Those who are not acquainted by experience with the difficulty of giving stability to tents of large dimensions, and the greater difficulty of erecting awnings, when, on account of the purpose for which they are intended, no support can be applied in the centre, may not fully estimate the difficulty of erecting and managing these velaria. Strength was necessary, both for the cloth itself and for the cords which strained and supported it, or the whole would have been shivered by the first gust of wind, and strength could not be obtained without great weight. Many of our readers probably are not aware, that however short and light a string may be, no amount of tension applied horizontally will stretch it into a line perfectly and mathematically straight. Practically the deviation is imperceptible where the power applied is very large in proportion to the weight and length of the string. Still it exists; and to take a common example, the reader probably never saw a clothes-line stretched out, though neither the weight nor length of the string are considerable, without the middle being visibly lower than the ends. When the line is at once long and heavy, an enormous power is required to suspend it even in a curve between two points; and the amount of tension, and difficulty of finding materials able to withstand it, are the only obstacles to constructing chain bridges which should be thousands, instead of hundreds of feet in length.
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.