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the one above that still less, and so on, till the upper one, having no weight over it, would be in its natural state. This is the case with the air, or atmosphere, that surrounds our earth, and accompanies it in its motion round the sun. On the tops of lofty buildings but still more on those of mountains, the air is found to be considerably less dense than at the level of the sea.
* The height of the atmosphere has never yet been exactly ascertained; indeed, on account of its great elasticity, it may extend to an immense distance, becoming, however, rarer, in proportion to its distance from the earth.
“ It is observed, that at a greater height than 45 miles, it does not refract the rays of light from the sun; and this is usually considered as the limit of the atmosphere. In a rarer state, however, it may extend much farther. And this is by some thought to be the case, from the appearance of certain meteors which have been reckoned to be 70 or 80 miles distant, and whose light is thought to depend upon their coming through our atmosphere.
“ It might easily be proved by calculation, that a cubic inch of such air as we breathe, would be so much rarefied at the altitude of 500 miles, that it would fill a sphere equal in diameter to the orbit of Saturn."
EDWARD.-“ And now, my dear Sir, one question more upon the subject of air, or rather of the atmosphere. How is the blue colour of the sky accounted for ; for I have always understood that air is itself colourless and invisible ?"
DR. WALKER.-“ So it is; and its invisibility is one of its most astonishing properties, that an element so wonderfully powerful, as to be capable of reducing, in a few short hours, not only the proudest work of man to a mass of floating shapeless wrecks, but also to produce the most astonishing effects on the other elements, should be so strongly felt, and yet unseen : that its voice should be heard in the whispering breeze, and in the howling blast ; that it should thus be felt and heart, and yet invisible to the eye, is sufficient to raise in the mind of uncultivated man extraordinary ideas of its apparently magical virtues. It is not therefore at all astonishing that it should have become an object of devout adoration. Its effects on the sea we have lately witnessed. Nor are they less fatal on the land. The monarch of the forest bows his lofty head reluctantly to the blast, until prostrate, at length, he falls, level with the dust. Rocks even yield to its invisible agency; while in the sandy desart the Mecca pilgrim sees with despair the first faint symptoms of the approaching whirlwind in the "black red ether;" when, as Thomson beautifully and fearfully describes it:
“ Straight the sands
Swept up, the whole continuous wilds arise.” « Fire, again, without air could not exist. Let but the smallest spark appear, and by its potent influence quickly the flames extend; and smoking ruins and houseless wanderers stand on every side the sad memorials of its fatal and assisting agency.
But in contemplating one of its properties, I have almost lost sight of your principal question,--the colour of the sky. This blue colour is occasioned by the vapours which are always mixed with air, and which have the property of reflecting the blue rays more copiously than any other. This has been proved by the experiments which M. Saussure made with his cyanometer, at different heights above the surface of the earth. This instrument consisted of a circular band of paper, divided into fifty one parts, each of which was painted with a different shade of blue: beginning with the deepest, mixed with black, to the lightest, mixed with white. He found that the colour of the sky always corresponded with the deepest shade of blue the higher the observer was placed above the surface of the earth; consequently, at a certain height the blue will disappear altogether, and the sky assume black tints; that is to say, will reflect no light at all. The colour becomes always lighter in proportion to the vapours mixed with the air; hence the blue colour is evidently owing to them.”
SECTION IV. THE MAGNET; OR, MARINER'S COMPASS. EDWARD.-" How little we think of the wonderful operations of nature that are carrying on around us ! Was it not very extraordinary, Sir, that the compass should turn round completely."
Dr. WALKER.-" Most extraordinary. I cannot account for it, unless there was a quantity of iron about the compass box; (which, by the bye, should never be the case,) now this being acted upon by the electric fuid, might possibly produce this invertion of the loadstone one of the most singular and beneficial gifts of Providence. Of its nature and properties you have, of course, some knowledge ; but as the evening is wet, and books are scarce, we will amuse ourselves with discussing its wonderful properties.
“ The natural magnet or loadstone, a hard mineral body of a dark brown, is found, when examined, to be an ore of iron. It is found in various countries, (Norway produces a good deal of it,) generally in iron-mines, and of all sizes and forms.
6. This singular substance was known to the ancients, who had remarked its peculiar property of attracting iron, though there is no evidence that they were acquainted with the wonderful property which it also has, of turning to the pole when suspended, and left at liberty to move freely.
« Upon this remarkable principle depends the construction and use of the mariner's compass, an instrument which gives us such infinite advantages over the ancients. It is this enables the mariner to conduct his vessel through vast oceans out of the sight of land, in any given direction; and this directive property also guides the miner in his subterranean excavations, and the traveller through deserts, which otherwise would be impassable.”
EDWARD.- “ I am sure we have experienced the value of it. You used to tell me that I must want a thing to know its value.”
Dr. WALKER.-“ Most true, Edward. It is not precisely known when and by whom this directive property of the magnet was discovered. The most probable accounts seem to prove, that it was known 'early in the 13th century; and that the person who first, in E:rope, made mariners' compasses, was a Neapolitan of the name of Flavio, or John de Gioja, or Giova, or Gira.
“ Before that period, sailors scarcely ever ventured out of sight of land, and in the longest voyages contented themselves with going round the coasts, making by that means their voyages much longer. In the night, and when necessity obliged them to lose sight of the shore, their only guides were the stars, and when these were obscured by clouds, they were absolutely without resource.
" While navigation continued so limited, men never would have ventured
such voyages as those to the West Indies, America, and the South Seas, and the existence of those countries would probably have been still unknown to us.
“ We, cannot, therefore, think too highly of this extraordinary instrument, which has so much enlarged our stock of knowledge, and procured for us so many new enjoyments.
« The natural loadstone has also the quality of communicating its properties to iron and steel; and when pieces of steel, properly prepared, are touched, as it is called, by the loadstone, they are denominated artificial magnets, which are even capable of being made more powerful than the natural ones, and as they can be made of any form, and are more convenient, they are now universally used, so that the loadstone, or natural magnet, is only kept as a curiosity.
“ An artificial magnet, fitted up in a proper box, for the purpose of guiding the direction of a traveller, is called a magnetic needle, and the whole together, is called the mari
“ All magnets, whether natural or artificial, are distinguished from other bodies by the following characteristic properties, which appear to be inseparable from their nature; so that no substance can be called a magnet, unless it be possessed of all these properties. A magnet attracts iron. When a magnet is placed so as to be at liberty to move freely in every direction, it turns, so that its ends point towards the poles of the earth, or very nearly so; and each end always points to the same pole. This is called the polarity of the magnet : the ends of the magnet are called poles, and they are respectively the north and south pole of the magnet, according as either points to the north or south pole of the earth. When a magnet places itself in this direction, it is said to traverse.
“. When the north pole of one magnet is presented to the south pole of another magnet, these ends attract each other but if the south pole of one magnet be presented to the south pole of another, or the north pole of one to the north pole of another, these ends will repel each other.
“ From these criteria, it is easy to determine the names of the poles of a magnetical bar, by applying it near a suspended magnet whose poles are known. You will observe, however, when a magnet is so situated, as to be at liberty to
move itself with sufficient freedom, its two poles do not lie in a horizontal direction, but it generally inclines one of them towards the horizon, and elevates the other pole above it. This is called the inclination or dipping of the magnet; and any magnet 'may, by proper methods, be made to impart those properties to iron or steel.”
At this moment the servant entered with a packet of letters from England. “ Well, Colin,” said Dr. Walker, “ these letters are the signal for our departure: are you willing to continue your journey, or has the storm damped your ardour for seeing strange countries ?' " Colin ne'er thinks of danger when it is gone by," replied the Highlander, “and where you maun gang, Colin will gang too.'
" To-morrow then we recommence our tour," resumed the Doctor, and on the morrow they set off for the silver mines in the vicinity of Konigsberg. During their first day's journey, Dr. Walker amused his pupil with a description of the Whirlpool of Maelstroom, or Moskoestrom.
" When I knew," said “the good man, that we were drifting to the north, instead of the south, in our little boat, I must confess I felt some little alarm, for that whirlpool is by no means an agreeable sort of spot to be whirled into. We were, it is true, at a great distance from it; but it would have been very near to imagination, if I had known we had been within 100 miles of it. On the coast of Norway, latitude 67, you will in the map find this dreadful vortex. The island of Moskoe, from whence this stream derives one of its names, lies between the mountains Heşleggen, in Lofoden, and the Island Ver, which are about one league distant, and between the island and coast on each side this dreadful mass of water makes it way. It is nearly 400 fathom deep between Moskoe and Lofoden ; but between Moskoe and Ver it is too shallow to admit the smallest ship. When it is flood, the stream rushes up the country between Lofoden and Moskoe with a boisterous rapidity; and when it is ebb, re