Beautiful Sission Theodolite Circa 1740

Starting in the early 16th Century Mathematical Practitioners envisioned an instrument that could measure both horizontal and vertical angles. Such an instrument is pictured in the illustration to the below left - the illustration was published in 1512. The instrument pictured to the below right was made in England in 1590. These dual purpose instruments were unsuccessful from a commercial perspective, however. I suspect that these somewhat unwieldy instruments could not compete with a much more managable Second Generation Dou style Theodolite, which when hung vertically, could measure vertical angles perhaps more accurately that the dual purpose instrument. Plus, I also suspect there wasn't much demand for vertical measuring devices, so an unwieldy dual purpose instrument would have been overkill most of the time.

This equation hold true until roughly 1725, when Sisson placed a telescope on top of a Second Generation Theodolite, and Wyld thereafter published a book showing the instrument. The Third Generation - Altazimuth Theodolite was born. This wonderful dual purpose instrument slowly replaced its competitors - Second Generation Theodolites, Circumferentors, Plane Tables, etc as the Surveyor's instrument of choice. John Dollond's commercialization of the Achromatic lens in the 1750s, which made telescopes much more useful, meaningfully enhanced the value of Telescopic Altazimuth Theodolites. Along with the dual purpose American Transit, the dual purpose Third Generation Theodolite dominated the market for surveying instruments in the 19th and 20th centuries.

George Adams, the famous English maker of instruments, stated the value of Telescopic Altazimuth Theodolites very bluntly in his late 18th century books/catalogues:

"Theodolites with telescopic sights are, without doubt, the most accurate, commodious, and universal instruments for the purposes of surveying, and have been recommended as such by the most expert practitioners and best writers on the subject, as Gardiner, Hammond, Cunn, Stone, Wyld, Waddington, &c."

Telescopic Altazimuth Theodolites are wonderful instruments, especially those made between 1725 and 1800. As discussed below, you see a good deal of variation in the pre-1800 Third Generation instruments. The variations pretty much disappeared by 1800, however, when the popular form of the Theodolite took over and almost all the Theodolites looked alike for the next 75 to 100 years.

Take a look at the pics below. You will see that the early Telescopic Altazimuth Theodolite makers basically replaced the sight vanes from a Second Generation Theodolite instrument with a telescope and vertical arc setup. This created one fairly common early variation - a convertible instrument. With this variation, a surveyor could use the theodolite either with the two or four sight vanes to measure just horizontal angles, or with the telescope to measure both horizontal and vertical angles. Please take a look at my Circa 1760 Cole Theodolite to see a really nice Convertible Third Generation Theodolite.

Another variation found on pre-1800 Third Generation instruments - the different vertical arcs used. Compare the arcs on the instruments pictured below. The arc's range from +/- 30 degrees to 270 degree arcs, with some of the arcs upside down.

The base of the instrument was yet another variation found on pre-1800 Third Generation instruments. Compare the bases of the instruments pictured below. Some have a flat plate base, while others have spoked bases.

Finally, note the difference in the amount of gearing each instrument offers. The more precise and therefore expensive instruments have gearing, while the lesser models don't have gearing.

Below are some of the important books related to the Third Generation of Theodolites.

Rathborne's1616 book shows an altazimuth instrument but unfortunately the book doesn't describe the instrument. As noted above, while some altazimuth instruments were made in the 17th century, the instruments didn't really catch on.

Altazimuth theodolites became commercially viable when makers added a telescope and vertical arc to the basic design of the simple theodolite. Two books published 6 years apart were instrumental in evolving theodolites from a horizontal angle measuring device to a full blown horizontal and vertical measuring device. Wyld first published his book in 1725 that set forth the basic design of an altazimuth theodolite. Not to be outdone, Hammond published his version of the altazimuth theodolite in the 2nd Edition of his book in 1731.

I also included a book by George Adams, the famous English maker of fine instruments. The book is his 1797 second edition of Geometrical and Graphical Essays, which discussed a variety of instruments, including the Second Generation and Third Generation Theodolites. (Adams refers to these as Common and Improved Theodolites, respectively, and discusses the Common Theodolite on page 223 et seq. and the Improved Theodolite on page 231 et seq.)