If the heavenly bodies went round in circles, their notes would never vary, as the distances would always be the same; but if they go round in ellipses, their notes will rise and fall with every variation in the distances. And as soon as Kepler had discovered that the Earth and other planets make ellipses round the Sun, he set to work to ascertain how far their notes run up and down the scale; and he published his results in 1619 in his _Harmonice Mundi_, V. 4-9. According to this, Saturn’s note went up and down a major third, and Jupiter’s went up and down a minor third; and Jupiter’s note at its lowest was an octave above Saturn’s at its highest. Similarly, the rise and fall was a fifth for Mars, a semitone for the Earth, and practically nothing for Venus, as its ellipse is nearly circular, whereas the long ellipse of Mercury produced a rise and fall of an octave plus a minor third; and between these rising and falling notes there were clear intervals of a major sixth from Mercury to Venus, a minor sixth from Venus to the Earth, a fifth from the Earth to Mars, and two octaves plus a minor third from Mars to Jupiter. And of course the trebles played their scales much faster than the basses, as they go round the Sun in much less time. Kepler took all this quite seriously, and was convinced that some such ratios must exist, as the Creator was a neat hand at geometry, “Deus nihil sine geometrica pulchritudine constituerit,” V. 4. It was the irony of Fate that in pursuing this absurdity he discovered a great truth--the Third Law of Motion. These great Laws are not always put before young minds with due simplicity: we obscure them by our jargon. All children know that if they spread a pat of butter on a slice of bread, the bigger the slice is, the thinner the butter will be. We express this by saying that the thickness of the butter varies inversely as the surface of the slice. They can see that the same thing would happen if they had to butter the outside of a roll or dumpling that was as round as a Dutch cheese. We say, as before, that the thickness of the butter varies inversely as the surface of this globe of bread; and as the surface of a globe varies directly as the square of the distance between the surface and the centre, we end by saying that the thickness of the butter varies inversely as the square of the distance. Young minds understand the butter. Put ‘the force of attraction’ for ‘the thickness of the butter,’ and they will understand the Law of Universal Gravitation, as discovered by Sir Isaac Newton with the assistance of an apple.