sixinches, was much the same as if a lessintense reflection were evenly diffusedover the same surface, and was readilyseen over a great distance (Figure 4).

In using these reflectors, two or morefaces as described might be set up on acommon base, arranged in regular polyg-onal order or so disposed as to providethe most advantageous reflections in thedirections from which it was to beobserved. Hilgard reported that severalof these reflectors had been used in thefield and found to be quite satisfactory.The tubes were bent into the requiredform by being curved around a woodendisk eight feet in diameter, and they wererelatively inexpensive to construct. Thesilvering was accomplished by knownmethods; he cautioned that the tubesshould not have bores too small for facil-ity of filling them with silvering fluids.The tubes then were arranged in simplewooden frames and secured in positionwith putty or plaster. Eventually theheliotrope in its various forms was made

obsolete during the twentieth centurywith the advent of aerial surveying.

A Resourceful SolutionOn occasion the principle of the silveredglobe was duplicated by surveyors in thefield even when the instrument itself wasnot available. A group of engineers of theTopographical Division of the U.S.Geological Survey, upon finding themselveswithout a heliotrope instrument and beingstranded from a source of supply, as oftenhappened, found a ready solution—by usingtheir discarded beverage containers. G.S.Druhot, a topographical engineer with theU.S. Geological Survey, reported such anincident some years later. He described howit happened in 1925, while he was engagedin mapping in western Colorado as one of a

party of the Survey’sTopographical Division.Under the charge of H.H.Hodgson, they wereextending a net of triangu-lation to control themapping.

The surveyors experi-enced difficulty in seeingsome of the signals of theextended net, and discussedthe practicability of usingsome reflective surface, butrealized that they had nei-

ther a heliotrope nor any other suitablemeans to solve the problem. Then one of therod men, who was fresh from a chemistrycourse in college, suggested a solution.Between them they had several empty spher-ical flasks that were eight or ten inches indiameter, and the rod man tried his hand atsilvering them, with success. These flaskswere then raised by the surveyors on severalmountain peaks and found that they servedtheir purpose admirably. Until, that is, someroving hunters out after game in the regionhad spotted them and used the glitteringglobes for target practice!

Suggestions and assistance received fromDavid Doyle, Sharon Faber and EdMcKay of NOAA, and Steven C. Turner,Smithsonian Institution, are greatly appre-ciated and gratefully acknowledged.(References for this article may be found atwww.theamericansurveyor.com.)

Silvio Bedini is a Historian Emeritusof the Smithsonian Institution. He isthe author of more than 300 articlesand monographs published in schol-arly periodicals, and is presentlycompleting his 23rd book.

“The new instrumentsrequire a man of someintelligence to attend them,and to replace them aboutevery four minutes . . .”

—Ferdinand Hassler, 1837

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