Читать книгу The dawn of astronomy. A study of the temple-worship and mythology of the ancient Egyptians онлайн

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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.

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.