“To prevent confusion, therefore, in our ideas, it became necessary to distinguish these two kinds of motion from each other by appropriate terms; and, accordingly, where there has been an actual change of place, in the common meaning of the term, the motion which produced it is termed ABSOLUTE motion; whereas, on the contrary, when the situation has been only relatively changed, by an alteration in the position of surrounding bodies, the motion is said to be RELATIVE.”
“Surely, papa,” said Louisa, “no person can ever mistake relative for absolute motion; what then is the use of such frivolous distinctions? When a body really moves, we can observe it in the act of changing its place, and no difficulty can arise about the matter.”
“Nothing, my dear, is more fallacious than our vision; the earth appears motionless, and the sun and stars seem as if they revolved round it; but it is scarcely necessary for me to inform you that our globe is constantly moving with considerable velocity, while the sun remains at rest.--Mr. Sadler, the famous aëronaut,” continued Mr. Seymour, “informed me, that he was never sensible of the motion of the balloon in any of his excursions, but that, as he ascended into the air, the earth always appeared as if sinking beneath him, and as he descended, as if rising to meet him.”
Mr. Twaddleton here observed, that he had heard a very curious anecdote, when he was last in London, which fully confirmed the truth of Mr. Sadler’s statement. “An aëronaut,” said he, “whose name I cannot at this moment recollect, had recently published a map of his voyage, and, instead of proceeding in any one line of direction, his track absolutely appeared in the form of circles, connected with each other like the links of a chain: this occasioned considerable astonishment, and, of course, some speculation, until it was at length discovered, that his apparent journey was to be attributed to the rotatory motion of the balloon, which the voyager, not feeling, had never suspected.”
“And what,” asked Tom, “could have been the reason of his not having felt the motion?”
His father explained to him, that we are only conscious of being in motion when the conveyance, in which we are placed, suffers some impediment in its progress. “If,” said he, “you were to close your eyes, when sailing on calm water, with a steady breeze, you would not perceive that you were moving: for you could not feel the motion, and you could only see it by observing the change of place in the different objects on the shore; and then it would be almost impossible, without the aid of reason and experience, to believe that the shore itself was not in motion, and that you were at rest; I shall, however, be able to explain this subject more clearly by an optical toy which I have in preparation.”
Mrs. Seymour here repeated the following passage from that interesting novel “Anastasius,” which she observed was beautifully descriptive of the illusive appearance to which their papa had just referred:--
“The gradually increasing breeze carried us rapidly out of the Straits of Chio. The different objects on the shore,--mountains,--valleys,--villages,--and steeples,--seemed in swift succession, first advancing to meet us, then halting an instant alongside our vessel, as if to greet us on our passage, and, lastly, again gliding off with equal speed; till, launched into the open main, we saw the whole line of coast gradually dissolve in distant darkness.”
“That is indeed a beautiful and very apposite illustration,” said Mr. Seymour; “and I think Louisa will now admit, that it is not quite so easy, as she at first imagined, to distinguish between Absolute and Relative motion.”
As the children now understood what was meant by the term Motion, their father asked them whether they could tell him what produced it.