In these erections the piers are raised to a considerable height, that a sufficient depth may be allowed for the curve of the chains without depressing the roadway. Ten times--a hundred times the power which was applied to strain them into that shape would not suffice to bring them even so near to a horizontal line but that the most inaccurate and unobservant eye should at once detect the inequality in their level; and the chains themselves would probably give way before such a force as this could be applied to them. The least diameter of the Coliseum is nearly equal in length to the Menai bridge; and if the labor of stretching cords over the one seems small in comparison with that of raising the ponderous chains of the other, we may take into consideration the weight of cloth which those cords supported, and the increase of difficulties arising from the action of the wind on so extensive a surface. In boisterous weather, as we learn from Martial and other authors, these difficulties were so great that the velum could not be spread. When this was the case the Romans used broad hats, or a sort of parasol, which was called _umbella_ or _umbraculum_, from _umbra_, shade. We may add, in conclusion, that Suctonius mentions as one of Caligula's tyrannical extravagances, that sometimes at a show of gladiators, when the sun's heat was most intense, he would cause the awning to be drawn back, and, at the same time, forbid any person to leave the place. The difficulty of the undertaking has given rise to considerable discussion as to the means by which the Romans contrived to extend the velum at such a height over so great a surface, and to manage it at pleasure. Sailors were employed in the service, for the Emperor Commodus, who piqued himself on his gladiatorial skill, and used to fight in the arena, believing himself mocked by the servile crowd of spectators, when once they hailed him with divine honors, gave order for their slaughter by the sailors who were managing the veils. [Illustration: COLISEUM OF ROME.] Concerning the method of working them no information has been handed down. It is evident, however, that they were supported by masts which rose above the summit of the walls. Near the top of the outer wall of the Coliseum there are 240 consoles, or projecting blocks of stone, in which holes are cut to receive the ends of spars, which ran up through holes cut in the cornice to some height above the greatest elevation of the building. A sufficient number of firm points of support at equal intervals was thus procured; and, this difficulty being overcome, the next was to stretch as tight as possible the larger ropes, upon which the whole covering depended for its stability.