Imatges de pÓgina
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I have endeavoured to keep my Readers of this fort every where in View and I hope nothing will be found here above their Reach; except, perhaps, in the Ninth and Tenth Chapters of the Second Part; where, indeed, they are supposed to have a little previous Knowledge of the Doctrine of Angles: But fo little, and fo presently to be attained, that it need hardly be mentioned as an Exception to my general Defign.

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The Method in which I have handled the Doctrine and Ufe of the Globes, in the first Part, is fomewhat fingular. For moft, if not all, Writers on that Subject, have treated on both Globes, as preparatory even to learning Geography; whereby, in my opinion, they have made the Threshold to that Science, much higher than it need be. I fee no Occafion to perplex a young Mind with Right and Oblique Afcenfion, Defcenfion, Afcenfional Difference, and many other puzzling Matters, which have no Relation to Geography, before he can be led into an Acquaintance with Lands and Seas, Kingdoms and Provinces. I have, therefore, in the first Place, explained the Terrestrial Globe only, as if there were no fuch

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Thing as a Celestial one; after which I would propofe that the young Student fhould read fome Syftem of Geography, before he proceeds to the Celestial Globe.

For the Ufe of the Globes, I have given only Twenty Problems, out of the Multitude that are to be found in almost every Book on that Subject: But I conceive thefe Twenty are fo far fufficient, that if a Perfon is Mafter of them, he will be able readily to work any that he meets with elsewhere. The best Collection of this Sort, that I know of, is in Dr. Watts's Knowledge of the Heavens and Earth made eafy; to which, therefore, I would refer the Learner, for his further Practice and Entertainment.

A good deal of the Second Part is adapted to the Use of the Orrery, for which Purpose it was drawn up; and, perhaps, an İnftrument of that Sort may be needful to illustrate some of the Chapters, and to render them eafily intelligible, to the Perfons whose Instruction is here chiefly aimed at. However I would propose the following cheap and eafy Contrivances to those who are not furnished with that noble Instrument; which may ferve tolerably well instead of it.

To help the Imagination to conceive of the double Motion of the Earth, viz. its annual Motion round the Sun, and its diA 3 urnal

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urnal Motion on its own Axis. Take a round Ball, or Globe, of an Inch or more Diameter, made of Ivory, or some other pretty heavy Substance, to represent the Earth ; let it be painted half black, and half White ; fix a Hook, or little Staple, any where in the Circle that divides the Black from the White Hemisphere; tie a String to it, of two Feet, or more, in Length; twist the String, so that when it is let go, and the Ball suspended by it, it will untwist, and thereby turn the Ball twiftly round; then holding the End of the String directly over a Candle (to reprefent the Sun) so that the Ball may hang as low as the Flame of the Candle, give it a circular Motion round the Candle, to represent that of the Earth round the Sun; and the untwisting of the String will, at the fame Time, give it one round its Axis, representing the diurnal Motion of the Earth. The Use of painting the Globe Black and White, in this Experiment, is to make the diurnal Motion visible, which otherwise, when the Ball moves swiftly round, would hardly be seen.

Again, to help the Imagination to conceive, how the annual Motion of the Earth occasions the lengthening and shortening of Days, and the Change of Seasons. Take another Ball, or Globe, of about two, or three, Inches Diameter ; let it be 4

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painted all over White ; only let the Poles be marked upon it, and the Equator, Tropics, and Polar Circles, be drawn with Black; make a Spot also to mark the Latitude of any given Place, (suppose London ;) let a Hole be bored through the Globe from Pole to Pole, into which put a Spindle, or Axis, on which it may turn round. Then setting a Candle directly before you, on a Table, in a darkened Room, hold the Axis of the Globe, (which must, therefore, be longer than its Diameter, in order to your holding the Globe upon it,) not perpendicular to the Table, but inclined to it in an Angle, as near as you can guess, of 66 Degrees ; and let the Globe be held as high from the Table as the Flame of the Candle. First, hold the Globe with its North Pole from you, at a Distance from the Candle, on the left Hand of it; fo that the Circle of Illumination may pass through both Poles : It then represents the Position and Place of the Earth at the Vernal Equinox. Move the Globe towards you,

in a Circle parallel to the Table, observing to keep the Axis in the same Inclination, and always parallel to itself, or pointing the same Way; when the Globe is brought directly betwixt you and the Candle, it represents the Position and Place of the Earth at the Summer Solstice ; and, if the Axis of the Globe is duly

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inclined to the Table, the enlightened Hemisphere will just take in the whole Northern Frigid Zone. Carry on the Globe to the right Hand of the Candle, directly oppofite to the Place where you held it at first, and it will reprefent the Autumnal Equinox; when it is moved to be directly beyond the Candle, it represents the Winter Solftice. And, by turning the Globe on its Axis, in feveral Parts of the Circle, in which it is moved round the Candle, you will fee how the Days lengthen and shorten in that Place whofe Latitude is marked upon it. Perhaps it may be of some Ufe to draw a Circle with Chalk, upon the Table, round the Candle, as its Center, over which you are to move the Globe. Or a Circle made of Wood, Paftboard, or Paper, divided and marked with the twelve Signs of the Zodiac, and laid upon the Table, may ftill do better.

By Means of this Globe, and the other Black and White one, now supposed to represent the Moon, (which should therefore be made less than the Earth,) the Phases of the Moon may be fhewed in the feveral Parts of her Orbit round the Earth. Let the white Hemisphere represent that Hemifphere, and Face of the Moon, which is always obverted to the Earth; and the Light of the Candle will fhew how much

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