Construction Details of Rittenhouse Compasses

In the field of American surveying instruments, the name Rittenhouse commands a great deal of attention and well-deserved respect. Unlike earlier Colonial makers, the number of examples of the work of brothers David and Benjamin Rittenhouse is sufficient enough to allow study of developmental characteristics.

From an original list of known Rittenhouse instruments in museums, as well as those in private hands provided by Deborah Warner of the NMAH (National Museum of American History, Smithsonian Institution), further research has revealed additional instruments. To date, 65 instruments have been documented. Based on personal experience in disassembly and inspection of 20 Rittenhouse instruments, a questionnaire was developed to provide a database intended for categorization of similarities of construction details into approximate time periods. As of this time, 40 questionnaires have been returned.

Most craftsmen "sign" their work in many ways other than with their actual signatures. This unwritten story can hold evidence, much like that of the written one contained in the shop records of the clock and instrument maker Daniel Burnap. Unwritten clues may be present in the instruments; one must dig deeper to uncover them. This article is intended to deal with the inspection of the Rittenhouse body of instruments, seeking mechanical as well as artistic "signatures." This author's frame of reference is that of a craftsman, one who has developed the skills necessary to the instrument maker such as needle and hub making, dividing the needle ring into degrees, and engraving in the 18th century style.


Began His Career as a Clockmaker
David Rittenhouse was born April 8, 1732 in Roxborough Township, Philadelphia County. Around the age of 17 he constructed a clock with wooden gears. His father, recognizing his son's potential, helped David build a collection of tools necessary for clock making. After construction was completed on a small workshop at the family's Norriton farm, David began making and selling clocks. Subsequently it appears that he trained his younger brother, Benjamin, to acquire the skills necessary to help with this line of work. Thereafter both men worked simultaneously in Norriton.

At the request of influential individuals who recognized his potential, based on his earlier computations of the 1769 Transit of Venus, as well as his proposed orrery, David moved to Philadelphia in 1770. His champions felt that David's presence in Philadelphia would be more beneficial to the Colonial scientific community if David resided in Philadelphia rather than in the farmlands of Norriton.

His younger brother Benjamin was born in 1740, and worked with David in Norriton until 1764-1766. When David married in 1766, Benjamin and his parents moved to Worchester and David was then given exclusive rights to the Norriton farm site.

Of all the known Rittenhouse compasses, only two are signed. "D. or David Rittenhouse, Norriton fecit," denoting the Norriton location. None are signed "B." or "Benjamin Rittenhouse" with the Norriton location. One may assume that the Norriton compasses are the earliest known examples of David's work and are probably those referred to during brother Benjamin's period of instruction.

Upon closer inspection of the instruments by David and Benjamin, there are several patterns of construction details that emerge as indicants of the transition from the earlier to the later instruments. The areas of investigation fall into the following categories:

1) Style and mechanical characteristics of the socket on the compass
2) Treatment of the inside vertical wall of the needle ring
3) Signature and engraving style
4) Needle ring engraved number style
5) Sight vane and thumbscrew variations

For illustrations of the terms in this article, see Figure A. There are fundamentally three distinct styles of sockets on all documented Rittenhouse compasses (see Figure B):

1) Fabricated, straight side with straight reinforcing rim.
2) Fabricated, straight side with radiused reinforcing rim
3) Cast, tulip profile socket

The first two styles are constructed in the following manner (see Figure C): 1) Brass is hammered into a cylinder over an anvil or "stake" form. 2) and 3) The joint is cut and filed smooth to make a clean vertical joint, which is then brazed shut. 4) Once brazed, the piece receives its final shape on the lathe. This style is economically made on a "one off" or as needed category. There is no mass production aspect to this technique other than making several at a time.

The third style, or "tulip," is created in the following manner (see Figure D): 1) Beginning with two semicircular cast pieces, two joints are filed smooth to form a full cylinder. 2) They are brazed together. 3) The final shape is formed on the lathe to its curvilinear profile. This style has two vertical joints 180 degrees opposed to each other, in contrast to the earlier styles that have only one vertical joint. This style lends itself to more of a mass production category and is more economical once the manufacturer has sufficient prospects of orders to produce the patterns and molds necessary for a "production run" of many castings to be assembled and finished as needed to fill orders.

Chronological Facts
Both of the two known Norriton compasses signed "D. Rittenhouse" or "David Rittenhouse" have the Style 1 Fabricated Socket—straight side with straight reinforcing rim. Of the two known Philadelphia compasses signed "D. Rittenhouse" or "David Rittenhouse" both have the Style 2 Fabricated Socket—straight sided with radiused reinforcing rim.

When Style 1 and Style 2 Fabricated Sockets are viewed side by side, the Style 2 Fabricated Socket shows a refinement of design perhaps influenced by the more cosmopolitan environment of Philadelphia.

None of the work signed "David Rittenhouse" or "D. Rittenhouse" contain the third or Cast Tulip socket. There is another category of David Rittenhouse compasses which are signed in a manner unlike the ones previously discussed and feature different engraving styles. By contrast, Benjamin Rittenhouse's sockets include the full range of all three socket styles.

Considering the fact that Benjamin's early work most likely was influenced by David's early Norriton work, it is quite probable that David's Norriton style of socket was copied in Benjamin's earliest work. Of the 19 documented compasses signed "Benjamin Rittenhouse," or "B. Rittenhouse," seven have the Style 1 Fabricated Socket and eight have the Style 2 Fabricated Socket, with one of these dated "Fecit 1786." Four have the Style 3 Cast Tulip Socket. One of the four compasses with the Cast Tulip sockets is dated "Fecit Anno 1790," and a second is engraved "John Dewitt 1792" on the reverse side of the main plate. All of the documented Rittenhouse & Potts, Rittenhouse & Evans, and Rittenhouse & Shower compasses have the Style 3 Cast Tulip Socket. This information is subject to future revision as more compasses are documented.

Timeline of Compasses
In an attempt to project a timeline on this group of compasses, the data seem to suggest that Benjamin's earliest work, considering the socket construction alone with his brother David's influence factored in, is represented by the Style 1 Fabricated Socket, followed by the Style 2 Fabricated Socket. Coincidentally, there are two similar Benjamin Rittenhouse compasses; one signed "Phildelphia Couy" and the other signed "Philada County." One of these has the Style 1 Fabricated Socket, the other has the Style 2 Fabricated Socket. According to a useful reference provided by Robert Miller, Benjamin's home in Worcester was located in Philadelphia County until 1784 when this area became a part of Montgomery County. Thus, the two Philadelphia County compasses appear to predate 1784. Because there exists a dated 1786 compass with the Style 2 Fabricated Socket and a compass dated 1790 with a Cast Tulip Socket, one might assume that the transition from fabricated to cast sockets took place somewhere between 1786 and 1790. After 1790, all of the subsequent compasses appear to be constructed with the Cast Tulip Socket. Reference to Bruce Foreman's book, The Clockmakers of Montgomery County, reveals that, "At Worcester he [Benjamin] had several journeymen clockmakers working with him. William Lukens Potts was so employed from 1796 to 1798, followed by Benjamin Evans from 1798 to 1801, and David Rittenhouse (the younger), from 1800 to 1802."

Treatment of the Vertical Wall of the Needle Ring
Because of the limited number of David Rittenhouse compasses available for thorough inspection, the following description of needles and their construction is limited to the instruments of Benjamin Rittenhouse and his associates.

The steps necessary to construct a needle assembly (needle and hub) are as follows (see Figure E): 1) The needle blade is blanked out in the rough, assuring that the center as well as the ends are in proper alignment. 2) The hub is constructed from a brass casting with two opposed ears or horns. There are some exceptions to this style of hub; some Rittenhouse compasses have a circular needle hub with hardened steel centers. 3) The cast center of the hub is drilled out and a hardened polished center is pressed into the casting, allowing the hub and needle to rotate smoothly. As illustrated in Figure F: 1) The hub and needle are assembled together and the needle is final-finished to provide an accurate balance and alignment corresponding to the center pivot and graduations of the needle ring. 2) The needle is bent or swept upward to align the ends with the upper horizontal plane of the needle ring.

There is, however, an inherent shortcoming to the dimensional clearance between the end of the needle and the vertical wall of the needle ring. When the needle is upswept, the clearance between the end of the needle and the inside diameter of the verticle wall of the needle ring is necessarily increased. This property enhances the likelihood of error when the instrument is read by its user.

As a remedy to this dilemma, albeit a minor one, it is possible that a craftsman of Benjamin Rittenhouse standards conceived of a method to correct the situation. Of all the 18 inspected compasses (not including David's for this portion of the study), 11 have the vertical wall at 90 degrees, one of which is dated "Fecit 1786" as seen in Figure G1, and the remaining seven have what we will call a relieved vertical wall as seen in Figure G2. With the relieved aspect of the vertical wall (average angle of 85 degrees), the maker is afforded the luxury of the additional dimension to make and balance the needle. Then, when the needle is upswept, the shortened overall length of the needle corresponds more accurately with the smaller inside diameter of the top surface of the needle ring. As an example, a Rittenhouse and Potts vernier compass with a relieved needle ring has an average clearance of .003 from needle end to inside diameter of needle ring; and a B. Rittenhouse compass with a Style 1 Fabricated Socket and vertical wall needle ring has an average clearance of .020.

It is interesting to note that of the seven instruments with the relieved needle ring wall, none are of the Style 1 Fabricated Socket style; two are of the Style 2 Fabricated Socket; and the remaining five are all of the Style 3 Cast Socket. Due to the difficulty of obtaining this measurement (the glass ring has to be carefully removed), only eighteen of the instruments to date have been recorded with this measurement. The information suggests that sometime after 1786 this relieved wall concept became accepted by Benjamin and his associates as an aid to accurate reading of the degrees on the scale. Hopefully, in time, more compasses will be inspected to confirm or refute this supposition.

Keep in mind that these findings are based on existing instruments that have been made available for study and inspection to the author, as well as information gathered from questionnaires.

Best Examples for Study
However, potential questions remain. Among these are: How does one identify a Rittenhouse needle to differentiate it from a replacement? No needles inspected to date (many of which were viewed under a stereo zoom microscope) have engraved names or initials as signatures. The Benjamin Rittenhouse needles that I place into the original category have a consistency regarding the degree of delicacy to the hand work that was generated by the craftsman doing the work. The best examples for study are the refined curves of the horns on the hub as well as the profile end of the needle, as seen in Figures H1 and H2. Most telling is the beautiful undercutting radius to the bottom of the needle which gives a knife edge to the decoration without sacrificing strength (see Figure H3).

Many "replacement" needles are missing these details of refinement and are quite cumbersome in comparison to those of the more skilled craftsman, whose quality of work is often invisible to the owner but resides as a tangible reward in the heart of the original maker.

This research is in its preliminary stages. Many important compasses are maintained in museums whose curators have yet to reply to the questionnaires, due undoubtedly to an unavailability of personnel. As more instruments fall into the documented category, the statements and conclusions made in this article may require later revision. I would like to thank all the private owners, institutions, and museums who did return the questionnaires that provided the data upon which my conclusions are based, and a special thanks goes to all those who sent photos.

If anyone reading this article owns or knows of a Rittenhouse compass that has not been documented, please contact me at for a questionnaire. I will at a later date consider other aspects of these compasses such as engraving styles, numbering and division styles, thumb screw variations, as well as other 18th and early 19th century American makers. Currently I am working on an illustrated book on the art of early American surveying instruments.


Jeff Lock is a professional restoration specialist working in wood and metal with thirty years experience in the trade. He is currently doing historical research and documentation on significant Colonial and early American surveying instruments. He can be reached at for questions or comments regarding this subject.

Note: All illustrations by author.

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