vertical boring into
this open space. It would thus be found how far the point from which the
plumb-line was let fall lay, either to the east or to the west of the
optical axis of the underground tunnel, and therefore how far to the
east or to the west of the centre of the open mouth of this tunnel. Thus
the true direction of a north-and-south line from the end of the tube to
the middle of the base would be ascertained. This would be the meridian
line of the pyramid's base, or rather the meridian line corresponding to
the position of the underground passage directed towards the Pole-star
when immediately under the pole.

A line at right angles to the meridian line thus obtained would lie due
east and west, and the true position of the east-and-west line would
probably be better indicated in this way than by direct observation of
the sun or stars. If direct observation were made at all, it would be
made not on the sun in the horizon near the time of spring and autumn,
for the sun's position is then largely affected by refraction. The sun
might be observed for this purpose during the summer months, at moments
when calculation showed that he should be due east or west, or crossing
what is technically the _prime vertical_. Possibly the so-called azimuth
trenches on the east side of the great pyramid may have been in some way
associated with observations of this sort, as the middle trench is
directed considerably to the north of the east point, and not far from
the direction in which the sun would rise when about thirty degrees (a
favourite angle with the pyramid architects) past the vernal equinox.
But I lay no stress on this point. The meridian line obtained from the
underground passage would have given the builders so ready a means of
determining accurately the east and west lines for the north and south
edges of the pyramid's base, that any other observations for this
purpose can hardly have been more than subsidiary.

It is, of course, well known that there is precisely such an underground
tunnelling as the considerations I have indicated seem to suggest as a
desirable feature in a proposed astronomical edifice on a very noble
scale. In all the pyramids of Ghizeh, indeed, there is such a tunnelling
as we might expect on almost any theory of the relation of the smaller
pyramids to the great one. But the slant tunnel under the great pyramid
is constructed with far greater skill and care than have been bestowed
on the tun