20th Century Gunner's Quadrants

I attended a number of talks at the XXV Scientific Instrument Symposium in Krakow (2006) which discussed the beautiful and elaborate 16th century gunner's quadrants or levels. However, the shear beauty, decoration and craftsmanship of these devices called into question whether they were actually used for combat or more for prestige and "gentlemen's" entertainment. The use of a quadrant for artillery in combat was further brought into doubt since very few unadorned instruments have been discovered from early historic weapons collections.

Since I own a couple of 20th century gunner's quadrants of fairly high precision I offered to provide data on these instruments and ask some friends as to whether they were in fact used to the levels of accuracy and precision implied by their manufacture.

The photos and measurements I made for my instruments are shown below for a Model 1918 (WW I?) and a Model M1 (WW II). As to their use I provided the paraphrase of an interview I made of a colleague who had used quadrants during the Viet Nam era, which I reproduce below the photos.

Both instruments consist of a cast bronze frame with machined seats for precision steel registration blocks. The instruments have a total range of 1600 mils (90°), 0-800 mils on the front (shown for both instruments, using the two registration blocks on the bottom in the photos) and 800-1600 mils on the back, as shown for the Model 1918 instrument (using the other two registration blocks now on the bottom in this photo). Both instruments were originally finished in heavy "military green" paint, however the M1 was stripped many years ago to show the underlying bronze. The scales of both instruments have a nickel-silver finish and are attached with brass rivets. The dimensions for both instruments are given in the labeled photo for the 1918 model.

It is interesting that the Model 1918 quadrant was manufactured by Central Scientific Co. (Cenco), a major supplier of scientific and science education equipment in the 20th century, apparently aiding U.S. military efforts by using their facilities to make these precision instruments. Finally, I have seen nearly identical European quadrants at military surplus stores in the U.S. in the 1970's.

Model 1918 Gunner's Quadrant

The instrument is operated by pushing in the plunger on the arm and aligning it with the scale and releasing it to engage the teeth on the arc. The scale indicates the angle by 10 mil divisions. The fine-scale on the curved arm is graduated every 0.2 mil with each whole mil numbered. The final setting is made by sliding the level vial along the arm to bring the bubble into the center. Adding the reading on the arm to the arc value gives the final reading.

Model M1 Gunner's Quadrant

Photo of Quadrant in 0–800 mill orientation, level vial cover closed

The Model M1 is operated as described for the Model 1918 above, except that the micrometer dial is used for fine settings instead of sliding the level along the beam.

Use with Artillary

According to Dr. Milton Boyd, Emeritus Professor of Biology at Humboldt State University and a former U. S. Army artillery officer and artillery instructor in the Viet Nam era, the gunner’s quadrant was used both predictively and to maintain range.

Basically he said there was a steel plate attached to the side of a howitzer (both towed and mobile guns) where the quadrant would be placed by the gunnery sergeant. I don’t recall the details of the gunnery crew, but the short of it is that after being given the range to a target a special purpose slide rule would be used to calculate the angle. The quadrant would then be set to the angle and used to adjust the gun for a “range shot.”

Of course theory rarely gives a shot right on target, so the gun would then be readjusted with the quadrant based on the range shot to reset the gun “for effect.” Once the gun was accurately set for range the quadrant was used between shots to maintain the proper setting of the gun. This is necessary as, particularly for towed guns, the gun moves with each shot and has to be rest.

Dr. Boyd assured me that the quadrants were used to their full precision (they are graduated to 2/10 of a mil, with 6400 mil to a circle, 1 mil =3.375 arc min, so 2/10 mil is approximately 40 seconds of arc). In rapid fire, after the range is accurately set, the gunnery sergeant would then just check the gun periodically with the preset quadrant, having the gun reset if necessary to bring the bubble level into position.

So, at least in the twentieth century, the gunner’s quadrant was used as a mathematical instrument to set guns according to predication, to nudge the guns onto the correct range based on how close the range shot hit, and finally, to maintain the range once accurately set.