The Arabs had a strong taste for mathematics. Here again they carried further the old Greek investigations. In arithmetic they used the so- called "Arabic" figures, which were probably borrowed from India. The Arabic numerals gradually supplanted in western Europe the awkward Roman numerals. In geometry the Arabs added little to Euclid, but algebra is practically their creation. An Arabic treatise on algebra long formed the textbook of the subject in the universities of Christian Europe. Spherical trigonometry and conic sections are Arabic inventions. This mathematical knowledge enabled the Arabs to make considerable progress in astronomy. Observatories at Bagdad and Damascus were erected as early as the ninth century. Some of the astronomical instruments which they constructed, including the sextant and the gnomon, are still in use. [28] ROMANCE AND POETRY In prose and verse there are two Moslem productions which have attained wide popularity in European lands. The first work is the _Thousand and One Nights_, a collection of tales written in Arabic and describing life and manners at the court of the Abbasids. The book, as we now have it, seems to have been composed as late as the fifteenth century, but it borrows much from earlier Arabic sources. Many of the tales are of Indian or Persian origin, but all have a thoroughly Moslem coloring. The second work is the _Rubáiyát_ of the astronomer-poet of Persia, Omar Khayyam, who wrote about the beginning of the twelfth century. His _Rubáiyát_ is a little volume of quatrains, about five hundred in all, distinguished for wit, satirical power, and a vein of melancholy, sometimes pensive, sometimes passionate. These characteristics of Omar's poetry have made it widely known in the western world. [29] ARCHITECTURE Painting and sculpture owe little to the Arabs, but their architecture, based in part on Byzantine and Persian models, reached a high level of excellence. Swelling domes, vaulted roofs, arched porches, tall and graceful minarets, and the exquisite decorative patterns known as "arabesques" make many Arab buildings miracles of beauty. Glazed tiles, mosaics, and jeweled glass were extensively used for ornamentation. From the first the Arab builders adopted the pointed arch; they introduced it into western Europe; and it became a characteristic feature of Gothic cathedrals. [30] Among the best-known of Arab buildings are the so-called "Mosque of Omar" at Jerusalem, [31] the Great Mosque of Cordova, and that architectural gem, the Alhambra at Granada. Many features of Moorish art were taken over by the Spaniards, who reproduced them in the cathedrals and missions of Mexico and California.