[17] See Ibn Hisham i. 54, Azraki p. 80 ('Uzza in Batn Marr); Yakut iii. 705 (Otheyda); Bar Hebraeus on Psalm xii. 9. Stones worshipped by circling round them bore the name _dawar_ or _duwar_ (Krehl, _Rel. d. Araber_, p. 69). The later Arabs not unnaturally viewed such cultus as imitated from that of Mecca (Yaqut iv. 622, cf. Dozy, _Israeliten te Mekka_, p. 125, who draws very perverse inferences). [18] The old _kiswa_ is removed on the 25th day of the month before the pilgrimage, and fragments of it are bought by the pilgrims as charms. Till the 10th day of the pilgrimage month the Ka'ba is bare. [19] Before Islam the Ka'ba was opened every Monday and Thursday; in the time of Ibn Jubair it was opened with considerable ceremony every Monday and Friday, and daily in the month Rajab. But, though prayer within the building is favoured by the example of the Prophet, it is not compulsory on the Moslem, and even in the time of Ibn Batuta the opportunities of entrance were reduced to Friday and the birthday of the Prophet. [20] See De Vogué, _Syrie centrale: inscr. sem._; Lady Anne Blunt _Pilgrimage of Nejd_, ii., and W. R. Smith, in the _Athenaeum_, March 20, 1880. [21] Ibn Jubair speaks of fourteen steps, Ali Bey of four, Burckhardt of three. The surrounding ground no doubt has risen so that the old name "hill of Safa" is now inapplicable. [22] The latter perhaps was no part of the ancient omra; see Snouck-Hurgronje, _Het Mekkaansche Feest_ (1880) p. 115 sqq. [23] The 27th was also a great day, but this day was in commemoration of the rebuilding of the Ka'ba by Ibn Jubair. [24] The sacrifice is not indispensable except for those who can afford it and are combining the hajj with the omra. [25] On the similar pelting of the supposed graves of Abu Lahab and his wife (Ibn Jubair, p. 110) and of Abu Righal at Mughammas, see Nöldeke's translation of Tabari, 208. MECHANICS. The subject of mechanics may be divided into two parts: (1) theoretical or abstract mechanics, and (2) applied mechanics. 1. THEORETICAL MECHANICS