16— 
AR in time of « Hydre, as I believe, some time after Stonehenge 
was built. But the present ar of a Hydre is 9" 22™ 31:55, and 
the difference between these two Ar’s— 
SARs silos}: 
and 7 l= 315 


is? 2) 11™"0"5 == 1179075 
The question, therefore, is how many years does this difference 
of AR represent? The present registered annual precession in 
time of a Hydre is 2°95, or in motion 44’'-25, and its declination 
is 8" 13™ 58.8. The annual variation in the precession of a star 
depends on several co-ordinates, namely, its declination and its ar, 
and of these the former is of more account than the latter ; also 
on its position as regards the solstitial colure. The North 
Polar Distance of stars having ar between 6" and 18" 2,800 
years ago was less, and Declination higher than now ; the case being 
reversed as to stars in the opposite half of the heavens. The whole 
results from an exceedingly slow motion in space of the Pole 
of the Earth round the Pole of the Ecliptic in a circle having 
a radius of about 23° 28’, one consequence of which is that the 
plane of the Ecliptic becomes less oblique by an amount of less 
than half a second in a year, which causes the precession of the 
Equinoxes and partial glacial periods. To endeavour by mathe- 
matical method to find out truly the precessions we require would 
be very laborious and might land us in confusion, therefore I 
propose, as our best plan for calculating the mean annual 
precession of our star, to search in any good catalogue of 
stars for a star having right ascension as nearly as may be 
corresponding with the angle which the line from No. 60: 
to the South mound bears from the East and declination higher 
than that of a Hydre now. If we could find such a star we 
might expect that his annual precession, as given in the catalogue, 
would be the same as that of a Hydre at the distant time 
sought, and we might safely use, for the mean annual precession 

