+44 Scientific Proceedings, Royal Dublin Society. 
been larger in the past, and may be traced back from the present 
5° 9' until it was 6° or 7°. This was a maximum inclination, and 
in the more remote past the inclination was less, and initially was 
very small or zero. 
3. The inclination of the proper plane to the ecliptic must have 
been greater in the past, and may be traced back from its present 
8" until it was in very early times about 11° 45’. It is possible 
that initially this inclination was less, and that the 11° 45’ was a 
maximum value. 
4. The eccentricity of the orbit must have been smaller in the 
past. Hither at one time it had a minimum value, before which 
it had a maximum value, and, again, earlier it was very small or 
zero, or else the maximum value never occurred, and the eccentri- 
city has always been increasing. The history of the eccentricity 
depends on the nature of the tides in the Earth, but the 
former of these alternatives seems the more probable. | 
At the present time the Earth rotates in twenty-four hours, its 
equator is inclined at anangle of 9" to a plane which Mr. Darwin 
calls “the proper plane of the Earth” (the mean equator). This 
proper plane is inclined at an angle of 23° 28’ to the ecliptic, and 
its intersection with the ecliptic is the equinoctial line. He finds 
that the frictional tides in the Earth have caused changes which 
may be summarised as follows :— 
5. The day must have been shorter in the past, and it may be 
traced back from its present value of twenty-four hours until 
initially it was from two to four hours in length. It was then 
identical with the Moon’s period of revolution as described in (1). 
6. The inclination of the equator to the Earth’s proper plane 
must have been larger in the past, and may be traced back from 
the present value of 9” until it was about 2° 45. This was a 
maximum inclination, and in the more remote past the inclina- 
tion was less, and initially it was very small or zero. 
7. The inclination of the Earth’s proper plane to the ecliptic 
must have been smaller in the past, and may be traced back from 
its present value of 23° 28’ until initially it was 11° 45’, or, perhaps, 
somewhat less. It was then identical with the proper plane of 
the lunar orbit, and this is true whether or not 11° 45’ was a 
maximum inclination of the lunar proper plane to the ecliptic as 
described in (1). 
