Diagram showing arc of fall trajectory.
“If you imagine that the kite, under such circumstances, would fall upon the spot directly under it, you are much deceived: recollect that, if the string should snap, the kite would be abandoned to two forces, those of the wind, and its own gravity; and you will perceive that, under such circumstances, it could not obey either of them, but would fall in an intermediate or diagonal direction. This fact will be rendered apparent by the annexed diagram.B Amay be supposed to represent the force and direction of the wind acting upon the kite, andB Dthose of its gravity; then it is evident that, under the influence of these joint forces, it would describe the diagonal, and, for reasons already explained, that line must necessarily be thecurveB F.”
“Come,” said the vicar, “before Tom draws down his kite, let us send up amessenger.”
“What may that be?” asked Louisa.
“A piece of paper or pasteboard, which, on being introduced upon the string, is blown along the line up to the kite.”
Themessengerwas accordingly prepared, and being placed upon the string, it ascended as Mr. Seymour had anticipated. While this operation was in progress, the vicar stood earnestly gazing upon the kite, and, at length, burst forth in the following animated soliloquy:--
“Assuredly, this must be acknowledged as a most beautiful and imposing toy! Fastidious or insensible must be that person, who does not feel exhilarated as he gazes on the kite, proudly floating under the canopy of heaven, and reflecting the departing smilesof the evening sun, after it has ceased to cheer us below.”
“It has been said,” observed Mr. Seymour, “to the disparagement of kite-flying, that as soon as the machine has been raised into the air, and all the string let out, the excitement of the sport is at an end, and that as nothing further can be achieved, the interest of the performer from that moment begins to languish; now at this period themessengerwill open a new source of pleasure and instruction, and may, by a little ingenuity, be made to afford a great diversity of amusement. I have therefore provided myself with several varieties of this machine. Here is one in the form of a dragon, which, as it ascends, produces a very striking and almost magical effect. See, there it goes!”
The children were delighted, for the string upon which it was carried became at a certain height invisible; so that the figure appeared like a monster hovering in the air.
“I will now show you awingedvariety of this apparatus, which we will name theBrompton Messenger.[44]It consists of a hollow cylinder of stiff pasteboard, or thin wood, the diameter of which is sufficiently large to allow its free revolution round the string of the kite. To this cylinder are attached several flappers, or sails, in an oblique direction, like those of a windmill, each of which is covered with paper of a different colour. The action of the wind upon those oblique surfaces necessarily occasions a rapid rotation, upon a principle which I shall presently explain; and the beautiful effect thus produced, as the whirling body ascends, must be seen before it can be appreciated. I have some other contrivances of a similar nature, which it is my intention to prepare for your future amusement.”(41)
“Has the kite ever been applied to any useful purposes?” asked Tom.
“Certainly,” answered his father. “It was by means of the kite that Dr. Franklin was enabled to demonstrate the identity of electricity and the cause of lightning, and thus to disclose one of the most awful mysteries of nature.”
“Pray do tell us something about this electrical kite, papa,” said Louisa.
“Not at present, my love; it would divert us too much from the subjects in which we are engaged: at some future period I shall have much pleasure in introducing you into these fairy regions of philosophy.”
“I just now remember reading in Miss Edgeworth’s Harry and Lucy,” said Louisa, “something about a kite and Pompey’s pillar.”
“I am glad that you have reminded me of that story,” replied Mr. Seymour: “I will relate it to you. Some English sailors laid a wager, that they would drink a bowl of punch on the summit of Pompey’s pillar. Now, that pillar is almost a hundred feet high, and it is quite smooth, so that there was no way of climbing to the top, even for sailors, who are such experienced climbers: so they flew their kite exactly over the pillar, and when it came down on the opposite side, the string lay across the top of the capital. By means of this string, they pulled a small rope over, and by this a larger one, that was able to bear the weight of a man; a pulley was then fastened to the end of the large rope, and drawn close up to the upper edge of the capital; and then, you perceive, they could easily hoist each other up. They did more, for they hoisted the English flag on the top, and then drank the bowl of punch, and won their wager.”
“That is a very good story,” said the vicar; “but I cannot help regretting that so much ingenuityand labour should not have had a nobler end to accomplish.”
“There is some truth in that observation,” said Mr. Seymour, “and I will, therefore, relate another story which shall be more congenial to your heart, and in which the kite will present itself in a more interesting point of view; for, instead of enabling the sailors to drink a bowl of punch at an altitude otherwise inaccessible, we shall find it engaged in rescuing them from the horrors of shipwreck.”[45]
“Pray proceed, papa.”
“No, my dear, upon reflection, I think it will be better that we should postpone the story, until your return to the lodge, when you shall read it in ‘Harry and Lucy.’ I will also point out to you, in the same work, an account of a new and useful application of the messenger, which will prove to you how successfully the faculties of youth may be increased and improved by those very amusements which are too generally regarded as idle and unprofitable: I shall at the same time exhibit one or two experiments in illustration of the nature and causes of wind.”
“Shall we not return immediately?”
“No, my dear; it would not be in my power to attend you at present; but join me in the library after dinner; Mr. Twaddleton will now accompany me to the village, and do you remain and enjoy the amusement of your kite.”
It was not until the evening, that Tom and his sisters requested their father to fulfil the promises he had made them in the morning.
“You told us,” said Louisa, “that you would give us some information about the wind; the subject has been puzzling me ever since, for I cannot make out the cause of it.”
“Wind, my love, is nothing more than air in motion; and is produced by a large volume of it flowing in a current, or stream, from one place, or region, to another, and with different velocities.”
“And what can produce these currents?” asked Tom.
“After the explanation of the action of the pump, I do not think that I shall have much difficulty in making you understand the nature of the operations by which wind is occasioned. Suppose a partial vacuum should be formed in any region, would not the neighbouring air immediately rush in to supply the deficiency and restore the balance?”
“Undoubtedly; from the pressure of the air behind it.”
“Heat,” continued Mr. Seymour, “will produce a partial vacuum, by rarefying the air, and thus rendering it lighter; in consequence of which, it will ascend, and the colder air will rush in to supply its place.”
“I do not exactly see why the rarefied air should ascend,” observed Louisa. “It appears to offer an exception to the general law of gravity.”
“Not at all; on the contrary, its ascent is occasioned by the force of gravity: in the first place, however, to prove the fact that heated air does actually ascend, we have only to observe the direction of smoke, as it issues from the chimney; this consists of minute particles of fuel carried up, by a current of heated air, from the fire below; and as soon as this current is cooled by mixing with that of the atmosphere, the minute particles of coal fall, and produce the small black flakes which render the air, and every thing in contact with it, so dirty in a populous city.”
“But I want to know, papa, what it is which causes the hot air to ascend?”
“The greater weight of the cold air above it, whichgets, as it were, beneath the lighter air, and obliges it to rise; just in the same way as a piece of cork, at the bottom of an empty vessel, is made to rise to the surface of the water which may be poured into it.”
“Now I understand it; pray, therefore, proceed with your account of the wind. You have just said that heat rarefies the air, and causes it to ascend.”
“And thus produces a current of air, or awind.”
“Is heat, then, the cause of wind, papa?” asked Tom.
“It is one great cause; but there are, probably, several others; I will, however, exemplify this subject by an experiment.”
So saying, Mr. Seymour produced a water-plate, a large dish, and a jug filled with cold water. The bell was rung, and the servant entered with a tea-kettle of boiling water.
The large dish was then filled with the cold, and the water-plate with the boiling fluid.
“Let this large dish represent the ocean,” said Mr. Seymour, “and this water-plate, which I will now place in its centre, an island in that ocean; for the land, from receiving the rays of the sun, will be more heated than the water, and will consequently rarefy the air above it.--Now, Tom, light me the wax taper.”
“I have done so.”
“Then now blow it out.”
“I cannot imagine what you are about, papa;--‘Light the candle, and then blow it out!’--but it still smokes, shall I put the extinguisher over it?”
“By no means; give it to me, and observe what will happen when I carry it round the edge of the dish.”
“The smoke goes to the centre,” exclaimed Tom.
“Showing, thereby, the existence of a current towards the water-plate, or island; in consequence ofthe air above it having been heated, and therefore rarefied. This explains in a very satisfactory manner, a fact which may be constantly observed in our own climate, viz. a gentle breeze blowing from the sea to the land in the heat of the day. Upon the same principle it is, that most of the winds in different parts of the globe may be readily accounted for.”(42)
“I suppose,” said Tom, “that the air must rush with great velocity, in order to produce wind.”
“A very general error prevails upon this subject,” replied his father; “the rate of motion has been greatly exaggerated. In a brisk gale, even, the wind does not travel with such velocity, but that it may be easily traced by the eye; and the sailor is able to watch its progress by the ripple which it produces on the sea.”
“Has, then, the rate of its motion ever been estimated?” asked Louisa.
“When its velocity is about two miles per hour, it is only just perceptible. In a high wind, the air travels thirty or forty miles in the same period. In a storm, its rate has been computed as being from sixty to eighty miles. It has also been ascertained, by experiment, that the air, as it rushes from a pair of blacksmith’s bellows, has not a velocity above that of five and forty miles in the hour.”
“At what rate should you think the air travelled this morning, when we flew our kite?” enquired Louisa.
“I should think at about five miles an hour, for it was a pleasant but gentle breeze.”
41.The author has been thus minute, in order to afford his young friends clear directions for constructing a kite, and which, as far as he knows, are not to be found in any work hitherto published; and he will also avail himself of this opportunity to state, that the thin glazed linen of various colours, commonly known to haberdashers by the name of lining, has been found to be the best material as a covering for the kite. It is not only more durable than paper, defying the most boisterous wind, but superior to it as being secure from the effects of a shower of rain.
41.The author has been thus minute, in order to afford his young friends clear directions for constructing a kite, and which, as far as he knows, are not to be found in any work hitherto published; and he will also avail himself of this opportunity to state, that the thin glazed linen of various colours, commonly known to haberdashers by the name of lining, has been found to be the best material as a covering for the kite. It is not only more durable than paper, defying the most boisterous wind, but superior to it as being secure from the effects of a shower of rain.
42.Those readers, who are inclined to enter more deeply into the subject, may consult, with advantage, a memoir on the kite, by Euler, published in the Transactions of the Academy of Berlin for the year 1756.
42.Those readers, who are inclined to enter more deeply into the subject, may consult, with advantage, a memoir on the kite, by Euler, published in the Transactions of the Academy of Berlin for the year 1756.
43.See p.60.
43.See p.60.
44.From associations of an interesting nature connected with the residence of the author’s children.
44.From associations of an interesting nature connected with the residence of the author’s children.
45.Transactions of the Society for the Encouragement of Arts, vol. xli.; and Miss Edgeworth’s Harry and Lucy, vol. iv. p. 288.
45.Transactions of the Society for the Encouragement of Arts, vol. xli.; and Miss Edgeworth’s Harry and Lucy, vol. iv. p. 288.
Children playing with paddles and a shuttlecock.