THE BRITANNIA AND CONWAY TUBULAR IRON BRIDGES. 
339 
effects of a fall that took place a quarter of a century ago by a magnetic experiment made 
now.” I believe that this point has really been gained. 
The tubes of the Britannia Bridge were built and riveted with their lengths very 
nearly in the magnetic E.-W. direction, in which state the terrestrial magnetism would 
have very little influence on them. It seems probable, therefore, that the magnetism 
which they now possess has been received entirely since their establishment on their 
piers. 
With regard to the anomaly of the longitudinal forces in the Conway Bridge, 1 find 
that the tubes were built and riveted with the Holyhead end about 48° 50' west of the 
magnetic N. ; in this state the tubes would receive much longitudinal magnetism 
with the sign that is shown in our observations. Each tube was then mounted with its 
axis nearly magnetic E.-W. ; and it appears that the forces subsequently acting upon 
it have not sensibly altered the longitudinal magnetism impressed on it during its 
construction. 
1872, October 21. 
