Изображения страниц

along the hempen string that held the kite, and was received by a key tied to the extremity of it. That part of the string, which the doctor held in his hand, was of silk, that the electric fire might stop at the key, and not reach his body.

He found that the string would conduct electricity even when nearly dry, but that when it was wet, it would conduct it quite freely; so that it would stream out plentifully from the key, at the approach of a person's finger. At this key he charged phials, and from electric fire thus obtained he kindled spirits, and performed all the common electrical experiments."

EDWARD. And was it from this discovery of the sameness of lightning and electricity, that Dr. Franklin contrived the method of securing buildings from the dreadful effects of lightning in a thunder-storm ?"

Dr. Walker.-"Yes. With regard to thunder itself or the sound or noise we hear, it is perfectly harmless. It is the lightning that does the mischief." But to the doctor's invention, which is simply that of fixing a pointed iron rod higher than any part of the building, and joining to the lower end of it a wire, which communicated with the earth ; this rod the lightning seizes upon, in preference to any other part of the building, and descends along it and the wire till it reaches the earth, where it is instantly dissipated without doing any harm. All public buildings, and especially all magazines, ought to have such an apparatus for defending them from lightning, and in the present state of science we should suppose all have. “ The fire of electricity is very

different from common fire, and operates in a very different manner. It has been known to melt a sword in the scabbard, without injuring the scabbard itself; and to melt money in a man's pocket, without burning his clothes. In a word, it seems to be of such a nature, that it can easily penetrate through porous bodies without affecting them, and spends all its force upon those that are hard and solid.

“ The experiment of drawing lightning from the atmosphere by means of an electric kite, is attended with danger. It proved fatal to Abbe Richman, who, in 1753, was killed by a flash of lightning, which he drew from the clouds, in an experiment he was making at Petersburgh.

“ Électricity has been applied to some medical purposes,

[ocr errors]

with so much success, that it may now be considered as part of the science.

“ 'Thunder is the noise produced by the motion of lightning, and the reason why we do not hear the dreadful noise of the thunder, as soon as we see the lightning, is, because sound is longer in arriving to our ears, than light to our sight.

“ Light moves almost instantaneously. Sound moves no more than 1142 feet in a second. That light moves much faster than sound, any one may satisfy himself, by observing a gun discharged at a distance; for he will see the fire long before he hears the sound.

“ The continuation and repetition of the sound are caused by a kind of echo formed in the clouds, to which many

hard bodies

upon the earth may contribute, which return those rollings we hear after a great clap of thunder.

“ A thunder bolt is nothing but a more solid and most rapid flame, which, with incredible swiftness flies from the clouds to the earth, and through every thing standing in its way, being interrupted by nothing. It sometimes kills men and animals, burns and overthrows large trees and buildings, and sets fire to every thing in its way."

EDWARD.~" Of its power we have indeed had melan. choly proof."

Dr. WALKER. “ Yes, and that of the winds. The effect of the latter, which is an invisible agent in the hands of Providence, appears more wonderful than that of lightning. In the course of a few hours one of the proudest works of man is reduced by its almost magical effects to a floating wreck; and yet this wind is nothing more than the common air put violently in motion, and this is occasioned chiefly by heat.

EDWARD.- :-“ By heat, Sir! I am sure we were cold enough in the storm." Dr. Walker." That is very true ; but I will explain

that its violent motion is produced by heat. When any part of the air is heated by the sun, by any electric matter, or by any other heat, it will swell and thereby affect the adjacent air; and so, by various degrees of heat in different places, there will arise various motions of the air.' Have you never observed that there is a light breeze at sun-rise, and cannot you now account for it by the rarefication of the air by the sun?”

and prove

this to you,


EDWARD.--" Yes, Sir, now you point it out to me.”

DR. WALKER.-“ Well, when the air is much heated, it will ascend towards the upper part of the atmosphere, and the adjacent air will rush in to supply its place, and therefore there will be a stream or current of air from all parts, towards the place where the heat is. And hence we see the reason, why the air rushes with such force into a glasshouse, a tile-kiln, or towards any place where a great fire is made; and also why smoke is carried up a chimney, and why the air rushes in at the key-hole of a door, or any small chink, where there is fire in the room. in general, we may take it for granted, that the air will press towards that part of the world where it is most heated.

“ The winds, you know, are divided into four principal ones, the north, south, east, and west, which receive their names from the four quarters of the world.

“ From the Frigid Zone comes the north wind, which is consequently the coldest. The south wind is the warmest, and particularly in the summer, because it comes from the Torrid Zone, over countries where the sun is most vertical. The east wind is the dryest; because it comes across the vast continent of Asia, which is but little watered by rivers or

The west wind often blows us rain ; because, as it crosses the great Atlantic ocean, it attracts a great quantity of vapours. Now when these impetuous winds happen to meet, the greatest inconveniences follow. The sulphureous exhalations from the south, torrents of nitre from the north, and watery vapours from every side, become indiscriminately blended together in one confused mass. From hence proceed tempests, thunder, rain, hail, and whirlwind.

“ The velocity of wind is computed to be at the rate of 50 or 60 miles an hour, in a great storm ; that of a common brisk wind is about 15 miles an hour; and some winds move not even one mile in that space of time. A person, therefore, on horseback, and even sometimes on foot, may be said to outstrip the wind; for, if he moves faster than the wind, which is very possible, he will have a wind in his face, though he move in the same direction with the wind.

“ Besides these, there are certain winds, called Tropical Winds, which blow almost always from the same point of the compass. They are of three kinds :

“ The general trade Winds which extend to nearly thirty degrees of latitude on each side of the equator, in the At


lantic, Ethiopic, and Pacific Oceans. On the north side of the equator, they blow from the north-east; on the south side, from the south-east; and near the equator, from almost due east.

“ The Monsoons, or shifting trade winds, which blow six months in one direction, and the other six months in the opposite direction. These are mostly in the Indian, or Eastern ocean, and do not reach above two hundred leagues from the land. Their change is at the vernal and autumnal equinoxes, and it is accompanied with terrible stornis of thunder, lightning, and rain. The Monsoons are occasioned by the cold air moving towards those places, in which the air is rarefied by the heat of the sun, in order to restore its equilibrium.

« The Land and Sea Breezes, which are periodical winds, and blow from the land, from night to about mid-day, and from the sea, from about noon to mid-night. These winds do not extend above two or three leagues from the shore.

“ Beyond the latitude of thirty degrees north and south, the winds, as we daily perceive in Great Britain, are more variable, though it may be observed, in general, that the ten. dency of the wind is from a colder region to that which is hotter.

“ But perhaps before I had explained the cause of wind, I should have spoken of the nature of air itself; and though this may be attended with a repetition of what I formerly told you, yet it cannot be avoided.

6 The air is a fluid in which we live and breathe : it entirely envelopes our globe, and extends to a considerable height around it. Together with the clouds and vapours that float in it, it is called the Atmosphere. As it is possessed of gravity, in common with all other fluids, it must press upon bodies in proportion to the depth at which they are immersed in it; and it also presses in every direction, in common with all other fluids.

“ It differs from all other fluids in the four following particulars :-1. It can be compressed into a much less space than what it naturally possesses ; 2. It cannot be congealed or fixed, as other fluids may; 3. It is of a different density in every part upward from the earth's surface, decreasing in its weight, bulk for bulk, the higher it rises; 4. It is of an elastic, or springy nature, and the force of its spring is equal to its weight.

People who are unacquainted with the principles of natural philosophy, would not suppose that the air by which we are surrounded, is a material substance, like water, or any other visible matter. Being perfectly invisible, and affording no resistance to the touch, it must seem to them extrà. ordinary, to consider it as a solid and material substance; and yet a few simple experiments will convince any one that it is really matter, and possesses weight, and the power of resisting other bodies that press against it.

" Take a bladder that has not the neck tied, and you may press the sides together, and squeeze it into any shape. Fill this bladder with air, by blowing into it, and tie a string fast round the neck; you then find that you cannot, without breaking the bladder, press the sides together, and that you can scarcely alter its figure by any pressure. Whence then arise those effects? when the bladder was empty, you

could press it into any form; but the air with which it is filled, prevents this: the resistance you experience when it is filled with air, proves that that air is as much matter as any other substance that we are acquainted with.

“ The atmospheric fluid, or common air, is composed chiefly of two gases, or aëriform fluids ; one of which is capable, by respiration, of contributing to support animal life; and in which metals are calcinable, and combustible bodies may burn. The other, on the contrary, is endowed with directly opposite qualities : it cannot be breathed by animals, neither will it admit of the combustion of inflammable bodies, nor of the calcination of metals.

“ The base of the former, which is the respirable part of the air, is called Oxygen, from two Greek words, signifying to produce acidity, because one of the most general properties of this base is to form acids, by combining with many different substances. The union of this base with caloric, is called Oxygen Gas, which is the same that was formerly called Pure, or Vital Air.

“ As all the parts of the atmosphere gravitate, or press upon each other, it is easy to conceive, that the air next the surface of the earth is more compressed and denser than what is at some height above it ; in the same manner as if wool were thrown into a deep pit until it reached the top. The wool at the bottom having all the weight of what was above it, would be squeezed into a less compass; the layer, or stratum above it, would not be pressed quite so much ;

« ПредыдущаяПродолжить »