Note 103 — Chapter XVIII

The effect of sunshine in deflecting the bridge is very curious. When the first main tube was tested, ballast-wagons loaded with iron were drawn into the centre and left standing there. The first 20 tons increased the deflection an eighth of an inch, and with 50 tons the deflection was 9 inches. After standing all night, the deflection in the morning was found to be only 8-3/8 inches. How was this to be accounted for? Mr. Clark says: "This was attributed at the time to an error made in the reading; but this, and many other anomalies in the deflection, were afterward fully accounted for by local changes of temperature. A gleam of sunshine on the top of the tube raised it on one occasion nearly an inch in half an hour with 200 tons at the centre, the top plates being expanded by increase of temperature, while the lower plates remained constant from radiation to the water immediately beneath them. In a similar manner, the tube was drawn sidewise to the extent of an inch from the sun shining on one side, and returned immediately as clouds passed over the sun, being, in fact, a most delicate thermometer in constant motion, both vertically and laterally."