obliquity of the orbit, fixed the length of the year and verified the precession of the equinoxes. In chemistry they were far advanced, having discovered the properties of sulphuric acid, nitric acid and alcohol. In mechanics they anticipated Newton's discovery and touched upon the Einstein theory, or the beginnings of it, when Al-Hazen made the discovery of the curvilinear path of a ray of light.
As for agriculture, the world will not overlook the fact that the Arabs introduced superior methods of irrigation, that they understood the skilful employment of manures, the improved breeding of horses and camels, the culture of rice, sugar and coffee. In the world of manufacture, their activity is shown in the making of leather, silk, wool and cotton goods and sword blades of superb temper.
THE RETREAT FROM EUROPE
As the Christian states grew in power, the Moslem holdings were reduced. In the latter part of the thirteenth century the only portion of what is now Spain that was left to the Moors was the province of Granada. The Christian forces took Toledo in 1085 and then fell in turn Saragossa, Valencia, Seville and Murcia, so that the Mahometan stronghold became the city of Granada. To protect that from the invader, a double circuit of walls was built. The city itself was one of great grandeur with its fortress and palace, its thousand towers and its famous Alhambra, "the finest home ever
