Signed in Script
W. & L.E. Gurley Co., Troy, N.Y.
It is sized between the large Gurley vernier compasses and their smaller pocket-sized counterparts and has sights a whopping 9 inches tall. Editions of Gurley's Manual were researched for every decade between 1855 and 1941 and none were found to show or describe this instrument. It is difficult to estimate the year of manufacture although it seems to be no earlier than the 1870's and is probably later. It has no serial number and there are few dependable guidelines for dating this unique piece.
The magnetic needle is 4 3/4 inches long. By comparison, Gurley's large-sized vernier compass with 6-inch needle has sights that stand 7 3/4" high. The south vane on this compass has two swing-away peep sights, one at the bottom and one at the top. The north vane has divisions to plus or minus 38º of arc engraved in the sides for determining vertical angles. The corresponding ranges on the Gurley large vernier compass with 6-inch needle are only plus or minus 22º, or about 58% of the vertical angle range this has.
Gurley's large vernier compasses have sighting arms of lengths between 12 1/2" and 15 1/2" while the pocket vernier compasses have no extended sighting arms at all. This instrument has extended sighting arms of total length of 9 inches giving it 50% greater sight radius than that of the largest corresponding pocket compass. With the folding sights it seems to be something that would be carried in a leather field case with shoulder strap. Its intended utility is in mountainous or mining work where steep vertical angles are encountered, but the compass has other advantages as well. It has much sturdier construction and greater accuracy than the pocket instruments, yet is easier to transport in the field than the large compasses with their mahogany cases. It thus offers a practical middle ground between portability and accuracy when used in more conventional applications.
The compass weighs 6 pounds. This compares to 10 1/2 pounds for Gurley's large vernier compass with 6 inch needle and 2 3/4 pounds for their largest pocket vernier compass. There is a socket on the underside center of the sighting base that fits Gurley's standard large compass spindle for use with a tripod or Jacob's staff. There is also a rack-and-pinion operated declination scale that reads by vernier to one minute. It is set by a knob also located on the underside and a second knob locks the setting. A third knob on the underside lifts the needle off the center-pin. There are two hexagonal spirit levels located inside the compass box and both are full of clear fluid. The compass dial with half-degree divisions is in particularly nice condition.
There are no small tools, case, stand, or ball joint spindle.
