The third, viz. that the squares of the periodic times are proportional to the cubes of the mean distances... History of Astronomy - Page 53 by George Forbes - 1909 - 200 pages Full view - About this book
...times. And he had found, near the end of his life, a third law connecting the different orbits. (3) The squares of the periodic times are proportional to the cubes of the semi-axes major (or of the mean distances). 81. Attempts had even been made to explain the planetary...
The third, viz. that the squares of the periodic times are proportional to the cubes of the mean distances... 
