... The gravity-gradient force on the Moon. 4b Craters of the Moon--why round, why middle higher. ... Intuitive concept of 2nd law--planet or satellite slows down when further away, relation to conservation of energy and analogy to motion of thrown stone. 12a Example of non-algebraic equation (Kepler's) . ... Principle of rocket motion. ... Design of house windows, taking advantage of the Sun's motion. 4a Poem by Vachel Lindsay. 4b The story of exploration of the Moon using space vehicles. ...