Harry Hess didn't actually *prove* the theory of plate tectonics, but his work with sonar and magnetometers was crucial in providing evidence that supported the theory.

Here's how:

* Sonar: Hess used sonar to map the ocean floor. This allowed him to discover features like the Mid-Atlantic Ridge, a huge underwater mountain range. He also found deep trenches and other features that couldn't be explained by the existing theory of continental drift.

* Magnetometers: Hess used magnetometers to measure the magnetic field of the ocean floor. This revealed a pattern of stripes of alternating magnetic polarity. This pattern, which was later explained by the theory of seafloor spreading, supported the idea that new oceanic crust is being formed at mid-ocean ridges and then moves away from these ridges.

Hess's work, alongside that of other scientists like Alfred Wegener and Tuzo Wilson, contributed significantly to the development and acceptance of the theory of plate tectonics. However, it's important to remember that it's a complex theory supported by a variety of scientific disciplines and data, and the development of plate tectonics was a collaborative effort.