On the other hand, when once the efficiency of a
contrivance has been established, with good workmanship it will
be easy afterwards to ascertain the degree of perfection which
will suffice for its due action.
324. It is partly owing to the imperfection of the original
trials, and partly to the gradual improvements in the art of
making machinery, that many inventions which have been tried, and
given up in one state of art, have at another period been
eminently successful. The idea of printing by means of moveable
types had probably suggested itself to the imagination of many
persons conversant with impressions taken either from blocks or
seals. We find amongst the instruments discovered in the remains
of Pompeii and Herculaneum, stamps for words formed out of one
piece of metal, and including several letters. The idea of
separating these letters, and of recombining them into other
words, for the purpose of stamping a book, could scarcely have
failed to occur to many: but it would almost certainly have been
rejected by those best acquainted with the mechanical arts of
that time; for the workmen of those days must have instantly
perceived the impossibility of producing many thousand pieces of
wood or metal, fitting so perfectly and ranging so uniformly, as
the types or blocks of wood now used in the art of printing.
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