To study motion of object on inclined plane Galileo performed two experiment. First Experiment :
As shown in figure, he arranged two plane at same inclination and allowed spherical object to move along incline following were his observation.
$(1)$ A sphere moving down on a incline has accelerated motion hence its velocity increases.
$(2)$ A sphere moving upward on incline has retreated motion hence its velocity decreases.
$(3)$ For sphere moving on horizontal surface it is intermediate condition from this Galileo concluded that a body moving on frictionless horizontal surface there is no acceleration or retardation. He concluded that body will continue motion with constant velocity.
Second Experiment :
As shown in figure, two plane with some slope are considered.
When a body is allowed from rest to move on the slope its velocity increases and on opposite slope its velocity decreases.
If both plane are smooth then height attended on second plane will be equal to height of first plane from where it was dropped. (It can be less but can never be more the height from where it was dropped.)
In ideal condition when there is no friction height attained by ball will be equal to initial height.