Tron Bridges on the Principle of Tenacity. 389 
common fuel of any kiad, and increase the value of the ashes as 
a manure. The mud swept up in the streets of London, and 
other paved towns, will answer admirably in lieu of mar! or clay, 
or with a little clay to give it adhesion, as such mud must un- 
avoidably contain a considerable quantity of iron, 
It may be observed, for the advantage of those unacquainted 
with chemistry, that sand is principally an oxide, that carbon. at- 
tracts oxygen from almost every known substance at elevated 
temperatures 5 and oxygen being the cause of combustion, every 
substance containing it, which can be decomposed, becomes a 
supporter: these balls therefore will prove of material conside- 
ration as a cheap fuel, aud will, I judge by estimation, if judici- 
ously managed, produce a saving of one half the quantity of coals 
used in general. To thousands this must be of valuable consi- 
deration. 
When the fire is at a sufficient temperature, it will be of still 
further advantage to sprinkle it occasionally with water; this will 
afford additional oxygen and produce increased effect: for this 
purpose a smail watering-pot ma become a part of the apparatus 
purp 8-} y Pp pp 
usually attached to a common grate. The balls also will pro- 
duce a greater effect moderately damp, than when perfectly dryy 
and when formed they should nat be too much compressed. 
I remain, sir, 
Your obedient servant, 
Hampshire, Noy. 3, 1817. A SUBSCRIBERs 
P.S. Balls of a similar composition haye been used in Wales 
from a very remote period. 
IRON BRIDGES ON THE PRINCIPLE OF TENACITY. 
The following is an explanatory statement, which has been put 
in circulation, of the principle of tenacity, on which the iron 
bridges projected by Mr. Dodd, engineer, over the ‘Thames at 
Hammersmith aud Rotherhithe, are designed to be executed. 
In the construction of the newly invented iron bridges, on what 
is termed the principle of tenacity, the objects are, to form and 
adjust their several parts with a particular view to that important 
quality of the metal, which disposes it, on being stretched, not 
merely to resist and keep its hold, but to appear to draw or pull 
in a direction opposite to that in which the force that sets upon it 
is applied. 
In the construction of other iron bridges the metal is employed 
like any common hard and bulky substance that is capable of 
having its pieces connected together ; and the several pieces of 
it are so arranged to rest and press against each other, as if they 
possessed no other property than their solidity, extension, and 
weight. In the Southwark bridge, for instance, we see the plates 
Bb3s of 
