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arch, and the reason why this makes the arch durable is this. We
know very well that if the arch is loaded with an excess of weight
above its quarter as a b, the wall f g will be thrust outwards
because the arch would yield in that direction; if the other quarter
b c were loaded, the wall f g would be thrust inwards, if it
were not for the line of stones x y which resists this.
7
87.
PLAN.
Here it is shown how the arches made in the side of the octagon
thrust the piers of the angles outwards, as is shown by the line h
c and by the line t d which thrust out the pier m; that is they
tend to force it away from the centre of such an octagon.
7
88.
An Experiment to show that a weight placed on an arch does not
discharge itself entirely on its columns; on the contrary the
greater the weight placed on the arches, the less the arch transmits
the weight to the columns. The experiment is the following. Let a
man be placed on a steel yard in the middle of the shaft of a well,
then let him spread out his hands and feet between the walls of the
well, and you will see him weigh much less on the steel yard; give
him a weight on the shoulders, you will see by experiment, that the
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