the star would reappear at Gr and P before it disappeared
at the altar. We have thus a sort of double
observation of the same meridian transit.
If we admit that these meridian lines were used
for the observation of stars in this way, and if we can
determine what star or stars were observed, the time
that has elapsed since they were observed admits of
calculation. The apparent altitude of the middle of
the stone as seen from the centres of the arcs is 7-|0,
and the latitude of Zimbabwe is 20° 16' SO", so that we
want stars having a north polar distance of about 28°.
Owing to the changing direction of the pole of the
earth, which produces the phenomenon of the precession
of the equinoxes, the declinations and right ascensions
of all the stars are undergoing a slow but regular
change ; but there are no stars of the first magnitude
which have had approximately this polar distance
since any probable date of the foundation of Zimbabwe.
Of stars of the second magnitude there, are four,
and of the third magnitude many more, which may
have been used, and they would all serve for widely
different periods. In order to enable :us to select the
proper star from this number we must, have its right o
ascension, and this we may yet hope to get when we
have the date of some important yearly festival at
Zimbabwe, and the hour at which the star would
be wanted on the meridian on the night of this
festival.
There are two other places where the meridian
transits of stars: have been watched at Zimbabwe, and
in these cases it is still the same portion of the heavens
L