WILLIAM BUSHNELL "Bush" OSBORNE, Jr., as a young graduate forester, went to work for the U.S. Forest Service in 1909 on the Oregon National Forest at Mount Hood.
In 1911 he invented an alidade he called the "firefinder" and tested it at 8 locations in Oregon and SW Washington. The original instrument was 14" across, round, with a map of the surrounding area, and each of 360 degrees etched around the rim. This disc was secured to an 8-sided 1/8" steel base which was in turn secured firmly to a tree stump. A brass sighting mechanism consisting of a rear vertical slit and a front vertical horse hair stretched tight, pivoted precisely in the center of the circle. The geographical location of the lookout point was situated exactly in the center of the circular map. An arrow etched beneath the roar sight corresponded with the compass reading when the sights were lined up on a distant smoke. This first Osborne Firefinder was commercially produced in 1913 by Fred Leupold and Adam Volpel (Leupold-Volpel & Co.) at their scientific instrument manufacturing facility in Portland. The U.S. Forest Service purchased fewer than 100 of these 1913 Model Osborne firefinders.
In 1914 the center-pivot sighting mechanism was abandoned in favor of a circular outer ring with the original fore and aft sights affixed to it. A cast iron base with a round recessed rim permitted the sights to be moved freely around the rim. 0° was positioned true south from the lookout location. To prove this instrument, "Bush" Osborne placed it atop Oregon's tallest volcano, Mount Hood the following summer. In one month lookout Elijah "Lige" Coalman spotted and reported 131 fires with this instrument!
In 1915, Osborne again modified his "firefinder" sighting mechanism. The 1915 model featured a 0-power scope similar to those used on a rifle, instead of the original fore and aft upright sights, to peer through. Precisioned calibration permitted obtaining azimuth readings to l/60th of 1°; and a vertical angle reading in 1/10th of 1° accuracy. Several hundred of the 1915 Osborne Firefinders were produced by Leupold-Volpel Co. In 1917, Bush Osborne again gave his Firefinder a radical change. Its width was enlarged from 14" to 24" across. Its weight was increased from 10 pounds to 70#, including the 3-railed track it sat upon. It featured rugged alidade sights front and back, using the concept of the 1913-1914 models; however, the front sight is equipped with a thin brass tape which could be moved up and down freely with a thumb wheel. As this wheel was turned back and forth while the observer sighted through the rear sight slot at the distant topography, a pencil attached to a sliding gear-driven arm could draw the panoramic features of the horizon. Thus, a panoramic picture could be drawn to accompany "seen area" maps made by the firewatcher. About a hundred 1917 Osborne Firefinders were manufactured by Leupold-Volpel.
In 1934, Osborne again radically changed only the sighting assembly, to include a far more simple mechanism. The rear sight now included "+" and "-" vertical scales and two sets of cross threads of horse hair, so that accurate vertical readings could be made on fires both below and above the lookout's elevation. Leupold-Volpel Co. (known since 1942 as Leupold & Stevens) manufactured more than three thousand of the lighter weight 55 pound 1934 Osborne Firefinder, until it ceased to produce the instrument in 1989. For a time prior to that, the A. Lietz Co. of San Francisco also manufactured the Osborne Firefinder in small quantity. The 1934 Osborne Firefinder is the most widely used fire plotting instrument in the world today. It is in use across America, and in many foreign nations on at least four continents. They were last featured for sale new in the 1991 Forestry Suppliers catalog for $3,495 each.
by Ray Kresek, Fire Lookout Museum, Spokane, WA
The Vertical Angle scale on an Osborne Firefinder was meant for use in conjunction wih the Panoramic Photo set for that lookout. The set of three panoramic photos have a very thin horizontal line which represents 0, or exact same elevation on distant hills as that of the lookout. The sliding peepsight on the rear sight registers + and - readings. + would be higher than you; - lower. This reading, along with the azimuth reading on a smoke base allowed the dispatcher to pull the Pan set for your lookout, and with a transparent scale, plot the exact location on the photo. These photo sets measure 6" tall by 36" wide; contact prints same size as the infrared film negative; incredible imagery, for the 1930s when most of them were filmed. With a magnifying glass, dispatchers could actually see the single tree 4 miles away that was burning! It was so good, I don't know why they don't use it anymore....
The metal tape extending across the map on the Osborne is nothing more than a steel tape measure, in inches starting at the center (lookout location). When using a standard 1/2" scale Forest map, it is easy to determine distance to the smoke.