412 Formation of Concrete Matter from Atomic Origins. 
" see that the mutual action between the Moon and the Earth 
" must tend, in virtue of the tides, to diminish the rapidit} r 
" o£ the Earth's rotation, and increase the moment of the 
" Moon's motion round the Earth/'* ....'•' The tidal defor- 
" matron of the water exercises the same influence on the 
" Moon as if she were attracted not precisely in the line 
" towards the Earth's centre, but in a line slanting very 
" slightly, relatively to her motion, in the direction forwards. 
u The Moon, then, continually experiences a force forward in 
" her orbit by reaction from the wafers of the sea, Now, it 
" might be supposed for a moment that a force acting 
" forwards would quicken the Moon's motion ; but, on the 
" contrary, the action of that force is to retard her motion. 
" It is a curious fact easily explained, that a force continually 
" acting forward with the Moon's motion w T ill tend, in the 
" long runj to make the Moon's motion slower, and increase 
u her distance from the Earth." * 
§ 34. Thus we see that in the present regime the Moon is 
getting farther and farther from the Earth, and the Earth's 
rotational velocity is becoming less and less ; the sum of 
moments of momentum being thus kept constant. Hence in 
more and more ancient times, the Moon must have been 
nearer and nearer to the Earth, and the Earth's rotational 
velocity must have been greater and greater. Trace then 
the course of motions backwards for a sufficient number of 
millions of years, and we find the Earth much hotter than at 
present, and in the semi-solid semi-plastic condition described 
in § 28 above. The distance of the Moon from the Earth must 
then as now have been increasing, and the Earth's rotational 
velocity then as now diminishing. But these two changes 
must have been much more rapid then than now, because of the 
viscosity of the semi-solid material of the Earth, and because 
of the Moon's shorter distance from the Earth and therefore 
greater gravitational influence on the Earth, then than now. 
Sir George Darwin had perfect right to trace the regime 
backwards, until there was contact and continuity between 
the Earth and Moon. The continuity of the whole mass must 
have come to an end with the sudden and violent catastrophe 
described in § 28 above, to which Sir George Darwin boldly 
went back with sober truthfulness. It is conceivable that 
meteorites large or small may have at various times pro- 
duced disturbances ; but I cannot see any probability for any 
* Quoted from Sections 7 and 14 respectively of an address by 
Sir "William Thomson, to the Geological Society of Glasgow, " On Geo- 
logical Time," Feb. 27, 1868 ; republished in Lord Kelvin's ' Popular 
Lectures and Addresses,' vol. ii., see pp. 21 and 33. 
