rithmetic.

But we cannot proceed a single step without deviating from common
language; if the theory of the balance, or the lever, is to be
explained, we immediately speak of _space_ and _time_. To persons not
versed in literature, it is probable that these terms appear more
simple and unintelligible than they do to a man who has read Locke,
and other metaphysical writers. The term _space_ to the bulk of
mankind, conveys the idea of an interval; they consider the word
_time_ as representing a definite number of years, days, or minutes;
but the metaphysician, when he hears the words _space_ and _time_,
immediately takes the alarm, and recurs to the abstract notions which
are associated with these terms; he perceives difficulties unknown to
the unlearned, and feels a confusion of ideas which distracts his
attention. The lecturer proceeds with confidence, never supposing
that his audience can be puzzled by such common terms. He means by
_space_, the distance from the place whence a body begins to fall, to
the place where its motion ceases; and by time, he means the number of
seconds, or of any determinate divisions of _civil_ time which elapse
from the commencement of any motion to its end; or, in other words,
the duration of any given motion. After this has been frequently
repeated, any intelligent person perceives the sense in which they are
used by the tenour of the discourse; but in the interim, the greatest
part of what he has heard, cannot have been understood, and the
premises upon which every subsequent demonstration is founded, are
unknown to him. If this be true, when it is affirmed of two terms
only, what must be the situation of those to whom eight or ten unknown
technical terms occur at the commencement of a lecture? A complete
knowledge, such a knowledge as is not only full, but familiar, of all
the common terms made use of in theoretic and practical mechanics, is,
therefore, absolutely necessary before any person can attend public
lectures in natural philosophy with advantage.

What has been said of public lectures, may, with equal propriety, be
applied to private instruction; and it is probable, that inattention
to this circumstance is the reason why so few people have distinct
notions of natural philosophy. Learning by rote, or even reading
repeatedly, definitions of the technical terms of any science, must
undoubtedly facilitate its acquirement; but conversation, with the
habit of explaining the meanin