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These are the later-style lights that overcame the two major defects
of the original embedded-directly-in-concrete kind—liability to fracture
due to compressive forces, and difficulty of replacing lenses—using the
same mechanism: the glass lens sits inside and is protected by
a cast iron cup. The cup takes the compressive forces, and the lens is only
caulked into the cup where it is isolated from stresses and can be easily
removed and replaced.
"In this construction, the
glass—both square lenses and round lenses—is armored against both
expansion pressure and the assaults of traffic. Each lens is caulked with
tar and brimstone compound in a cast iron galvanized ring, or bottomless
cup. This is built right into the concrete slab, being perfectly embedded
with a permanent bond." —Sweet's, 1927-28
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