Another notable engineering feature are the chains of huge eyebars under the roadway that tie together the ends of each arch; the tied-arch technique resisted the outward horizontal thrust of the arches and thus allowed the piers to be much smaller and more economical.
The text describes a specific engineering design element: chains of large eyebars connecting the ends of arches to resist outward horizontal forces on the structure. This allowed for smaller and more cost-effective piers.