Military Bridges^ and the Passage of Rivers, 287 



domes were used in buildings long before their properties were 

 investigated on scientific principles. The invention of the arch 

 is frequently attributed to the Greeks, as it has been discovered 

 in several of the most ancient temples of Greece, but nothing of 

 the kind is to be found in any of the ancient monuments of Per- 

 sia or Egypt. Most of the apartments of the ruins of ancient 

 Egypt are covered with a single stone ; and, in the galleries of 

 the pyramids, of which the roofs consist of numerous pieces, 

 their peculiar construction renders it highly probable that the 

 builder was ignorant of the arch. The Greeks seem, therefore, 

 entitled to the honour of the invention. The arched dome, 

 however, appears to have been invented by the Romans. In 

 the later monuments of Italy, the Etruscan dome, and the Gre- 

 cian temple are combined, as in the famous Pantheon, even in 

 its most ancient form. 



In modern times, mathematical and mechanical principles 

 have been applied to the investigation of the properties of 

 bridges. Dr Hook proposed the inverted catenary as the best 

 form of an arch, in which all the strains would balance one an- 

 other. This curv* may be applied to most cases that occur, 

 though it has seldom or never been used in practice. The cir- 

 cular arch has been, from the simplicity of its construction, most 

 generally introduced. Of late years, however, the elliptical 

 arch, from its convenience in keeping the crown of the bridge 

 low to suit the level of the road, and its superior elegance, has 

 been much employed. When the semiellipse forms the arch, 

 this curve rises more rapidly at the haunches than the semicir- 

 cular, of the same span, and thus, besides the advantage of be- 

 ing lower at the crown, the under part of the arch forming the 

 voussoirs, is by that means much more capacious, and conse- 

 quently better fitted to admit a free passage of the water in 

 high floods. 



Permanent bridges have generally been constructed of stone, 

 sometimes of wood ; and, for temporary purposes, of boats, es- 

 pecially in military operations, which require them to be speedily 

 erected, and as quickly removed. Suspension bridges have 

 been long used in India and America ; and of late years they 

 have been introduced in Europe, both for civil and military 

 purposes, with considerable advantage. These methods should 



