Notices respecting New Books. 419 



nuous surfaces of sheet iron ; whereas in the French invention they 

 are formed of iron bars taking the shape of a series oi parallelopipeds. 

 The top and bottom parts of Dr. Guyot's bridge are connected 

 together by a series of pillars, so that the roadway seems to be 

 standing upon, and supported by, the arch beneath ; whereas in the 

 English bridges, an unbroken surface of sheet iron connects the top 

 and bottom parts so perfectly as to render them portions of an in- 

 tegral structure. 



3. The leading elements of strength in the two structures are not the 

 same. — In the French invention, the arched structure at the bottom, 

 composed of regular voussoirs, constitutes the main element of 

 strength, the top and sides being subsidiary parts of the construction ; 

 and hence the forces of resistance to rupture mainly depend upon the 

 principle of the arch. In fact, owing to the imperfect connexion be- 

 tween the voussoirs at the bottom and the rectangular parallelopipeds 

 at the top, as well as from the want of a complete cooperation in the 

 forces of resistance at the top and bottom parts, these forces seem to 

 be acting, to a certain extent, independently of each other. Now 

 in Mr. Fairbairn's model tube, the top cells constitute the distin- 

 guishing feature of the construction ; and the side plates, forming 

 a perfect connexion between these top cells and the plates at the 

 bottom, transform the whole structure into a perfect hollow beam, 

 subject to the ordinary laws of compression and extension. Here 

 the resistance of the cells at the top to compression, and the re- 

 sistance of the bottom plates to extension, were found alone suffi- 

 cient, without the aid of any subsidiary or auxiliary force, to ensure 

 the stability of the structure. In Dr. Guyot's bridge, therefore, the 

 cellular structure at the bottom forms the essential part of the con- 

 trivance, whereas in the tubular beams tlie bottom part may be com- 

 posed of simple plates*. 



4. The English tubular bridge has been for some time constructed 

 on a scale of surprising magnitude ; its powers of resistance have 

 been fully tested, and in all respects it has fully realized the most 

 sanguine expectations of its projectors, and it will no doubt stand 

 for a future age to contemplate, as a proud and enduring monument 

 of the genius and practical skill of British engineers. The French 

 scheme has yet to undergo the ordeal of actual construction on a 

 scale sufficiently large to test its efficiency ; and we may fairly ob- 

 serve, that it is still problematical whether it will prove itself to be 

 so perfect a structure as the English tubular bridge. 



* The bottom part of the model tube of the Conway and Menai bridges 

 had no cellular structure. 



8F2 



