By Jim Gehring
Originally published in The Tool Shed, Number 120 (February 2002)
In its most general sense, the term "inclinometer" refers to any device for measuring with precision the angle between a surface and a line that is perpendicular to a line through the earth's center of gravity – or, in other words, level. The difference between a level and an inclinometer is that while a level tells the user whether or not a surface is level (or plumb), an inclinometer tells the user exactly how far out of level (or plumb) it is.
In addition to ones used by carpenters or machinists, various forms of inclinometers are widely used in other fields – by plumbers, surveyors, miners, skiers, artillery gunners, sailors (to tell how far over a vessel is listing), and even chiropractors, who use a device called an inclinometer to measure the angles of the spine. Today we tend to think of "inclinometers" as carpenters tools and "clinometers" in the context of surveying or scientific instruments. However, the nineteenth century meaning of the terms was exactly the opposite. Knight's comprehensive American Mechanical Dictionary , published in 1877, defined an inclinometer primarily as an instrument to detect the inclination in the earth's magnetic field, and gave as the best known example the "dipping needle," a form of vertical compass widely used in mining. A secondary definition given was that of an instrument for measuring the slope of an embankment, incline, using a level attached to a "base piece which lies on the slope."
The same work defined clinometer primarily as an instrument used in determining the slope of cuttings and embankments, illustrating the definition with an instrument very similar to many of the gravity inclinometers illustrated in this article. The now obsolete “batter-level” was given as a synonym. A secondary definition for "clinometer" was a "carpenter's tool for leveling up sills and other horizontal framing timbers." In fact, the terms "inclinometer" and "clinometer" have become more or less synonymous, and this article will use the term "inclinometer" exclusively.
Inclinometers fall roughly into two categories: those that use a gravity-actuated pendulum or other indicator, and those that use a spirit vial. (The use of the term "gravity" to describe only the former class of tools is somewhat inaccurate, since all inclinometers are based on gravitational force, but it is a well established and convenient convention.)This article will concentrate on gravity inclinometers. Gravity inclinometers are direct descendants of the oldest form of leveling device, the plumb bob. A plumb bob can be converted to a level by attaching it to a horizontal bar to form a plumb square, and from that it only requires the addition of a scale along the horizontal bar to measure the exact grade of the slope on which the horizontal bar rests. Although a plumb square is accurate, it is not particularly convenient or portable, and one of the principle themes of the development of gravity inclinometers has been the attempts of inventors to adapt the mechanism of the plumb square into an instrument that can be safely carried and easily used.
One possible approach is to enclose a small plumb bob in a case mounted on the level stock. Figure 1 shows a traditional plumb square, along with a user-made device that includes a small plumb bob in the teardrop shaped case that folds flat against the stock for carrying and can be rotated to determine the slope of the surface. It lacks only a scale to make it a true inclinometer. As far as I know, the only commercially produced inclinometer to use this enclosed pendulum mechanism was patented in 1887 by Oscar B. Fuller of Burlington, Kansas, and manufactured by the Pittsburg Novelty Works in Pittsburg, Kansas. The term "novelty" is particularly appropriate in this case as the level is quite rare; a contemporary advertisement for it is shown in figure 2, and a photo of the level itself (from Don Rosebrook's collection), is in Sandor Nagyzalanczy's The Art of Fine Tools. (My thanks to Roger Smith for sending me this ad, and the one for the Melick Clinometer in figure 12).
Most gravity inclinometers utilized some form of rotating pointer or weighted wheel in lieu of the inherently fragile enclosed plumb bob. Some of the earliest examples of the rotating pointer are found in inclinometers manufactured by Rufus Porter, the inventor, artist and Scientific American founder, who is sometimes referred to as the American da Vinci. Porter inclinometers (see figure 3) have a distinctive square shape and were decorated with an engraving by Porter showing the device in use. They are highly prized by collectors. Although marked "Patent Applied For," it does not appear that Porter actually received a patent for this device.
The weighted pointer rotating around a central pivot point in the middle of a scale was used in several other gravity inclinometers. Some of the patented variations of Porter's basic design are illustrated in figures 4, 5 and 6. William Gibson of Enid, Oklahoma Territory, patented a "Combined Level and Plumb" in 1907 (figure 4). It used a matched pair of pointers supported by a central bearing to overcome the problem of friction between the point and the face of the scale and to produce an inclinometer that could be read from either side. Henry Loeschner of Fredericktown, Missouri, patented a "Protractor Level" in 1921 (figure 5), in which the needle was suspended from the top against a semi-circular scale, and which also included four separate level vials (three of which are missing in this example) located with the inclinometer in a separate compartment that could be removed from the level stock for independent use.
James T. Reed of Akron, Iowa, patented a level in 1923 that was subsequently manufactured by the Union Gravity Level Co. of Sioux City (figure 6). The patent papers describe a rotating needle suspended by bearings in the glass cover plates, and a worm gear adjustment mechanism. However, the actual production model suspended the rotating pointers through a central brass disc on which the scales are engraved. The level also features a stock made of aluminum rather than wood or cast iron.
It will be noted that all of these levels were patented and manufactured by small entrepreneurs located outside of the toolmaking hubs of New England. Neither Stanley nor Starrett ever manufactured a gravity inclinometer of this type. Perhaps the most successful of the rotating-needle type inclinometers – at least in terms of surviving examples – was Edward Helb's "Combined Level and Grade Finder" patented in 1904. Helb's device (figure 7) which was manufactured in Railroad, Pennsylvania, and is often mistakenly identified as a tool used primarily in laying railroad tracks, used a simple rotating needle against a cardboard scale functionally identical to Porter's. However, it had a much more elaborate scale that purportedly allowed the user to perform various elaborate calculations, as shown in the excerpt from the instruction book included in figure 7, and also featured a sighting-tube and compass for surveying purposes.
The weak point of all these inclinometers was friction at the point at which the needle rotated (and in some cases between the needle and the face of the scale), which caused inaccuracy that would only increase as the instrument was used. The needle was also susceptible to damage when the tool was transported. Various methods of overcoming one or both of these shortcomings were also tried. Alvan B. Ewing of Lewisburg, Tennessee, patented a "Plumb-Level" in 1889 (figure 8) that used a heavier pendulum enclosed within the stock of the level with a pointer that could be read through a slot in the top (or a window in the side for plumb applications). Instruments that used a heavier pendulum suspended from bearing points in the glass covers, such as the apparently unpatented example shown in figure 9, which was manufactured in Decatur, Illinois, were more accurate but even more susceptible to damage during transport. Perhaps the most extreme example of accuracy at the expense of delicacy is the "Hight Micrometer Level" manufactured by the Automatic Level Co. of Cleveland, Ohio (figure 10), which used a suspended plate inside a large enclosed semi-circular scale. The Hight level, which was apparently never patented, also included sights and was probably intended more for surveying than carpentry.
The device that used the most elaborate mechanism for insuring smooth and accurate operation of the rotating pointer was the "Gravity Level" invented by Thomas F. Deck of Toledo, Ohio, which was covered by two separate patents issued in 1896 and 1905. The Deck inclinometer (figure 11) used an elaborate set of internal roller bearings, and also featured a locking button that could be used to lock the needle in place to prevent damage during transport. The second Deck patent provided for an internal pendulum suspended below the center of the scale, with the motion of the pendulum conveyed to the rotating needle through a gear train.
The best and most practical of the gravity inclinometers – as well as the only one to be so described by its inventor - was the "clinometer" invented in 1889 by William B. Melick of St. Louis (figure 12). In lieu of a needle rotating against a fixed scale, Melick's instrument contains a rotating scale and fixed pointers. The scale is powered by an internal pendulum weight fastened to the back, and is suspended from a bearing supported in the front by the diagonal cross-piece. There were also two buttons (only one shown in the patent drawings) that could be depressed to lock the scale in place for easier reading, although they did not allow it to be locked during transport (unlike the Deck design). This was an altogether sturdier and more practical design, and was marketed both in a traditional wooden stock (see the advertisement at the bottom of figure 8) and in an elaborate cast-iron mantle clock design.
Finally, mention should be made of what I believe to be the only gravity inclinometer that was specifically designed to be mounted on another level – the "Attachment for Levels" patented in 1908 by David Southward and manufactured by the Superior Level Co. of Pittsburg, Pennsylvania (figure 13, right). The Pittsburg (Kansas) Novelty Works manufactured a variation of the Fuller patent that was designed to be mounted on a rule, but it lacked an inclinometer scale. Although spirit-levels designed to be mounted on a rule are common, the Southward attachment, with its thicker bracket, was specifically intended to be mounted on a common wood-stock spirit level. The design of its pendulum and pointer are actually superior to most of the full-size inclinometers shown above. Next to the Southward patent in figure 13 is the most modern gravity inclinometer in my collection, a mantle clock design which appears to date from the 1940s or 1950s. It is marked "Jos. Behr & Sons, Inc." of Rockford, Illinois, but I don't know whether that is the manufacturer or whether it is an advertising level.
Although gravity inclinometers are fascinating in their variety and the ingenuity of their inventors, they were destined to be supplanted by devices that used the increasingly accurate spirit vials that became available near the end of the nineteenth century, just as the spirit level supplanted the plumb square. In the next article I will describe inclinometers that used different types of spirit vials.