VI. CHA P. combination of different laws, which it is neceffary to attend to, in order to be able to predict the refult. 1. The air is a heavy fluid, and the preffure of the atmosphere is measured by the column of mercury in the barometer. 2. The air is an elastic fluid; and its elasticity at the earth's furface (as it resists the preffure of the atmosphere) is measured by the column of mercury in the barometer. 3. In different fates, the elaftic force of the air is reciprocally as the spaces which it occupies. But, in this experiment, the mercury which remains fufpended in the tube, together with the elastic force of the air in the top of the tube, is a counterbalance to the preffure of the atmosphere; and therefore their joint effect must be equal to the preffure 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 my knowledge of the phyfical laws which are here combined, puts it in my power to foretel the refult, not only in this cafe, but in all the cases of a fimilar nature which can be fuppofed. The problem, in any particular instance, might be folved by making the experiment; but the refult would be of no use to me, if the slightest alteration were made on the data. It is in this manner that philofophy, by putting us in poffeffion of a few general facts, enables us to determine, by reafoning, what will be the refult of any supposed combination of them, and thus to comprehend an infinite variety of particulars, which no memory, however vigorous, would have been able to retain. In confequence of the knowledge of fuch general facts, the philofopher is relieved from the neceffity of treasuring up in 439 VI. his mind, all those truths which are involved in his principles, CHAP. 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 accefs to examine by immediate obfervation. There is, therefore, this important difference between a 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 reafon from a hypothesis a priori, we are almoft certain of running into error; and, confequently, 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. THERE are some sciences, in which hypothetical theories are more useful than in others; thofe sciences, to wit, in which we have occafion for an extensive knowledge and a ready recollection of facts, and which, at the fame time, are yet in too imperfect a state to allow us to obtain just theories by the method of induction. This is particularly the cafe in the science of medicine, in which we are under a neceffity to apply our knowledge, fuch as it is, to practice. It is alfo, in fome degree, the cafe in agriculture. In the merely fpeculative parts of phyfics and chemistry, we may go on patiently accumulating. facts, without forming any one conclufion, farther than our facts authorise us; and leave to pofterity the credit of establishing the theory to which our labours are fubfervient. But in medicine, in which it is of confequence to have our knowledge at command, it feems reasonable to think, that 9 hypo CHA P. combination of different laws, which it is 1 VI. in order to be able to predict the result. IT is in this manner th SECTION V. Continuation of the fame fubject.—Effects produced on the Memory by committing to Writing our acquired Knowledge. HAVING treated at confiderable length of the improvement of memory, it may not be improper, before leaving this part of the fubject, to confider what effects are likely to be produced on the mind by the practice of committing to writing our acquired knowledge. That such a practice is unfavourable, in fome refpects, to the faculty of memory, by fuperfeding, to a certain degree, the neceffity of its exertions, has been often remarked, and I believe is true; but the advantages with which it is attended in other respects, are so important, as to overbalance greatly this trifling inconvenience. It is not my intention at present to examine and compare together the different methods which have been propofed, of keeping a common-place book. In this, as in other cafes of a fimilar kind, it may be difficult, perhaps, or impoffible, to eftablish any rules which will apply univerfally. Individuals must be left to judge for themselves, and to adapt their contrivances to the particular nature of their literary pursuits, and to their own peculiar habits of affociation and arrangement. The remarks which I am to offer are very general, and are intended merely to illuftrate a few of the advantages which the art of writing affords CHA P. VI. CHAP. to the philosopher, for recording, in the course of his progress through life, the refults of his fpeculations, and the fruits of his experience. THE utility of writing, in enabling one generation to transmit its difcoveries to another, and in thus giving rise to a gradual progress in the fpecies, has been fufficiently illuftrated by many authors. Little attention, however, has been paid to another of its effects, which is no lefs important; I mean, to the foundation which it lays for a perpetual progrefs in the intellectual powers of the individual. It is to experience, and to our own reflections, that we are indebted for by far the most valuable part of our knowledge: and hence it is, that although in youth the imagination may be more vigorous, and the genius more original, than in advanced years; yet, in the cafe of a man of observation and inquiry, the judgment may be expected, at least as long as his faculties remain in perfection, to become every day founder and more enlightened. It is, however, only by the conftant practice of writing, that the refults of our experience, and the progress. of our ideas, can be accurately recorded. If they are trusted merely to the memory, they will gradually vanish from it like a dream, or will come in time to be fo blended with the fuggeftions of imagination, that we fhall not be able to reason from them with any degree of confidence. What improvements in science might we not flatter ourselves with the hopes of accomplishing, had we only activity and industry to treasure up every plaufible hint that occurs to us! Hardly a day paffes, when many fuch do not occur to ourselves, or are fuggefted by others: |