Galileo discovered that the speed of a ball rolling down an inclined plane increases at a constant rate, meaning it gains the same amount of speed each second. This is the foundation of what we now call uniform acceleration.

He didn't quantify the exact amount of speed gained per second, as that depends on the incline's angle. However, his experiments demonstrated that:

* The speed of the ball increased consistently over time.

* The steeper the incline, the faster the ball accelerated.

This groundbreaking observation led to the understanding of gravity's influence on falling objects and paved the way for Newton's laws of motion.