This seems, without doubt, to have been the primary motive of the Great Pyramid, and in a lesser degree of all pyramids, sepulchral mounds, and costly tombs. But the pyramids, and especially the Great Pyramid, are not mere piles of masonry heaped together without plan or design. On the contrary, they are all built on a settled plan, which implies an acquaintance with the sciences of geometry and astronomy, and which, in the case of the Great Pyramid, is carried to an extent which shows a very advanced knowledge of those sciences, and goes far to prove that it must have been used, during part of the period of its construction, as a national observatory. The full details of this plan are given by Procter in his work on the Great Pyramid, and although the want of a more accurate knowledge of Egyptology has led him into some erroneous speculations as to the age and object of this pyramid, his authority is undoubted as to the scientific facts and the astronomical and geometrical conclusions which are to be drawn from them. It appears that the first object of all pyramid builders was to secure a correct orientation; that is, that the four sides should face truly to the north, south, east, and west, or in other words that a line drawn through the centre of the base parallel to the sides should stand on a true meridian line. This would be a comparatively easy task with modern instruments, but before the invention of the telescope it must have required great nicety of observation to obtain such extremely accurate results in all the sides and successive layers of such an enormous building. There are only two ways in which it could be attempted--one by observing the shadow cast by a vertical gnomon when the sun was on the meridian, the other by keeping a standard line constantly directed to the true north pole of the heavens. In the case of the Great Pyramid another object seems to have been in view which required the same class of observations, viz. to place the centre of the base on the thirtieth degree of north latitude, being the latitude in which the pole of the heavens is exactly one-third of the way from the horizon to the zenith.