s, but even then we are likely to get a
cinder in one eye, to swallow germs by the dozens, and to get a gray
coating of plain, harmless dust. We welcome the rain that lays the dust,
or its feeble imitation, the water sprinkler, that brings us temporary
relief.

On the quietest day, even after a thorough sweeping and dusting of the
library, you are able to write your name plainly on the film of dust
that lies on the polished table. Take a book from the open shelves, and
blow into the trough of its top. This is always dusty. Where does the
dust come from? This is the house-keeper's riddle.

The answer is not a hard one. I look out of my window on a street which
is famous as the road Washington took on his retreat from White Plains
to Trenton. It has always been the main thoroughfare between New York
and Philadelphia, and now is the route that automobiles follow. A
constant procession of vehicles passes my house, and to-day each one
approaches in a cloud of dust. The air is gray with suspended particles
of dirt. The wind carries the successive clouds, and they roll up
against the houses like breakers on the beach. Windows and doors are
loose enough to let dust sift in. When a door opens, the cloud enters
and lights on rugs and carpets and curtains. Any ledge collects its
share of dust. The beating of carpets and rugs disturbs the accumulated
dust of many months.

[Illustration: In this lonely Arizona desert the wind drifts the sand
into dunes, just as it does on the toe of Cape Cod]

[Illustration: The Grand Canyon of the Colorado shows on a magnificent
scale the work of water in cutting away rock walls]

The wind sweeps the ploughed field, and takes all the dust it can carry.
It blows the finest top soil from our gardens into the street. It blows
soil from other fields and gardens into ours, so the level of our land
is not noticeably lowered. The wind strips the high land and drops its
burden on lower levels. This is one of the big jobs the water has to do,
and the wind is a valuable helper. To tear down the mountains and fill
in the valleys is the great work of the two partners, wind and water.

Dead, still air holds the finest dust, without letting it fall. The
buoyancy of the particles overcomes their weight. We see them in a
sunbeam, like shining points of precious metal, and watch them. A light
breeze picks up bits of soil and litter, from the smallest up to a
certain size and weight. If the velocity of the win