Evolution of the Transit

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Article taken from "Backsights" Magazine published by Surveyors Historical Society


by Dale R. Beeks

Of the varying types of surveying instruments available to collectors today, American transit instruments seem to be the most abundant.

The predecessor of the American transit was the prominent English theodolite, an instrument first appearing in the early 16th century, and the circumferentor compass, a variation of the American vernier compss.  The notable Philadelphia manufacturer, William J. Young, was the self-proclaimed, and generally accepted, inventor of the first American transit in the year 1831.  Young's new transit enabled the perpetuation of a straight line by means of turning, or "transiting" the telescope on its horizontal axis.  The theodolite was incapable of this simple motion, the telescope having to be reversed in its wye yokes, or a 180-degree horizontal angle turned to continue a line forward.  Young also eliminated the cumbersome rack-and-pinion motions associated with the early theodolites and opted for a clamp and slow motion tangent screw arrangement.

So successful were the sales of Young's transits in 1835 that in order to keep up with the demand he had to have several instruments made to his specifications by an English manufacturer.  Of the three dozen built, only a few proved to be usable.  The others were dismantled due to various problems, including iron fragments in the brass.

The evolution of the transit instrument brought about a few recognizable design changes that can assist the collector in dating instruments.  Early forms, from 1830 to the mid-nineteenth century, can be identified by several features.  Telescopes were fit with crosshair-adjusting capstan screws located toward the center of the telescope.  The telescope could only transit in one direction due to the longer length of the tube on one side of the axis.  Horizontal plates and housings were flat, and the vernier and graduated circles were often viewed through a cut-out within the compass housing.  Both upper and lower motions were fit with clamps and directly connected slow-motion tangent screws.

Young's early transits were fit with bull's-eye type level bubbles and outkeepers, the latter used to tally the number of chains measured, and his verniers were graduated to three minutes.  Most mid-nineteenth century instruments began incorporating horizontal circles and verniers viewable outside the housing.  Full vertical circles fit with clamps and slow-motion tangent screws and level tubes slung parallel to the telescope became numerous.

These few developmental features of instruments made from 1831 through 1875 were not confined to any specific time period.  Early features can be found on instruments dating from later periods.  Therefore, dating instruments by only observing one feature may give inaccurate results.

The last quarter of the nineteenth century saw a standardization in transit design.  Spring-opposed screws were invented in the year 1871 by Heller & Brightly and were incorporated by most manufacturers after 1885.  Crosshair capstan screws and primary optics tube adjustments were located closer to the eyepiece.  In most late-19th century examples, the heavy upper plate of the leveling base was eliminated and replaced with a cast lattice frame to support the screws.  These later recognizable features continued into the first half of the 20th century.

The transit, thoroughout its years of development, lent itself well to adaptations for specialty use.  In 1867 William Schmoltz adapted William Burt's solar attachment to the transit.  This allowed for location of the true meridian by solar observations, therefore bypassing the often inaccurate magnetic readings.  In 1869 Benjamin S. Lyman devised a solar apparatus that fir below the horizontal plate of the transit.  An inclined standard transit instrument with a solar attachment over a compass (somewhat like Burt's original solar compass design) was invented by R. R. Siebert of the U. S. Coastal Survey.

Other solar attachments were devised by various American inventors and manufacturers, including Buff and Berger (Pearson's, 1882), Brandis (1881), J. W. Holmes (1887), Gardam's (1881), Walter Scott's (1890), and Davis' solar transit (1894).  Two of the more popularly designed solar attachments, aside from the Schmoltz-type, are Smith's (1880) and Saegmuller's (1881) attachments.  Both were telescopic attachments, as opposed to peep-sights.  The Smith solar attachment fits onto the side of one standard, and the Saegmuller fits atop the existing telescope.  Both continued to be used by government agencies throughout the middle of the 20th century.

Another series of rare adaptations of the transit evolved for use in mines, since mine surveying presented special problems.  The conventional transit was unable to give readings of extreme vertical angles up and down steep shafts and slopes, because the line-of-sight would be obscured by the horizontal plate.  This problem led to some pretty radical changes, all of them designed so that the line-of-sight would bypass the horizontal plate and allow for vertical sightings.  Some of the more scarce designs include the inclined standard transit, Blattner's hinge-standard model, and Buff and Berger's duplex telescope-bearing transit designed in 1889.  Both instruments were fit with telescopic standards that extended beyond the vertical plane of the horizontal circle.  One form of mining instrument that gained popularity, probably due to its ease of ability to convert, was the auxiliary, or double-telescope mining transit.  This was basically a standard from of transit with a detachable second telescope and counter weight.  The points of attachment were at the ends of the telescope axis, outside the standards.  Other designs included attachment of the auxiliary telescope to the existing telescope as well as to the outside of the standards.

Other forms of mid-to-late 19th century transit variations include precise instruments used for geodetic, tunnel and astronomical applications.  These are usually very large instruments and, due to the fact that they were never made in large quantities, are very rare today.

Finally, materials used in the construction of instruments are of importance for identification purposes.  Although most transits primarily were made from brass, many manufacturers offered instruments made from aluminum, adding that they would cost 50% more than the standard transit, and that aluminum had several drawbacks, for example, poor bearing durability, difference in the coefficients of expansion, and cost.  It appears that few aluminum instruments were made prior to the use of aluminum alloys which began around 1900.  Examples were claimed to have been manufactured by the firm of W. & L. E. Gurley of Troy, New York, as early as 1875.  Very few examples exist today.

Many mid-to-late 19th century American transits are still available to the collector today, and while all the aforementioned specialized instruments are very rare, recognizing these forms may be advantageous for future highlighting of quality collections.



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