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a conversation may be heard at a very great distance over water. The fire of the English, on landing at Egypt, some years since, was heard more than 130 miles at sea; and it has been asserted that every word of a sermon was distinctly heard two miles distant over water. The influence that water has in propagating sound, is evident from the difference caused by a canal of water laid some years ago under the pit floor of the theatre of Argentino, at Rome, the voice having been since heard distinctly at the remotest part of the theatre, where it was before scarcely distinguishable. The canal is, notwithstanding, covered with a brick arch, over which there is a quantity of earth, independent of the floor. Sound is said to travel in water at the rate of nearly 5000 feet in a second; and a bell sounded under water may be heard at a very considerable distance: it does not produce a tone, but a noise like the clashing of knives.

Stone is considered to be, next to water, the best conductor of sound. Brick is nearly as good a conductor: a soft whisper has been conveyed by a garden-wall so as to be distinctly heard at the distance of 200 feet. Wood is also a good conductor, and from its vibratory nature, it is fittest for musical instruments, as well as for the lining of theatres, &c.

Sound, like light, may be collected into one point as a focus, and will be more audible there than at any other point. In buildings of an elliptical shape, a whisper in one of the foci may be distinctly heard in the other focus.

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The concert-rooms at Edinburgh are of an elliptical shape, and are so contrived that the musicians sit in one focus of the ellipse, and the audience in the other. person speaking in the lowest tone, in the whispering-gallery of St. Paul's, is distinctly heard on the opposite side: also a person in one of the recesses of Westminster Bridge readily hears another person speaking on the opposite

side.

When sound strikes any object, and is reflected back, it forms an Echo. Caverns, mountains, and buildings, are favourable to this reverberation; but unless a person stands more than 60 feet from the reflecting object, he will not be able to hear the echo of his own voice distinctly. The

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echo is in some situations repeated several times successively. Near Milan there is an echo that returns the sound of a pistol more than 50 times. An echo in Woodstock

Park returns 17 syllables in the day-time, when the air is brisk, and 20 in the night-time. The air at night being denser, the vibrations become slower, and a repetition of more syllables is heard.

For the purpose of the augmentation of sound, an instrument called a Speaking-trumpet has been invented, by means of which the sound is reflected from the sides of the tube, and prevented from spreading in the open air: it is much used at sea for hailing vessels at a distance. Hearing-trumpets collect and condense the sound, and thus convey it to the ear.

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Many amusing experiments and exhibitions have been at various times shown, depending on the peculiar nature of this branch of natural philosophy. The most striking of these was that termed the Invisible Girl, and which, in the beginning of the present century, was exhibited at different parts of England with considerable eclat. description of this lady may not be uninteresting to the youthful reader, it is attempted; although, being given quite from memory, after a period of more than 30 years, when the writer was a mere child, it is feared that it may be, in some respects, defective. On entering the exhibition-room, the spectator saw an instrument having the appearance of four brass trumpets, at right angles, communicating with a brass globe in the centre, about 10 inches or more in diameter. This was suspended by small cords or ribbons from four small pillars about one inch in diameter, with a cross-rail near the top and bottom, standing unsteadily in the centre of the room. The exhibiter

directed the attention of the spectators to the brass globe, representing to them that the lady was withinside, and that, by addressing her in any one of several languages, she would return an answer. Various questions were put in English, French, and Italian, to each of which an answer was heard from the trumpets, in a weak, feminine voice, issuing apparently from the globe in the centre; and as there was no visible communication between the globe and any other thing whatever, it was at the time a source

of surprise how the voice was conveyed there. It subsequently appeared that a communication was made by a pipe in an adjoining apartment, in which was a female properly instructed; the pipe being passed under the floor, and up one of the pillars, its orifice was placed directly opposite one of the trumpets, and the voice was thus conveyed backwards and forwards between the lady and her visiters.

METEOROLGY.

Constituent parts of the Atmosphere-Evaporation-Rain-Fog or MistDew-Snow-Hail-Coronæ, or Haloes-Parhelia, or Mock-suns-Fiery Meteors-Aeroliths-Aurora Borealis-Ignis Fatuus, or Jack with a LanternWind-Meteorological Instruments, &c.

METEOROLOGY is that part of Natural Philosophy which explains the various phenomena of the atmosphere, as the clouds, rain, hail, dew, &c.

The Atmosphere is a vast body of air surrounding the earth, and extending to about 45 miles above its surface. It has been found when decomposed, as was observed under the article Pneumatics, to consist of two gases, oxygen and azote, containing at the same time about one part in seventy of Aqueous Vapour independent of other substances. One cubic foot of air near the surface of the earth is found to weigh about one ounce and a quarter, being about 800 times lighter than water.

A certain process called Evaporation is continually going on, which supplies the air with aqueous vapour, ultimately to form rain, dew, &c. It is calculated that five thousand millions of tuns of water are carried off from the Mediterranean sea alone in a summer's day, and that twenty millions of tuns are carried off the Thames in the same time; also that one hundred thousand cubic miles of water are by this process annually taken up by the atmosphere, the greatest part of which, when it has arrived at a certain height, is condensed into clouds.

As it is through the agency of caloric that water becomes suspended in the air, it is natural to expect that when the caloric is by any means abstracted, a condensa

tion will take place, and that the aqueous particles will form drops. Something more than the mere parting with the caloric, is now thought necessary for the production of rain, and as it is known that electricity is carried off the earth by evaporation, it is generally understood that rain is in great measure an electrical phenomenon; so that when the clouds part with their electricity, which they may do in various ways, the result is a conversion of the aqueous vapour into drops of water, which, the atmosphere being incapable of supporting, falls in Rain.

Under the idea that Rain is caused by the disturbance of the electricity of the clouds, Sir Richard Phillips, many years ago, published a plan for fertilizing barren districts by erecting metallic rods on elevated spots, thus arresting the clouds and producing rain. It is well known that more rain falls in mountainous countries than in plains ; it is also surmised that the leaves of vegetables, particularly of trees, have a tendency to attract the electricity of the atmosphere, from the fact that woody countries receive most rain. Sir Richard attributes the sterility of certain countries to the cutting down of trees, and conceives that to this may be ascribed the present sterility of the once fertile, but now desert regions of Syria, Chaldea, and Barbary, and he ascribes the oases of the desert to the circumstance of a few trees having been accidentally suffered to grow in them.

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In our own country, as well as in others, most rain falls in places near the sea-coast, so that while the mean annual depth of rain at London is 23 inches, it is somewhat more than twice as much near the western coast. rain falls in the first six months of the year, than in the last six months, and the mean annual quantity is greatest at, or near, the equator, and diminishes towards the poles. In Grenada the annual depth of rain has been found to be 126 inches, when at England* it was 32 inches and at Petersburg 16 inches.

The whole of the vapours which are exhaled from the surface of the earth are not formed into clouds; sometimes, through an imperfect condensation, they remain suspended

*It is calculated that from 300 to 400 tuns of rain fall annually on an average on every acre in England.

in the form of Fog or Mist, sometimes through the coolness of the air a more perfect condensation takes place, and they fall in Dew.

Dews are found to be more copious in clear, than in cloudy weather, also in spring than in any other season, there being then more vapour than at any other time. Egypt and many other countries very much abound with dews, for the air is too hot in the day-time to constipate the vapours into clouds, it, therefore, never rains, but the nights being remarkably cold the vapours are condensed

in Dews.

If the temperature of the air be low, the vapours will sometimes become frozen before they are formed into drops, their specific gravity is then greater than that of the air, and they descend in Snow.

If the vapours are united into drops and become frozen in falling, they are called Hail. As Hail is often found to accompany thunder and lightning, it is considered an electrical phenomenon. Showers of hailstones of extraordinary magnitude have occasionally taken place, more particularly on the continent, which have carried with them death and devastation to an incredible amount. A most violent storm of hail fell on the army of Edward III, near Chartres, in France, when the hailstones were so large that they killed 6000 of his horses, and 1000 of his best troops. At Antwerp, in Holland, in 1776, hailstones fell as large as hens' eggs, and weighed three quarters of a pound; horses were killed, and the fruits of the earth destroyed. In France, in 1785, one hundred and thirty-one villages and farms were laid waste by a dreadful storm of this nature. In order to prevent the destruction occasioned by hail-storms, which often destroy the vintage in the south of France, an instrument called a paragrele has been invented, by means of which the electricity of the atmosphere is said to be disturbed, and hail-storms rendered less severe.

Sometimes a luminous ring or coloured circle appears round the body of the sun and moon, this is called a Co

* Hailstones are said to fall with a rapidity of, at least, 60 feet in a second.

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