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On

ally land.

is of Norwegian origin, Staffa being derived from staf, -a staff, or prop,-or, figuratively, a column.

“ The little island of Staffa is about three leagues northcast from Columb-kill; its greatest length is about an Eng. Jish mile, and its breadth is not more than half an one. the west side of the island is a small bay, where boats gener.

At a short distance froni Staffa, is a small island, called Buachaille, or the herdsmen, which is wholly composed of pillars, without any stratum above them.

« On proceeding to the north-west, you meet with the highest range of pillars, the magnificent appearance of which is past all description; here they are bare to their very base, and the stratum below them is also visible; in a short time it rises many feet above the water, the whole of this stratum slips gradually to the south-east, beyond this the pillars totally cease, and a brown rock appears until you approach the celebrated cave."

SECTION IV.

TWILIGHT-PHYSICAL PROPERTIES OF.

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OUR travellers staid so long in the interior of the natural columnar hall, that they thought it advisable not to return to Mull that evening, and accordingly the boatman rowed round towards the north of the island, and they took up their abode at the only house it contained. The evening being remarkably fine, Doctor Walker and his pupil amused them selves till a late hour, in wandering over the small but extraordinary isle of Staffa.

" I like to walk in the twilight,” said Edward, as they slowly returned to their inn. “Every thing is so quiet and so still. Pray, Sir, when is twilight said to begin? I have often wondered, when the sun was quite gone down and there was no moon, why it was not quite dark. I know it is so in stormy weather-but I mean now, in summer, when there is neither sun or moon, it is still light enough to walk about.'

DR. WALKER.-" The Crepusculum, or Twilight, is that faint light which we perceive before the sun rises and after he sets. It is produced by the rays of light being refracted in their passage through the earth's atmosphere, and reflected from the different particles thereof. The twilight is supposed to end in the evening when the sun is 18° below the horizon, or when stars of the sixth magnitude (the smallest that are visible to the naked eye) begin to appear; and the twilight is said to begin in the morning, or it is daybreak, when the sun is again within 18° of the horizon. The twilight is the shortest at the equator, and longest at the poles ; here the sun is near two months before he retreats 18° below the horizon, or to the point where his rays

are first admitted into the atmosphere; and he is only two months more before he arrives at the same parallel of latitude.

“ The benefits of twilight are obvious. A change so great, as from the darkness of midnight to the splendour of noon-day, would probably be injurious to the sight; and it would be unpleasant to all, and in many cases very danger. ous to travellers, to be involved in darkness without timely notice of its approach.

EDWARD. “I can understand how in all countries situated near the equator, twilight is of much shorter duration than it is in countries of high latitudes ; for at the equator the sun rises and sets perpendicularly, --but to places at a great distance from the equator, it rises and sets very obliquely; and hence it requires a longer time to go 180 below the horizon.

Dr. WALKER.-" At the latitude of 49. N. twilight continues the whole night on June 21st; and, at places still farther north, it continues the whole night, for a certain number of days before and after the summer solstice. At London there is no total darkness from May 28th till July 20th.

“ Twilight continues, at the north pole, from September 22d, when the sun sets, to November 12th,-a space of 51 days. Twilight first appears again there about the 30th of January, and continues till sun-rise on March 21st.

Thus, though the inhabitants (if any) at the north-pole never see the sun for 6 months, yet out of that time, they have twilight for 14 weeks. The time that they receive no light from the sun is only 12 weeks; and, during that time, the moon is 6 weeks above the horizon.

“ Now we are upon this subject,” continued the Doctor, " I will pursue it, while our lady hostess prepares our frugal supper; and first of all it will be proper to treat of the physical properties of light.

$ The physical properties of light are easily understood; and I will therefore now take leave to notice such of them

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as are most common; leaving, till we study, optics, such as are usually discussed in treating of that science.

“ It is generally allowed, that light consists of inconceivably small particles, which are projected, in all directions, with an aniazing velocity, from the luminous or radiant body.

“ This, however, is only an hypothesis, for the materiality of light is denied by some. But the theory of light which we have adopted, appears to be the most simple of any, and serves best to explain the phenomena of vision.

“ M. Romer was the first who observed the velocity with which light moves.

He discovered that it took but seven or eight minutes in passing from the sun to the earth. It must travel, therefore, at the rate of 150,000 miles in a second of time; a velocity so great, that, were it not for the extreme minuteness of its particles, our organs of vision would be de. stroyed by its impulse upon them.

“ The rarity of this Auid, and the minuteness of its particles, are not less matter of wonder than its velocity; for its rays cross each other in all possible directions, without the least apparent disturbance.

“ Make a small pin-hole in a piece of paper, and all the objects, such as the sky, trees, houses, &c. which you could see without the paper, will be distinctly seen through this tiny hole. The light proceeding from all these objects, passes at the same time through the hole in a great variety of directions, before it can arrive at the eye; yet it does not appear

that vision is disturbed by that means. · Set by night, a lighted candle on an eminence, it will be seen all round to the distance of half a mile; there is therefore no place within a sphere of a mile in diameter, in which the eye can be placed, where it will not receive some rays from this small flame,

“ The rays of light move always in straight lines, as may be evioced by the impossibility of seeing through a crooked tube.

“ Hence it follows, that the intensity of light decreases, the

square of the distance from the luminous body increases ; that is to say, if you remove an object to twice the distance from the luminous body, it will be enlightened only one-fourth part as much as before; if to three times the distance, it will be illuminated only one-ninth as much, and so on, in Geometric progression,

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* The chemical properties of light are not less astonishing than its physical properties; and it is now beyond doubt that light does have considerable influence upon many chemical processes, in the great laboratory of nature.

“ The effect of light upon vegetation, is well known. Many flowers follow the course of the sun; and plants that grow in houses, seem solicitous, as it were, to get at the light. Plants that grow in the shade, or in darkness, are pale, and without colour, and when this is the case they are said to be etiolated or blanched. Gardeners avail themselves of this fact, to render vegetables white and tender. The more plants are exposed to the light, the more colour they acquire.

« Vegetables are not only indebted to light for their colour: their taste and odour are derived from the same source: hence hot climates are the native countries of perfumes, odoriferous fruits, and aromatic resins.

“ The action of light on the organs of vegetables, causes them to pour out streams of pure air from the surfaces of their leaves, while exposed to the sun; whereas, on the contrary, when in the shade, they emit air of a noxious quality. Even animals, in general, droop when deprived of light; and it appears to be of great importance to the health and happiness of human beings. The darkness I lived in was the only thing I could not accustom myself to, says Trenck, in his description of his confinement.

“ Birds that inhabit tropical countries, have much brighter plumage than those of the North. This is also the case with insects; and the parts of fishes which are exposed to the light, such as the back, fins, &c. are uniformly coloured ; but the belly, which is deprived of light, is white in all of them.

“ Light has considerable influence upon the crystalliza. tion of salts. Many of which will not crystallize, except exposed to the light. Camphor kept in glass bottles exposed to light, crystallizes in symmetrical figures, on that side which is turned towards the light.

“ Many bodies, if exposed to light, either at high or low temperatures, combine with it, and emit it again, unde tain circumstances. These are called solar phosphori. Sub. stances of this kind have been prepared by chemists, and have the property of shining in the dark. “ But I must observe, that besides preparations of art,

various animal and vegetable substances seem to possess a great deal of this phosphorus. The glow-worm is a remarkable instance. Dead fish, rotten sea-weeds, and great numbers of insects, have this property in a great degree. Phosphorus is never met with pure in Nature. It is commonly found united to oxygen, in the state of phosphoric acid, which is found plentifully in different animal, vegetable, and mineral substances. You observe this piece of phosphorus is a yellowish semi-transparent substance, of the con=”. sistence of wax. It is luminous in the dark, at the common temperature of the atmosphere. You see it takes fire spontaneously, and burns rapidly in the open air, at 1220 of Fah. renheit, with a brilliant white Alame, and is converted into phosphoric acid.

“ The combustibility and luminous property of phosphorus, have given birth to various experiments, and the following will evince its characteristic properties in a pleasing manner.

- That phosphorus burns at the usual temperature, appears by writing with it upon black or purple paper, or any other smooth surface. The writing will be luminous in the dark, as if on fire. The fiery appearance vanishes by blowing upon it, but becomes visible again after a few seconds.

“All this you perceive is very plain, and I therefore trust you will not forget it. Make a brief entry of it into your journal, and I will revise it for you."

“ Ten thousand thanks, Sir," replied Edward, “I shall certainly note down, as far as I am able, the particulars of all you are so kind as to relate or explain to me.''

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SECTION V.

THE ISLES OF SKYE AND ORKNEYS.

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HAVING partaken of their frugal meal they retired to rest, and Colin, early in the morning having roused the travellers, led them with exultation to the parlour, where a Scotch breakfast awaited them. He had at Torbimore, a fine har. bour in the island of Mull, laid in a little store of provision, which he had reserved for the isle of Staffa, as he concluded that an island made of pillars could not be very productive

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