Elements of the philosophy of the human mind. By Dugald Stewart. Rev. and abridged, with critical and explanatory notes, for the use of colleges and schools. By Francis Bowen ...

ia:EMOtY, 303
cury in the barometer. 3. In different states, the elastic force
of the air is reciprocally as the spaces which it occupies. But,
in this experiment, the mercury which remains suspended in
the tube, together with the elastic force of the air in the top of
the tube, is a counterbalance to the pressure of the atmosphere;
and therefore, their joint effect must be equal to the pressure of
a column of mercury twenty-eight inches high. Hence we
obtain an algebraical equation, which affords an easy solution of
the problem. It is further evident, that mny knowledge of the
physical laws which are here combined, puts it in my power to
foretell the result, not only in' this case, butt in all the cases of a
similar nature which can be supposed. The problem, in any
particular instance, might be solved by making the experiment;
but the result would be of no use to me, if the slightest alteration were made on the data.
It is in this manner that philosophy, by putting us in possession of a few general facts, enables us to determine, by reasoning, what will be the result of any supposed combination of
them, and thus to comprehend an infinite variety of particulars,
which no IMemory, however vigorous, would have been able to
retain. In consequence of the knowledge of such general facts,
the philosopher is relieved from the necessity of treasuring up
in his mind all those truths which are involved in his principles,
and which may be deduced from them by reasoning; and he
can often prosecute his discoveries, synthetically, in those parts
of the universe which he has no access to examine by immediate observation. There is, therefore, this important difference
between the hypothetical theory and a theory obtained by induction; that the latter not only enables us to remember the facts
we already know, but to ascertain, by reasoning, many facts
which we have never had an opportunity of examining: whereas,
when we reason from a hypothesis a priori, we are almost certain of running into error; and, consequently, whatever may
be its use to the Memory, it can never be trusted to in judging
of cases which have not previously fallen within our experience.
In what sciences /ypothetical theories are most useful. - There
are some sciences, in which hypothetical theories are more use