C 202 J 
XV. On the mathematical theory of suspension bridges, with 
tables for facilitating their construction. By Davies Gilbert, 
Esq. V.P.R.S. &c. Communicated March 9, 1826. 
Read March 9, 1826. 
JVIy attention was first directed to a consideration of suspen- 
sion bridges, and of the catenary curve on which their theory 
depends, when the plan for making such a communication 
across the Menai Straits was submitted to the Commissioners 
appointed by Parliament to improve the communication by 
roads and bridges through Wales. It then appeared to me, 
that the proposed depth of curvature, was not sufficient for 
ensuring such a degree of strength and permanence as would 
be consistent with the due execution of a great national 
work. This opinion I advanced as a Member of the Com- 
mission. But wishing to take on myself the full responsibi- 
lity for such increased expense, as must of necessity be 
occasioned by enlarging the curvature, I also printed some 
approximations, hastily deduced, in the Quarterly Journal of 
Science ; and derived from them a confirmation of the opinion 
that had been given. The interval between the points of 
support and the road-way of the Menai Bridge has in conse- 
quence been augmented to fifty feet ; and it now possesses that 
full measure of strength, which experience has established as 
requisite and sufficient for works of iron not perfectly at rest. 
Since bridges of suspension are obviously adapted to very 
general use, I have flattered myself with the hope of doing 
something serviceable to the public, by expanding into tables 
