Benjamin C. B. Tilghman, of Philadelphia, once went into the lighthouse
at Cape May, and, observing that the window glass was translucent
rather than transparent, asked the keeper why he put ground glass in
the windows. "We do not," said the keeper. "We put in the clear
glass, and the wind blows the sand against it and roughens the outer
surface like ground glass." The answer was to him like the falling
apple to Newton. He put on his thinking cap and went out. It was
better than the cap of Fortunatus to him. He thought, "If nature does
this, why cannot I make a fiercer blast, let sand trickle into it, and
so hurl a million little hammers at the glass, and grind it more
swiftly than we do on stones with a stream of wet sand added?"
He tried jets of steam and of air with sand, and found that he could
roughen a pane of glass almost instantly. By coating a part of the
glass with hot beeswax, applied with a brush, through a stencil, or
covering it with paper cut into any desired figures, he could engrave
the most delicate and intricate patterns as readily as if plain. Glass
is often made all white, except a very thin coating of brilliant
colored glass on one side. This he could cut through, leaving letters
of brilliant color and the general surface white, or _vice versa_.
Seal cutting is a very delicate and difficult art, old as the Pharaohs.
Protect the surface that is to be left, and the sand blast will cut out
the required design neatly and swiftly.
There is no known substance, not even corundum, hard enough to resist
the swift impact of myriads of little stones.
It will cut more granite into shape in an hour than a man can in a day.
Surely no one will be sorry to learn that General Tilghman sold part of
his patents, taken out in October, 1870, for $400,000, and receives the
untold benefits of the rest to this day. So much for thinking.
Nature gives thousands of hints. Some can take them; some can only
take the other thing. The hints are greatly preferred by nature and
man.