Mr Stevenson's Description of Bridges of Suspension. 255 
these together give 100+ 47 — 147, or say 150 tons. Assuming 
the angle of suspension at 12 degrees, we find that the real 
weight is increased to about 370 tons of tension on the catena- 
rian chains. But as we find the strength of these chains to be 
equal to 1104 tons, there remains a surplus strength of about 
734, or say 700 tons, to resist any emergency beyond a weight 
of 50 tons. 
But the effect we have to provide against in bridges of sus- 
pension, is not merely what is technically termed dead-weight. 
A more powerful agent exists in the sudden impulses, or jerking 
motion of the load, which we have partly noticed in the descrip- 
tion of Dryburgh Bridge. The greatest trial, for example, 
which the timber bridge at Montrose, about 500 feet in extent, 
has been considered to withstand, is the passing of a regiment 
of foot, marching in regular lime. A troop of cavalry, on the 
contrary, does not produce corresponding effects, owing to the 
irregular step of the horses. The same observations apply to a 
crowd of persons walking promiscuously, or a drove of cattle, 
which counteract the undulating and rocking motion, observed 
on some occasions at the bridge of Montrose, when infantry has 
been passing along it. Hence also the effects of gusts of wind, 
often and violently repeated, which destroy the equilibrium of 
the parts of a bridge of suspension ; and the importance of ha- 
ving the whole roadway and side-rails framed in the strongest 
possible manner. 
Progress of Improvements.— -It is not a little curious and 
interesting to trace the discovery and progress of improvements 
in the several departments of the arts and sciences, nor will it be 
considered foreign to our purpose, if we notice an instance or two 
of this kind. In the case of impelling boats by steam, for example, 
we know that this was first suggested and pointed out by Jona- 
than Hulls of England, about the year 1735, and was applied to 
practical and extensive use many years afterwards, in the United 
States of America (as noticed in the Annals of Philosophy, vol.xiii. 
p. 279.) It has also long been known, that chain-bridges of 
great extent have been erected in the Chinese Empire ; and 
w r e have seen, that, about the year 1741, a bridge of this kind 
was constructed over the Tees, and has now formed a com- 
munication between the shires of Durham and York, for about 
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