CHAP. VI. RENNIE'S BRIDGES. 171 



Britannica,' and he believed he should be enabled to im- 

 part an additional value to his writings by throwing upon 

 them the light of Eennie's strong practical judgment. 

 He proposed to take a lodging in the immediate neigh- 

 bourhood of Rennie's house, then in the Great Surrey 

 Road, and to board with him during the day ; but Rennie 

 would not listen to this proposal, and insisted on being 

 the Professor's entertainer during the period of his visit. 

 One of the points which he particularly desired to 

 discuss with Mr. Rennie was the theory of the equi- 

 librium of arches a subject at that time very imper- 

 fectly understood, but which the young engineer had 

 studied with his usual energy and success. He had 

 clearly proved that the proper proportion and depth 

 of the key-stone to that of the extrados (or exterior 

 curve) should be in proportion to the size and form 

 of the arch and the materials of which it was com- 

 posed ; and he had also established the ratio in which the 

 arch-stones should increase from the key-stone to the 

 piers or abutments. Up to this time there had been no 

 rules laid down for the guidance of the engineer or archi- 

 tect, who worked very much in the dark as to principles ; 

 and it was often a matter entirely of chance whether a 

 bridge stood or fell when the centres were removed. 

 According to the views of Hutton and Attwood, the 

 weight upon the haunches and abutments, to put the 

 arch in a state of equilibrium so that it should stand, was 

 unlimited ; whereas Mr. Rennie established the limit to 

 which the countervailing force or weight on the extrados 

 should be confined. Hence he adopted the practice of 

 introducing a flat inverted arch between the extrados of 

 each two adjoining arches, (at the same time increasing 

 the width of the abutment,) the radii of the vous- 

 soirs or arch-stones being continued completely through 

 them. And in order to diminish the masonry, the 

 lower or foundation course was inclined also, thus 

 combining the work more completely together, and 



