think, evident that they saw no such necessity[46\3]. Their idea of mechanical action was not sufficiently distinct to enable them to see the absurdity of {281} supposing an intense pressure with no obstacle for it to exert itself against. [Note 45\3: _Hist. Ind. Sc._ b. iv. c. i. sect. 2.] [Note 46\3: See Prof. Powell, _On the Nature and Evidence of the Laws of Motion_. _Reports of the Ashmolean Society_. Oxford. 1837. Professor Powell has made an objection to my use of this instance of confusion of thought; the remark in the text seems to me to justify what I said in the History. As an evidence that the fish was not supposed to produce its effect by its muscular power acting on the water, we may take what Pliny says, _Nat. Hist._ xxxii. 1, 'Domat mundi rabiem, nullo suo labore; non retinendo, aut alio modo quam adhærendo:' and also what he states in another place (ix. 41), that when it is preserved in pickle, it may be used in recovering gold which has fallen into a deep well. All this implies adhesion alone, with no conception of reaction.] 3. We may trace, in more modern times also, indications of a general ignorance of mechanical truths. Thus the phrase of shooting at an object 'point-blank,' implies the belief that a cannon-ball describes a path of which the first portion is a straight line. This errour was corrected by the true mechanical principles which Galileo and his followers brought to light; but these principles made their way to popular notice, principally in consequence of their application to the motions of the solar system, and to the controversies which took place respecting those motions. Thus by far the most powerful argument against the reception of the Copernican system of the universe, was that of those who asked, Why a stone dropt from a tower was not left behind by the motion of the earth? The answer to this question, now universally familiar, involves a reference to the true doctrine of the composition of motions. Again; Kepler's persevering and strenuous attempts[47\3] to frame a physical theory of the universe were frustrated by his ignorance of the first law of motion, which informs us that a body will retain its velocity without any maintaining force. He proceeded upon the supposition that the sun's force was requisite to _keep up_ the motion of the planets, as well as to deflect and modify it; and he was thus led to a system which represented the sun as carrying round the planets in their orbits by means of a _vortex_, produced by his revolution. The same neglect of the laws of motion presided in the formation of Descartes' system of vortices. Although Descartes had enunciated in words the laws of motion, he and his followers showed that they had not the practical habit of referring to these mechanical principles; and dared not trust the planets to move in free space without some surrounding machinery to support them[48\3]. [Note 47\3: _Hist. Ind. Sc._ b. v. c. iv. and b. vii. c. i.]