B IU D G R. 



51 y 



not be in the least surprised, that not only grav-1 

 ' and shiver, but even large stones, are hlied in it, and 

 ihoved away by the vertical motion. 



For those, and other reasons, it is difficult to as- 

 certain the exact amount of the obstruction caused 

 by obliquity. Like every other department of our 

 inquiry connected with hydraulic pimciples, experi- 

 ments are yet wanting to assist us in making this a 

 subject of calculation. 



Instead, therefore, of occupying the reader's time 

 with a mathematical discussion, from which, at pre- 

 sent, little benelit can be derived, we conceived it 

 better to offer a few practical remarks on the me- 

 thods by which the chief difficulties of such a case 

 may be obviated. 



With this view we will venture to recommend, 

 that whatever the position of the bridge may be with 

 respect to the stream, the lengthway of the piers 

 and abutments should coincide with the direction of 

 the current as nearly as may be. A considerable de- 

 viation is perfectly practicable. We have formerly 

 shewn, that the position of the joints of the arch 

 may be varied, perhaps 30 from that of equilibra- 

 tion, before any slide is thereby produced among 

 the archstones : we surely have the same liberty 

 here, and this even when the coursing joints are ho- 

 rizontal. 



This advantage of friction may be even improved 

 by dowelling or other means, if thought beneficial. 

 But if the coursing-joints, instead of being made ho- 

 rizontal, be formed at right-angles to the side of the 

 bridge, we have the very same advantages that would 

 be obtained were the bridge perfectly direct. 



Did the subject admit of it, we would here give 

 the theory of those arches which are not straight on 

 the ground-plan, as the arched top of a bow-window, 

 and the like, some of which afford pretty examples 

 of the application of the principles of equilibration, 

 and all of which, by means of the gravity, friction, 

 or cohesion of their parts, admit to a certain extent 

 of being steadily and solidly constructed. But we 

 shall, at another opportunity, take up this subject, 

 and, in the meantime, we hasten to other matters more 

 intimately connected with our present inquiries. 

 The centres or framings of carpentry, on which the 



built, are, in many rase*, objects of great Theory. 

 : ytothc builder: they form one of the most ~v ~ 

 beautiful applications of the science of CARI-EXTHV. 

 But as the principles upon which the moie difficult 

 kinda are designed will not be clearly understood, 

 without entering into more detail than is consistent 

 with our present subject, we shall only offer in this 

 article some of the most approved designs, and re- 

 fer the reader for their explanation and discussion 

 of their comparative merits until we come to treat of 

 CARPENTRY.' 



To the same place we shall also refer the subject 

 of wooden and iron bridges, which intimately de- 

 pend on the same science ; their statical equilibrium 

 admitting of the introduction of a principle essen- 

 tially different from those employed in erections of 

 stone. 



The side-walls and wing walls of bridges consti- 

 tute a part not the least important of our subject. 

 They have, especially the latter, to resist the pres- 

 sure of a mass of earth behind them, funning the 

 roadway up to the haunches of the bridge. Little 

 has been written upon this subject in our language, 

 and that little has been of no practical benefit. Mul- 

 ler and some others give us a few theorems respect- 

 ing the thickness of the revetements in fortification*. 

 Some of our elementary writers inquire a little into 

 the same subject, as one case of the application of 

 the doctrine of the centre of gravity. Their results, 

 in almost every case, give a much greater thickness 

 for walls of this kind, than is ever found necessary 

 in practice. There can be no doubt that some prin- 

 ciple must be overlooked in these investigations, 

 which has a material effect in their application. We 

 cannot say that the inquiries of Belidor, reckoned 

 one of the first of scientific engineers, have been 

 more successful. In another part bf this work we 

 shall offer a theory of retaining walls, in general 

 founded upon principles that are perhaps new, and 

 that has been confirmed by the results of experi- 

 ment. This theory has been found to agree with 

 the ideas of practical men. It is capable of appli- 

 cation to .all cases of the kind, and it comprehei 

 them all. But in this place rt would be embracing 

 too large a field to bring it forward. ( \. x.) 



PART II. PRACTICE OF BRIDGE BUILDING. 



ice of W'TH regard to the practice of bridge building, the 

 ie chief objects may be arranged as follows : 

 -g- 

 SECT. I. 



I; The situation. 



2. The design. 



3. The materials. 



SECT. II. 



1. The foundations. 



2. The Piers and abutments. 



3. The Centres. 



4. The Arches. 



5. The Spandrels and wings. 



6. The Parapets. 



7. The Roadway. 



Pract i 



Bridge 

 Euilduig. 



SECT. I. 

 On the Situation, Design, and Material*. 



1. The situation is generally determined by local On tlie 

 circumstances, in a town by streets, and in the coun- 

 try by the roads adjacent. If the bridge is of a great 

 size, or the foundations difficult, it is frequently ad- 

 visable to choose the most favourable situation for 

 the bridge, even at the expence of changing the ap- 

 proaches. It is absolutely necessary that the at 



