Basically, the user fires a projectile—in Robins’ day, a cannonball or musket shot—at the pendulum. The faster it’s moving, the farther the pendulum will swing, pulling measuring tape along behind it. Robins determined that the distance the pendulum swings, along with its weight and that of the projectile, can be used to figure out the projectile’s velocity. Bullet speed doesn’t interest me, but a ballistic pendulum can measure the speed of any object—including a baseball. With baseball season in full swing, I set out to build one.