is the exact description of _the weaver's alarm_, mentioned in
the Philosophical Transactions which C---- had never seen or heard of.

Mr. ---- now showed us the patent extinguisher, which was much
approved of by all the rival inventors.

It is very useful to give children problems which have already been
solved, because they can immediately compare their own imperfect ideas
with successful inventions, which have actually been brought into real
use. We know beforehand what ideas are necessary to complete the
invention, and whether the pupil has all the necessary knowledge.
Though by the courtesy of poetry, a creative power is ascribed to
inventive genius, yet we must be convinced that no genius can invent
without materials. Nothing can come of nothing. Invention is nothing
more than the new combination of materials. We must judge in general
of the ease or difficulty of any invention, either by the number of
ideas necessary to be combined, or by the dissimilarity or analogy of
those ideas. In giving any problem to children, we should not only
consider whether they know all that is necessary upon the subject, but
also, whether that knowledge is sufficiently _familiar_ to their
minds, whether circumstances are likely to recall it, and whether they
have a perfectly Clear idea of the thing to be done. By considering
all these particulars, we may pretty nearly proportion our questions
to the capacity of the pupil; and we may lead his mind on step by step
from obvious to intricate inventions.

July 30th, 1796. L----, who had just returned from Edinburgh, and had
taken down in two large volumes, Dr. Black's Lectures, used to read to
us part of them, for about a quarter of an hour, every morning after
breakfast. He was frequently interrupted (which interruptions he bore
with heroic patience) by Mr. ----'s explanations and comments. When he
came to the expansive power of steam, and to the description of the
different steam engines which have been invented, Mr. ---- stopped to
ask B----, C----, and S----, to describe the steam engine in their own
words. They all described it in such a manner as to show that they
clearly understood the principle of the machine. Only the general
principle had been explained to them. L----, after having read the
description of Savary's and Newcomen's steam engines, was beginning to
read the description of that invented by Mr. Watt; but Mr. ----
stopped him, that he might try whether any person pres