METHODS OF DETERMINING THE ORIENTATION OF TEMPLES.
This brings us at once to a practical point. It will be asked, How can
such an inquiry be prosecuted? How can the amplitudes of the temples be
determined?
[Illustration: AZIMUTH COMPASS.]
Nothing is easier. An azimuth compass is all that is necessary for all
but the most accurate inquiries.
[Illustration: SECTION OF AZIMUTH COMPASS.
A, needle and card; P, prism; SV, directrix or frame carrying a
wire directed to the object and seen over the prism while the prism
reflects to the eye the division of the scale underneath it.]
The azimuth compass is an instrument familiar to many; it consists of
a magnetic needle fastened to a card carrying a circle divided into
360°, which can be conveniently read by a prism when the instrument is
turned toward any definite direction marked by a vertical wire. Its use
depends upon the fact that at the same place and at the same time all
magnetic needles point in the same direction, and the variation for the
true north and south direction is either supposed to be known or can be
found by observation.
[Illustration: THEODOLITE FOR DETERMINING AZIMUTH AND ALTITUDES.]
A theodolite armed with a delicately hung magnetic needle, which can
be rotated on a vertical axis, will do still better; it has first of
all to be levelled. There is a little telescope with which we can see
along the line. When we wish, for instance, to observe the amplitude
of a temple, the theodolite is set up on its tripod in such a position
that we can look along a temple wall or line of columns, etc., by means
of the telescope. We then get a magnetic reading of the direction after
having unclamped the compass; this gives the angle made between the
line and the magnetic north (or south), as in the azimuth compass.
What we really do by means of such an instrument is to determine
the astronomical meridian by means of a magnetic meridian. Here some
definitions will not be out of place.
The _meridian_ (_meridies_ = midday) of any place is the great circle
of the heavens which passes through the zenith (the point overhead) at
that place and the poles of the celestial sphere.
The _meridian line_ at any place is the intersection of the plane
of the meridian with the plane of the horizon at that place, or, in
other words, it is the line joining the north and south points. If
we have the proper instruments, we can determine the meridian line
astronomically at any place by one of the following methods:--
(1) If only an approximate position is required, the best means of
determining it is by fixing the direction of the sun or a star when it
has the greatest altitude. The instrument to be used for this purpose
would be a small theodolite with both a vertical and horizontal circle,
and provided also with tangent screws to give slow motion to each of
the circles as required.
By using stars of both high and low altitudes, a greater exactness
can be obtained, but, after all, the method only gives a first
approximation, as its weakness lies in the very slow change of altitude
as the meridian is approached.
(2) A much more accurate method is that of observing with an altitude
and azimuth instrument the azimuth (_i.e._, its angular distance east
or west of the north or south) of a star when at the same altitude east
and west of the meridian. If the mean of the two readings given by the
azimuth circle be taken, the resulting reading indicates the direction
of the meridian.
If we employ the sun in place of a star, its change of declination
during the interval between the observations must be taken into account.
(3) To find the meridian line by means of the pole star is a simple and
accurate method, as a value can be obtained at _any_ time at night by
a simple altitude, provided the time of observation is known.[8]
If these means of directly determining the astronomical meridian line
are not available, then we have to do it indirectly by using the
magnetic meridian in the first instance.
If we take a magnetic needle and balance it horizontally on a vertical
pivot, its ends will be directed to two points on the horizon. By
drawing a great circle through these two points and the zenith point of
the place, we obtain the _magnetic meridian_.
The _magnetic meridian line_ is the intersection of the plane of the
magnetic meridian with the plane of the horizon. The angle between the
astronomical and magnetic meridian lines is called the variation, E.
or W. according as the needle points to the W. or E. of true--that is,
astronomical--north at any particular place at any particular time. The
variation may vary from place to place, and always varies from time to
time.
The bearing required has, in the first instance, to be determined
by the instruments already referred to in relation to the magnetic
meridian.
Having made such an observation, the next thing we have to do is to
determine the astronomical or true north, which is the only thing of
value.
[Illustration: MAGNETIC MAP OF THE BRITISH ISLES, SHOWING THE VARIATION
AT DIFFERENT POINTS.]
If the magnetic variation has been determined for the region, we may
use a map. Such a map as that shown below gives us the lines along
which in the British Isles the compass variation west of north reaches
certain values. From such a map for Egypt we learn that in 1798 a
magnet swung along a line extending from a little to the west of
Cairo to the second cataract would have had a variation of 11½° to
the west; in 1844 of 8½° to the west; and at the present time the
variation is such that observations made along the same part of the
Nile valley will have a variation closely approximating 4½° to the
west. By means of such a map it is quite possible to get approximately
the astronomical bearings of all temples which were observed by the
French in 1798 or by the Germans in 1844, or which can be observed
in the present day, provided always that there is no local magnetic
attraction.
If we are not fortunate enough to possess such a map, the methods
previously referred to for obtaining the astronomical north must be
employed; observing the direction in which the sun culminates at noon
will give us the south point astronomically; from observations of the
pole star at night the astronomical north can also be determined. From
the former of these observations the magnetic variation is obtained
without any difficulty, even in the absence of accurate local time.
When this is available other methods are applicable.
It is sad to think how much time is lost in the investigation of
a great many of these questions for the reason that the published
observations were made only with reference to the magnetic north,
which is vastly different at different places, and is always varying.
Few indeed have tried to get at the astronomical conditions of the
problem. Had this been done with minute accuracy in all cases, either
by the French or Prussian Commissions to which I have referred, it is
perfectly certain that the solstitial orientation of Karnak and other
temples, which I shall have to mention, would have been long ago known
to all scholars.