One of the many challenges in using artillery is hitting the target! It’s not as simple as pointing a cannon at something and shooting. You have to take into account a lot of variables, including equipment; distance to target; ammunition; and the weather.
Gunner’s Quadrants are instruments that help you deal two of these, equipment and distance to target.
A Gunner’s Quadrant is used to measure the inclination of an artillery piece. Inclination is the angle of the artillery piece’s bore from level. Think of a cannon’s tube, or barrel, as a large straw. The inside of the straw is the tube’s bore and it’s always cylindrical. Since it is cylindrical, you can use either the top or bottom of the bore to measure its angle from level. “Level” is usually the plane a piece of artillery sits on, be it on open ground or in an emplacement of some sort.
Knowing inclination is important because it helps the Gunner, the person who’s usually in charge of an artillery piece, to “point” (aim) the cannon. If the Gunner wants to shoot at something far away, they “elevate” the tube. This means they raise the front of the tube to a higher angle. If you want to shoot at something close, you “depress” or lower the tube.
Artillerists and ordnance officers spent centuries figuring out how to make artillery more accurate. Early artillery was pretty crude and primarily used at short ranges. In an effort to shoot at things farther away and consistently hit them, artillerists learned they had to standardize equipment and processes. This meant eliminating some of variables mentioned earlier so you can predict what will happen when you shoot projectiles out of your cannon. They did this by imposing standard bore sizes; standard weights for artillery tubes; standard sizes and weights for projectiles; standard ways of making propellants (black powder) and breaking the propellants into small, usable grain sizes. Once you have some standardization, reducing or eliminating some of the variables, you can figure out how to hit targets with your artillery reliably and consistently.
Ordnance experts went on to create “gunnery tables” that detailed exactly how much propellant to use and the specific elevation or depression angles for each type of artillery piece in service to hit targets at predetermined distances. (Then you had to estimate your distance to the target, which is an entirely different matter…)
Now that you know you need to put a cannon’s tube at a certain angle to hit a target at a specified distance, how do you measure that angle? This is where the Gunner’s Quadrants come into play.
The quadrant is a simple device. It has a face that is 1/4 of a circle (hence quadrant), with reference lines marked along the two straight sides at a right angle and a scale marked to show every degree between 0 and 90. At the intersection of the two reference lines there is either an attached post or a hole drilled, where a plummet (a weight, also known as a plumb bob) is affixed. The line for the plummet is long enough to allow it to fall just beyond the curved edge of the face. A staff is attached to a straight side of the face, the one that’s marked with 90.
The staff is placed inside the artillery piece’s tube at the muzzle, so that the face hangs down just in front of the muzzle. The plummet is allowed to hang from the post or the hole. Once settled (no longer moving), you look at where the plummet’s line falls along the scale. The number it lands on is the inclination (degrees of elevation).
For example, if the tube is level, the Quadrant will read “0”; if it points straight up in the air, perpendicular to the ground (level) it will read “90”.
Although the Gunner’s Quadrant is a simple device, using it is not. Consistency is key. You have to place it in the bore in exactly the same place every time. And use it enough to be familiar with it, the plummet, and the scale. You also have to make sure the plummet settles, ideally stops moving, before taking a reading. You have to do everything the same way every time you use it.
This is easy in theory, incredibly difficult in practice. Especially under combat conditions when you’re in a hurry to make a good shot and someone is shooting back!
Despite the difficulties, Gunner’s Quadrants (along with other pointing devices) made it possible for artillerists to greatly improve accuracy and precision of shooting. They may not hit the target every time but they could hit it more consistently than ever. Gunner’s Quadrants were a standard piece of equipment issued with every American cannon and mortar until replaced by either pendulum type sights or specialized sights with screw mechanisms that incorporated both windage (side to side movement) and elevation (inclination) into one device. Even then, quadrants and later inclinometers remained on hand as backups to the improved sights.