136 LECTURE 



jured in appearance by pediments, cornices, columns, and pilasters, 

 which are entirely out of place. 



There are enough projections and other features which occur natur- 

 ally in the construction to give the necessary relief, and structures 

 depending upon these alone have always a neater, bolder character 

 than those which are encumbered and enfeebled by useless ornaments. 



In the United States but few, if any, very large stone bridges have 

 been built, and we must look to Europe for examples. The ancient 

 aqueducts, the characteristics of which are usually very high, thin 

 piers and small arches, are among the most beautiful of the remains 

 of Roman enterprise and magnificence, while the river bridges of the 

 same people, however good of their class, are surpassed by those 

 constructed in the sixteenth century and later, when larger spans, 

 lighter piers, and segmental and elliptical arches were introduced by 

 Ammanati, Perronet, and others. 



The ellipse or more properly the curve of several centres, Figure 59, 

 presents several advantages over the circular curves. Apart from 

 the increased water-way, its form is much more pleasing to the eye, 

 not only in its own variety, but because in it the broken line which 

 occurs where a segmental arch joins the face of the pier or the abut- 

 ment is avoided, and because the flatness of the arch near the top 

 gives an appearance of lightness which cannot be got in any other 

 way. The bridge of the Trinita over the River Arno, Florence, 

 Italy, built by Ammanati, in 1566, is the most graceful structure of 

 its kind that has yet been produced, and it owes its beauty principally 

 to the use of this curve. The piers of this bridge are, however, 

 thicker than they should be, since practice has reduced these parts 

 very much in later structures, with advantage both as to appearance 

 and to the water-way left beneath them. 



At the present time, when many large bridges are built in isolated 

 places for railway purposes, cheapness and quickness of construction 

 are matters of the first importance, and as these necessities have led 

 to the general use of wood and iron, we shall finish this subject by a 

 description and comparison of two of the most important works of late 

 date. When the Chester and Holyhead railroad was to be carried 

 across -the Menai straits, which separate the island of Anglesea from 

 the main land of Wales, a suspension bridge something like a famous 

 carriage bridge built by Telford at the same point, was proposed, but 

 the opinion of the distinguished engineer, Robert Stephenson, sup- 

 ported by nearly the whole of the English profession, was, that it was 

 impossible so to overcome the flexibility of a suspension bridge as to 

 fit it for railway trafiic, and the idea was accordingly abandoned. 

 Several plans were then proposed — among them that of a cast-iron 

 arch — but owing to the restrictions placed on the work by the admi- 

 ralty, some arrangement was required w^hich would avoid interference 

 for any length of time with the free passage of vessels through the 

 strait, and nothing like centring could be used. 



Mr. Stephenson first determined to have a wrought-iron truss with 

 strong top and bottom, and thin plate sides or web supported by 

 chains at the central point; the chains, however, were afterwards 



