Archimedes, while sitting in a bathtub, notices that his body becomes lighter as it pushes the water aside. This leads to the first true theory of buoyancy. (c. 250 BC)
Eratosthenes evaluates the diameter of the Earth by comparing the length of the lengthiest shadow of the day with the distance between that location and a place where the sun shines to the bottom of the well at midday (240 BC)
Ole Rømer uses the timing of the eclipses of the moons of Jupiter with respect their distance from earth to estimate the speed of light for the first time. He yields a value of 225,000 km/s (Actual value of 299,792 km/s) (1672)
Guglielmo Marconi demonstrates that radio signals can travel between two points separated by an obstacle. Marconi's servant is behind a hill 3 kilometers away and fires his rifle upon receiving the signals (1895).