This results from the fact that there is a slow change in what is called the obliquity of the ecliptic--that is, the angle between the plane of the earth's equator and the plane of the ecliptic; this change is brought about by the attraction of the other planetary bodies affecting the plane of the ecliptic. If these planes approach each other, the obliquity will be reduced; the present obliquity is something like 23° 27′; we know that 5,000 B.C. it was 24° 22′, nearly a degree more. A difference of 1° means, then, a difference of time of about seven thousand years. It may go down to something below 21°. Since the obliquity has been decreasing for many thousand years, a temple directed to the rising or setting sun at the solstice some thousands of years ago had a greater amplitude than it requires now. It will be readily understood that if the orientation of the temple and the height of the hills towards which it points be accurately known, knowing also the precise obliquity of the ecliptic at different epochs, we have an astronomical means of determining the date of the original foundation of the temple, supposing, of course, that it was founded to observe the solstice. But before I go into these matters it is essential that the evidence of Egyptologists should be considered. Very fortunately for us in these inquiries the temple of Amen-Rā is one of those most carefully studied by Mariette, so that the _dernier mot_ of the archæologist is at our disposal. Mariette, in his magnificent memoir on Karnak,[34] surpassed himself in the care and sagacity which he displayed in endeavouring to fix dates for the various structures in that wonderful temple-field, and among them the various parts of the temple of Amen-Rā. In his maps, to which I now refer, each part of the temple is coloured according to the supposed date of its building. He points out first of all that the inscriptions on the walls must be disregarded, as they could have been put there at any date after the temples were built. On this point I quote Mariette's own words:--[35]