The present year has been remarkable for the large number of machines invented for the purpose of superseding steam, in at least some of its lighter tasks. Many of these are due to French engineers; being further proofs, if any were required, of the great activity now displayed in France in all matters of mechanical invention. Two of these new engines are especially interesting as illustrating that all-important law in modern physics, the correlation or convertibility of forces. By this is meant that the forces of inanimate nature, such as light, heat, electricity--nay, even the muscular and nerve forces of living beings--have such a mutual dependence and connection that each one is only produced or called into action by another, and only ceases to be manifest when it has given birth to a fresh force in its turn. Thus motion (in the {30} shape of friction) produces heat, electricity, or light; heat produces light or electricity; electricity, magnetism; and so on in an endless chain, which links together all the phenomena of this visible universe. As a metaphysical principle, this is as old as Aristotle, and may be found dimly foreshadowed in the forcible lines of Lucretius: "--Pereunt imbres, ubi eos pater aether In gremium matris terrai praecipitavit; At nitidae surgunt fruges, ramique virescunt, Arboribus crescunt ipsae, fetuque gravautur, Hinc alitar porro nostrum genus atque ferarum. * * * * * * Haud igitur penitus pereunt quaecumque videntur, Quando aliud ex alio reflcit natura, nec ullam Rem gigni patitur, nisi morte adjuta aliena." [Footnote 8] [Footnote 8: Lucret. lib. i. 250-65.] But the rediscovery of this law, as a result of experiment, is due to English physicists of our own day; and it is so invariably true, and the produced force is always so perfectly proportioned to the force producing it, that some [Footnote 9] have gone so far as to revive a very old hypothesis in philosophy, supposing that all the forces of nature are but differently expressed forms of the Divine Will. [Footnote 9: Dr. Carpenter, Philos. Trans. 1840, vol. ii. ]