=136.= The kings of the Second Dynasty ruling Middle Egypt from This or Abydus, and those of the Fifth ruling Upper Egypt from the Isle of Elephantine, were probably related by blood to the powerful sovereigns of Lower Egypt, and the tombs of all three families are found in the neighborhood of Memphis. The structure of the Pyramids shows great advancement in science and the mechanical arts. Each is placed so as exactly to face the cardinal points, and the Great Pyramid is precisely upon the 30th parallel of latitude. The wonderful accuracy of the latter in its astronomical adjustments, has led a few profound scholars[14] of the present day to believe that it could only have been built by Divine revelation; not by the Egyptians, but by a people led from Asia for the purpose, the object being to establish a perfectly trustworthy system of weights and measures. =137.= The Arabian copper-mines of the Sinaitic peninsula were worked under the direction of the Pyramid kings. At this period the arts had reached their highest perfection. Drawing,[15] sculpture, and writing, as well as modes of living and general civilization, were much the same as fifteen centuries later. =138.= B. C. 2220. While a sixth royal family succeeded the pyramid-builders at Memphis, the second and fifth continued to reign at This and Elephantis, while two more arose at Heracleop´olis and Thebes; so that Egypt was now divided into five separate kingdoms, the Theban becoming gradually the most powerful. Thus weakened by division, and perhaps exhausted by the great architectural works which had withdrawn the people from the practice of arms, the country easily became the prey of nomad tribes from the neighboring regions of Syria and Arabia. These were called Hyk´sos, or Shepherd Kings. They entered Lower Egypt from the north-east, and soon became masters of the country from Memphis to the sea. =139.= SECOND PERIOD. B. C. 1900-1525. Native dynasties continued for a time to reign in Middle and Upper Egypt; and even in the heart of the Delta a new kingdom sprang up at Xo´is, which maintained itself during the whole time that the Shepherds were in the land. A large number of the enslaved Egyptians continued to cultivate the soil, paying tribute to the conquerors; and, in time, the example of their good order may have mollified the fierce invaders. The latter built themselves a strongly fortified camp, Ava´ris, in the eastern portion of the Delta, near the later city of Pelusium.