On pinions broad display'd, l. 375. The progressive motion of birds in the air is principally performed by the movement of their wings, and not by that of their tails as in fish. The bird is supported in an element so much lighter than itself by the resistance of the air as it moves horizontally against the oblique plain made by its breast, expanded tail and wings, when they are at rest; the change of this obliquity also assists it to rise, and even directs its descent, though this is owing principally to its specific gravity, but it is in all situations kept upright or balanced by its wings.
As the support of the bird in the air, as well as its progression, is performed by the motion of the wings; these require strong muscles as are seen on the breasts of partridges. Whence all attempts of men to fly by wings applied to the weak muscles of their arms have been ineffectual; but it is not certain whether light machinery so contrived as to be moved by their feet, might not enable them to fly a little way, though not so as to answer any useful purpose.[Back to Canto]
With laugh repress'd, l. 434. The cause of the violent actions of laughter, and of the difficulty of restraining them, is a curious subject of inquiry. When pain afflicts us, which we cannot avoid, we learn to relieve it by great voluntary exertions, as in grinning, holding the breath, or screaming; now the pleasurable sensation, which excites laughter, arises for a time so high as to change its name, and become a painful one; and we excite the convulsive motions of the respiratory muscles to relieve this pain. We are however unwilling to lose the pleasure, and presently put a stop to this exertion; and immediately the pleasure recurs, and again as instantly rises into pain. Which is further explained in Zoonomia, Sect. 34. 1. 4. When this pleasurable sensation rises into a painful one, and the customs of society will not permit us to laugh aloud, some other violent voluntary exertion is used instead of it to alleviate the pain.[Back to Canto]
With smile chastised, l. 434. The origin of the smile has generally been ascribed to inexplicable instinct, but may be deduced from our early associations of actions and ideas. In the act of sucking, the lips of the infant are closed round the nipple of its mother, till it has filled its stomach, and the pleasure of digesting this grateful food succeeds; then the sphincter of the mouth, fatigued by the continued action of sucking, is relaxed; and the antagonist muscles of the face gently acting, produce the smile of pleasure, which is thus during our lives associated with gentle pleasure, which is further explained in Zoonomia, Sect. 16. 8. 4.[Back to Canto]
How Oxygen, l. 13. The atmosphere which surrounds us, is composed of twenty-seven parts of oxygen gas and seventy-three of azote or nitrogen gas, which are simply diffused together, but which, when combined, become nitrous acid. Water consists of eighty-six parts oxygen, and fourteen parts of hydrogen or inflammable air, in a state of combination. It is also probable, that much oxygen enters the composition of glass; as those materials which promote vitrification, contain so much of it, as minium and manganese; and that glass is hence a solid acid in the temperature of our atmosphere, as water is a fluid one.[Back to Canto]
Two electric streams, l. 21. It is the opinion of some philosophers, that the electric ether consists of two kinds of fluids diffused together or combined; which are commonly known by the terms of positive and negative electricity, but are by these electricians called vitreous and resinous electricity. The electric shocks given by the torpedo and by the gymnotus, are supposed to be similar to those of the Galvanic pile, as they are produced in water. Which water is decomposed by the Galvanic pile and converted into oxygen and hydrogen gas; see Additional Note [XII].
The magnetic ether may also be supposed to consist of two fluids, one of which attracts the needle, and the other repels it; and, perhaps, chemical affinities, and gravitation itself, may consist of two kinds of ether surrounding the particles of bodies, and may thence attract at one distance and repel at another; as appears when two insulated electrised balls are approached to each other, or when two small globules of mercury are pressed together.[Back to Canto]
And Irritation moves, l. 64. Irritation is an exertion or change of some extreme part of the sensorium residing in the muscles or organs of sense in consequence of the appulses of external bodies. The word perception includes both the action of the organ of sense in consequence of the impact of external objects and our attention to that action; that is, it expresses both the motion of the organ of sense, or idea, and the pain or pleasure that succeeds or accompanies it. Irritative ideas are those which are preceded by irritation, which is excited by objects external to the organs of sense: as the idea of that tree, which either I attend to, or which I shun in walking near it without attention. In the former case it is termed perception, in the latter it is termed simply an irritative idea.[Back to Canto]
And young Sensation, l. 72. Sensation is an exertion or change of the central parts of the sensorium or of the whole of it, beginning at some of those extreme parts of it which reside in the muscles or organs of sense. Sensitive ideas are those which are preceded by the sensation of pleasure or pain, are termed Imagination, and constitute our dreams and reveries.[Back to Canto]