shown us a similar exploitation of the beaten paths
and of the road-mender's macadam. All these open-air builders, all
these erectors of monuments exposed to wind and weather require an
exceedingly dry stone-dust; otherwise the material, already moistened
with water, would not properly absorb the liquid that is to give it
cohesion; and the edifice would soon be wrecked by the rains. They
possess the sense of discrimination of the plasterer, who rejects
plaster injured by damp. We shall see presently how the insects that
build under shelter avoid this laborious macadam-scraping and give the
preference to fresh earth already reduced to a paste by its own
dampness. When common lime answers our purpose, we do not trouble about
Roman cement. Now Eumenes Amedei requires a first-class cement, even
better than that of the Chalicodoma of the Walls, for the work, when
finished, does not receive the thick covering wherewith the Mason-bee
protects her cluster of cells. And therefore the cupola-builder, as
often as she can, uses the highway as her stone-pit.

With the mortar, flints are needed. These are bits of gravel of an
almost unvarying size--that of a peppercorn--but of a shape and kind
differing greatly, according to the places worked. Some are
sharp-cornered, with facets determined by chance fractures; some are
round, polished by friction under water. Some are of limestone, others
of silicic matter. The favourite stones, when the neighbourhood of the
nest permits, are little nodules of quartz, smooth and semitransparent.
These are selected with minute care. The insect weighs them, so to say,
measures them with the compass of its mandibles and does not accept
them until after recognizing in them the requisite qualities of size
and hardness.

A circular fence, we were saying, is begun on the bare rock. Before the
mortar sets, which does not take long, the mason sticks a few stones
into the soft mass, as the work advances. She dabs them half-way into
the cement, so as to leave them jutting out to a large extent, without
penetrating to the inside, where the wall must remain smooth for the
sake of the larva's comfort. If necessary, a little plaster is added,
to tone down the inner protuberances. The solidly embedded stonework
alternates with the pure mortarwork, of which each fresh course
receives its facing of tiny encrusted pebbles. As the edifice is
raised, the builder slopes the construction a little towards the centre
and