Newton’s Gravity Law.- Newton proposed the idea of a invisible force called gravity which attract every object to each other in the universe. He discovered that this force is inversely proportional to the square of the distance between them, and directly proportional to the product of their masses.
To proof this, he used a device called the Cavendish weights. The Cavendish weights is formed by two spheres with the same mass (normally made of gold or platinum), attached to the ends of a light horizontal bar, suspended by its middle with a thin quartz string. A small mirror attached to the string reflects a light beam over a scaled ruler. To use the weights you need two big spheres (usually made of lead). You approach both of the lead spheres to the two gold spheres. The weights will turn and point the light beam to the correct number.