blown, or on an insect in all the pride and beauty of its colours; without observing immediately the different stages through which they have passed, the different forms they have assumed, the different changes they have undergone, and without descending to the seeds and principles from which they spring, and which upon examination will be found totally different both in form and colour. In like manner are the senses, the ultimate criteria of all physical knowledge, liable to be imposed upon and deceived in regard to the qualities and causes, the powers and operations of physical body'.

The senses are therefore to be assisted by various observations taken with diligence and circumspection, and to be undeceived by different analyses, which divest Nature of her external and compounded form, and lay open

'Edificium autem hujus universi, structura sua, intellectui, humano contemplanti, instar labyrinthi est; ubi tot ambigua viarum, tam fallaces rerum et signorum similitudines, tam obliquæ et implexæ naturarum spiræ et nodi, undequaque se ostendunt; iter autem, sub incerto sensus lumine, interdum affulgente, interdum se condente, per experientiæ et rerum particularium sylvas, perpetuo faciendum est.-Bacon. Nov. Org. Præf. See also Reid's Essays, vol. ii. p. 22, 290, &c.

her internal mechanism and construction. Their errors and misconceptions are to be corrected by the use of experiments of different kinds, which penetrate her inmost recesses, and descend to her remotest causes. By the application of such assistance they are enabled, not without difficulty', to leave behind the fallacious exterior, to pass from one phenomenon to another, and as far as human search can go, to judge of the elements of nature.

"The information which the senses give us," as the great friend and father of philosophers has observed3, "is to be examined and corrected by various methods; for though they deceive us on some occasions, they themselves discover the errors into which they lead. But whereas the errors lie imme

Quin etiam duces itineris (ut dictum est) qui se offerunt, et ipsi implicantur; atque errorum et errantium numerum augent. In rebus tam duris, de judicio hominum ex vi propria, aut etiam de felicitate fortuita, desperandum est. Neque enim ingeniorum quantacunque excellentia, neque experiendi alea sæpius repetita, ista vincere queat. Vestigia filo regenda sunt: omnisque via usque a primis ipsis sensuum perceptionibus, certa ratione munienda.-Ibid.

'See Lord Bacon on the Advancement of Learning, Distrib. Op. p. 15.

diately before us, the indications of them are to be sought at a great distance.

"The senses are subject to a twofold defect. They may either desert or deceive ús. Many subjects' elude their cognizance, however well they may be disposed and free from impediment, either from the tenuity of the whole object or the extreme minuteness of its parts, from the distance of its situation, the slowness or the velocity of its motion, its familiarity to the eye, and from many other And again, where they fully appre-.

causes.

Air and fire are bodies of the most universal extent and operation in the material system. And the great Boerhaave, speaking of the latter, makes the following observation :"So great is the power, so extensive the action, and so wonderful the manner wherein fire acts, that it was anciently held and adored as the supreme God by a nation reputed the wisest of all others. Thus some of the chemists, having found its extraordinary force, took it for an uncreated being, and many of the most eminent among them attributing all the knowledge they had acquired to this instrument, called themselves philosophers of fire, as thinking they could not be dignified by a higher title. There is however nothing more wonderful in the nature of fire, than that whilst it is the chief cause and principle of almost all the effects cognizable by our senses, itself is imperceptible by any sense, being so incomprehensible, by reason of its extreme minuteness, that it eludes our nicest research; so that with many it passes for a spirit rather than a body."-Boerhaave's Chemistry.

hend their object, they are not to be securely relied upon; for the testimony and information of the senses depend on the analogy and constitution of man, and not on those of the universe; so that to say that sense is the adequate measure or competent judge of things, is an assertion founded in mistake.

To obviate the imperfections of sense, philosophers are under the necessity, by much labour and attention, of calling in aid and assistance from every quarter, in order to supply the deficiency where the senses fail, and also to regulate and rectify them where they vary in themselves. This is effected not so much by the use of instruments as by the help of experiment. For experiments are much more penetrating and subtle than the senses, even when assisted by instruments of the most exquisite contrivance; I mean such experiments as are ingeniously invented, and applied with skill and address to the elucidation of the very thing which is the subject of inquiry. Philosophers do not therefore rely on the perception of the senses immediately applied, as in their natural and common exercise; but bring the matter of

judging to this issue, that the senses may judge of experiments, and experiments of things. Thus experiments serve in fact as thè religious guardians of the senses, from which every thing in sound philosophy is originally derived, and become the skilful interpreters of their oracles; so that, whilst others only pretend, the true philosopher in reality cultivates and supports the evidence of sense".

BY

SECT. III.

Of Physical Reasoning.

Y such experiments and observations in aid of the external senses skilfully chosen, artfully conducted, and judiciously applied, the philosopher advances from one stage of inquiry to another', by a slow but steady pace, in the rational investigation of the general causes of physical truth.

5 Consult the Preliminary Discourse of Sir J. Herschel on the study of Natural Philosophy; Playfair's Dissertation on Mathematical and Physical Science; Whewell's Bridgewater Treatise, &c.-Editor.

'Nonnulli qui experientiæ undis se commisere, et fere me