338 
THE ASTRONOMER ROYAL’S MAGNETICAL OBSERVATIONS IN 
have hacl square plates constructed of narrow planks, riveted together like those of the 
bridges, the trials of which were thus conducted : — First, the square plate was placed 
on the equatoreal plane of a “ Magnetic Anvil ” and carefully hammered, and was then 
placed under a prismatic compass with which a distant object was viewed, with its planks 
directed at one time N.E. or S.W., and at another time N.W. or S.E. ; it gave no sign 
of quadrantal deviation. Second, the plate was placed on the dip-plane and hammered 
with the same violence (as well as I could judge), with each of its four edges downwards 
in four different examinations, and after each hammering was placed under the compass, 
with the edge that had been lowest placed alternately E. and W. ; the deviations produced 
were sensibly the same (at least I could not certainly answer for any difference), whether 
the sides of the planks had been horizontal or vertical during the hammering upon the 
dip-surface. Both experiments appear to show that none of the magnetic results in the 
bridge-experiments can depend on the direction of the iron planks. 
I may, however, mention that in the horizontal structures forming the roof and floor 
of the bridge the planks are longitudinal, and that in the vertical side walls the planks 
are vertical. 
Postscript. 
Received October 22, 1872. 
With the hope of obtaining some information which might explain the anomaly in 
the amount of longitudinal disturbance in the Anglesey Water Tubes, I inquired of 
Edwin Clark, Esq. (under whose immediate superintendence the Britannia and Conway 
Bridges were constructed) whether there was any thing peculiar in the iron material of 
those tubes. Mr. Clark informed me that there was no known difference in the iron ; 
but he reminded me that the Eastern Anglesey Water Tube was the tube first raised, 
and that it was this tube which, in consequence of the bursting or rather longitudinal 
disruption of one of the cylinders of the hydrostatic press by which it was raised, suffered 
at one end a fall of 8 or 9 inches. The details of this accident are given in the work 
4 On the Britannia and Conway Tubular Bridges,’ pages 690 &c. There is no doubt that 
the strain then sustained by the tube greatly exceeded any other strain to which it has 
been exposed. On referring to the first Table, it will be seen that the times of vibration 
are disturbed in both the Anglesey Water Tubes, slightly more so in the eastern than 
in the western tube. If, therefore, the magnetic longitudinal influence exhibited within 
the western tube was really affected by this accident, and to the supposed amount (a 
thing on which I have no doubt), it would seem that the longitudinal effect of the 
eastern tube is nearly as great at an external point (at a distance equal to that of 
the collateral tubes in this bridge, 27 feet centre to centre) as in the centre of that 
eastern tube — a law which it is impracticable to verify in this instance, but which I think 
is very probably correct. 
Mr. Clark remarks, “ It would indeed be extremely interesting if you discovered the 
