« ForrigeFortsæt »
there is an analogy or similitude in some particulars, though in others a great diversity.* The telescope and microscope compared with the eye may be mentioned as examples. Bacon's ear-spectacle, (or ear-trumpet, as it is now named,) and the concave roof of the dungeons of Syracuse, commonly called the
ear of Dyonisius,' by which that tyrant used to listen to the words and whispers of his prisoners, are examples of a partial conformity with the construction of the human ear.
• Strictly speaking, Analogy signifies a similitude of proportions; but in a looser and translated sense it has been used not merely with reference to the relation of one quantity to another, but as signifying all similitude of relations whatsoever. In common discourse, things are said to be analogous when they are, in fact, only similar.—See Berkeley's Works, vol. 2, p. 63. In a short and luminous section, this great master of our language, and most acute logician, fully elucidates a subject which has produced not a little perplexity in the arguments of some of our theological writers. Bp. Berkeley's reasonings on this head have been ably illustrated by Bp. Copleston in his well-known · Four Discourses,' pp. 122-141.
But one of the most beautiful and striking illustrations of this class of instances is afforded by the interesting phenomena of light and sound. Sound as well as light is capable of being reflected from surfaces; and in passing from a dense to a rare medium both are refracted; each is impeded when traversing an atmosphere of variable density; feeble sounds which are distinctly heard in the night are drowned in the continual hum of day; and when the sun is up, the stars disappear. Both sound and light are subject to the laws of interference. Under certain known circumstances, two vibrations of sound will neutralize each other's effects, and produce silence; so in like manner, two rays of light will destroy each other, and produce darkness.* These remarkable analogies render it extremely probable, that as sound is produced by the
* See sir John Herschel's Treatise on Sound in the Ency. Metropol., and his Discourse on Nat. Phil. § 288.
communication of a vibratory motion from the sounding body to the atmosphere which transmits it to the ear; so light is in like manner communicated from a luminous body to the eye by means of a highly subtle and elastic ether, which is supposed to pervade
Of the great importance of attending to this class of analogous instances the discovery of the circulation of the blood affords another illustration; for it was by observing in the blood-vessels a contrivance similar to the valves employed in hydraulic machines for preventing the return of the water, and by reflecting on their use, that Dr. Harvey was led to this memorable discovery.
'I remember,' says Mr. Boyle, * 'that when
Boyle's Works, vol. 4, p. 539. In Aubrey's account of Walter Warner,—a contemporary of Harvey, and of some note in his day,—it is said that Warner told Dr. Pell, that from what had passed in a conversation between himself, Dr. Harvey, and Mr. Prothero, at the earl of Leicester's, he had no doubt that Prothero had first hinted the fact of the circulation of the blood I asked our famous Harvey, in the only discourse I had with him, (which was but a little while before he died, what were the things which induced him to think of a circulation of the blood? he answered me, that when he took notice, that the valves in the veins of so many parts of the body were so placed, that they gave free passage to the blood towards the heart, but opposed the passage of the venal blood the contrary way, he was incited to think, that so provident a cause as nature had not placed so many valves without design; and no design seemed more probable, than that, since the blood could not well, because of the interposing valves, be sent by the veins to the limbs, it should be sent through the arteries,
to Harvey. So Isaac Walton, in a letter to Aubrey, says, that lord Winchester, who was acquainted with Warner, informed him, that Warner himself claimed to be the first discoverer.--See Aubrey's anecdotes, printed in the collection of Letters from the Bodleian Library, vol. 2, pp. 418, 578. Such gossip as this will never lessen Harvey's fame.
and return through the veins, whose valves did not oppose its course that way.'
5. Instantiæ Potestatis, or instances of power, are those which are reckoned the masterpieces of art. Lord Bacon suggests that these should be thoroughly examined ; because they render the way to new discoveries and inventions more easy and feasible. *For if any one,' he says, ' after an attentive contemplation of such works as are extant, be willing to push forward in his design with alacrity and vigour, he will either advance them or apply them to some other purpose.' In illustration of this class, Bacon adduces paper as a singular and beautiful production of art. If he had lived in this day, with what delight he would have described the almost miraculous machine invented by the eminent mechanist, Mr. Dickinson, of Hertfordshire, by means of which a continuous stream of fluid pulp is not only made into paper, but actually dried, polished, and every separate sheet cut round the edges, and rendered