We have referred to the fact that the pagan Irish were skilled workmen in metal. This class of work received a new impetus with the introduction of Christianity. The pre-Christian craftsmen exercised their skill in ornamenting shields, swords, sword-hilts, chariots, brooches, bridles, etc. as we learn from the specimens that are preserved in our museums.[452] In addition to articles of this class the Christian artists, the majority of whom were ecclesiastics,[453] made crosses, croziers, chalices, shrines to hold books or relics as well as book satchels in which both metal and leather were used.[454] Specimens of these may be seen in the National Museum, Dublin.[455] For real artistic skill the most admired specimens are the Ardagh Chalice, the Tara Brooch, and the Cross of Cong.[456] The designs and styles of ornament used by the metal workers were similar to those of the artists who illuminated the MSS. Artistic skill in metal work was brought to its highest degree of excellence in the tenth and eleventh centuries and continued to flourish until about the end of the twelfth century, but gradually declined after that date owing to the general disorganization of society consequent on the Norman invasion.[457] STONE CARVING: The skill of the Irish artists in stone carving is seen at its best in the great stone crosses of which about 55 remain in different parts of Ireland. One peculiarity of the Irish (or Celtic) Cross is the circular ring round the intersections, thus binding the arms together. This peculiar form was developed in Ireland and, once developed, remained fixed from the eighth to the twelfth century. Of the 55 great crosses 35 are richly ornamented and eight have inscriptions bearing names of persons who have been identified as living at various dates from 904 to 1150 A.D.[458] The crosses have a style of ornamentation similar to that of the manuscripts and of the metal work. In addition to the ornamentation most of the crosses have groups of figures representing various events in Sacred History, such as, The Fall of Man, Noah in the Ark, The Sacrifice of Isaac, The Fight of David and Goliath, The Arrest of Our Lord, The Crucifixion, The Crucifixion of St. Peter (head downward), Eve Presenting the Apple to Adam, The Journey to Egypt. These sculptures are _iconographic_, _i.e._ they were intended to bring to the minds of the unlettered people the facts of Sacred History by vivid illustration. No doubt the preachers in their discourses directed the attention of their hearers to these representations; and perhaps they often lectured while standing at the foot of the cross with the people ranged in front, the preacher pointing to the sculptured groups as occasion demanded. It is probable that the figures were painted in brilliant colours like those used in illuminating the MSS. so that the people might see them more distinctly.[459] MATHEMATICS AND SCIENCE: