Concrete Matter from Atomic Origins. -All 
§ 31. Sir George Darwin, with comprehensively pene- 
trating dynamical insight, has traced the course of events 
following the stage reached in § 29. He has given a rea- 
sonable account of the evolution of the present eccentricity 
of the Moon's orbit ; and he has made the remarkable and 
important discovery that the axis of the Earth's rotation 
could not remain as it is at the stage of § 29, perpendicular 
to the plane of the orbital motion of Earth and Moon. 
We might readily enough work out the general character of 
the motions and transformations that would follow the stage 
of § 29, if the Earth's and Moon's rotational axes did in 
reality remain perpendicular to the plane of their orbital 
motions. But Darwin finds that this possible and easily 
understood association of motions would be unstable ; and 
that the slightest deviation from exact perpendicularity of 
the Earth's axis to the orbital plane would become, not 
diminished but, augmented by the Earth's viscous resistance 
against change of shape. With this hint it is almost as easv 
for us to see, by dynamical reasoning, that the Earth's axis 
must, through millions of years, have become more and more 
. . . 
oblique to the orbital plane, as it is, for us, with Archibald 
Smith's hint*. to see that a "teetotum" or a boy's spinning- 
top, having a well rounded bearing point, and set to spin at a 
sufficiently high speed round an axis oblique to the vertical, 
and dropped on a hard horizontal plane, w T ill in a short time, 
perhaps less than a minute, be found spinning round a fixed 
exactly vertical axis (" sleeping "), and will go on so for 
ever, if the materials of the top and plane are perfectly hard, 
and if there is no resistance of the air. 
§ 32. Darwin's theory of the birth of the Moon might 
seem improbable ; might seem even an extravagant attempt in 
evolutionary philosophy, insufficiently founded on knowledge. 
In reality it is rendered highly probable, it is indeed forced 
upon us, by tracing backwards to earlier and earlier times 
the dynamical antecedents of the present conditions of Earth, 
Moon, and Sun. 
§ 33. A hundred years before the doctrine of energy 
thoroughly entered the minds of mathematicians and natu- 
ralists, Kant made known the truth that the Earth's rotational 
velocity is diminished by tidal friction. When we consider 
the dvnamics on which this statement is founded, we see 
that it implies reactive forces gravitationally exerted on the 
Sun and Moon by terrestrial waters. Ignoring the Sun, as 
less influential than the Moon in respect to the tides, we 
* "Xote on the Theorv of the Spinning Top," Camb. Math. Journal, 
1839, vol. i. p. 42. 
