tects a circle around it, the radius of which is equal to twice its length above the building.

Some fishes have the power of giving electrical shocks, the effects of which are the same as those obtained by the friction of an electric. The best known of these are the Torpedo, the Gymnotus electricus, and the Silurus electricus.

The torpedo, when touched with both hands at the same time, the one hand on the under, and the other on the upper surface, will give a shock like that of the Leyden vial; which shows that the upper and under surfaces of the electric organs are in the positive and negative state, like the inner and outer surfaces of the electrical jar.

The gymnotus electricus, or electrical eel, possesses all the electrical powers of the torpedo, but in a much higher degree. When small fish are placed in the water with this animal, they are generally stunned, and sometimes killed, by his electrical shock, after which he eats them if hungry. The strongest shock of the gymnotus, will pass a short distance through the air, or across the surface of an electric, from one conductor to another, and then there can be perceived a small, but vivid spark of electrical fire; particularly if the experiment be made in the dark. Galvanism. See Chemistry.

MAGNETISM.

The native Magnet, or Loadstone, is an ore of iron, which is found in various parts of the world. Its color is iron black, its specific gravity from 4 to 5, and it is sometimes found in crystals. This substance without any preparation attracts iron and steel, and when suspended by a string, will turn one of its sides towards the north, and another towards the south.

It appears that an examination of the properties of this species of iron ore, led to the important discovery of the magnetic needle, and subsequently laid the foundation for the science of Magnetism, though at the present day magnets are made without this article.

The whole science of magnetism is founded on the fact that pieces of iron or steel, after being treated in a certain manner, and then suspended, will constantly turn one of their ends to

What animals have the power of giving electrical shocks? Is this electricity supposed to differ from that obtained by art? How must the hands be applied to take the electrical shock of these animals? What is the native magnet, or loadstone? What are the properties of the loadstone: On what is the whole subject of magnetism founded?

wards the north, and consequently the other towards the south. The same property has been more recently proved to belong to the metals nickel and cobalt, though with much less intensity.

The poles of a magnet are those parts which possess the greatest power, or in which the magnetic virtue seems to be concentrated. One of the poles points north, and the other south. The magnetic meridian is a vertical circle in the heavens, which intersects the horizon at the points to which the magnetic needle, when at rest, directs itself.

The axis of a magnet, is a right line which passes from one of its poles to the other.

The equator of a magnet, is a line perpendicular to its axis, and is at the centre between the two poles.

The leading properties of the magnet are the following. It attracts iron and steel, and when suspended so as to move freely, it arranges itself so as to point north and south; this is called the polarity of the magnet. When the south pole of one magnet is presented to the north pole of another, they will attract each other; this is called magnetic attraction. But if the two north or two south poles be brought together, they will repel each other, and this is called magnetic repulsion. When a magnet is left to move freely, it does not lie in a horizontal direction, but one pole inclines downwards, and consequently the other is elevated above the line of the horizon. called the dipping, or inclination of the magnetic needle. Any magnet is capable of communicating its own properties to iron or steel, and this again will impart its magnetic virtue to another piece of steel, and so on indefinitely.

This is

If a piece of iron or steel be brought near one of the poles of a magnet, they will attract each other, and if suffered to come into contact, will adhere so as to require force to separate them. This attraction is mutual; for the iron attracts the magnet with the same force that the magnet attracts the iron. This may be proved, by placing the iron and magnet on pieces of wood floating on water, when they will be seen to approach each other mutually.

The force of magnetic attraction varies with the distance in the same ratio as the force of gravity; the attracting force be

What other metals besides iron possess the magnetic property? What are the poles of a magnet? What is the axis of a magnet? What is the equator of a magnet? What is meant by the polarity of a magnet? When do two magnets attract, and when repel each other? What is understood by the dipping of the magnetic needle?

ing inversely as the square of the distance between the magnet and the iron.

The magnetic force is not sensibly affected by the interposition of any substance except those containing iron, or steel. Thus, if two magnets, or a magnet and piece of iron, attract each other with a certain force, this force will be the same, if a plate of glass, wood, or paper, be placed between them. Neither will the force be altered, by placing the two attracting bodies under water, or in the exhausted receiver of an air pump. This proves that the magnetic influence passes equally well through air, glass, wood, paper, water, and a vacuum. Heat weakens the attractive power of the magnet, and a white heat entirely destroys it. Electricity will change the poles of the magnetic needle, and the explosion of a small quantity of gun powder on one of the poles, will have the same effect.

The attractive power of the magnet may be increased by permitting a piece of steel to adhere to it, and then suspending to the steel a little additional weight every day, for it will sustain, to a certain limit, a little more weight on one day, than it would on the day before.

Small natural magnets will sustain more than large ones in proportion to their weight. It is rare to find a natural magnet, weighing 20 or 30 grains, which will lift more than thirty or forty times its own weight. But a minute piece of natural magnet, worn by Sir Isaac Newton, in a ring, which weighed only three grains, is said to have been capable of lifting 746 grains, or nearly 250 times its own weight.

The magnetic property may be communicated from the loadstone, or artificial magnet, in the following manner, it being understood that the north pole of one of the magnets employed, must always be drawn towards the south pole of the new magnet, and that the south pole of the other magnet employed is to be drawn in the contrary direction. The north poles of magnetic bars are usually marked with a line across them, so as to distinguish this end from the other.

How is it proved that the iron attracts the magnet with the same force that the magnet attracts the iron? How does the force of magnetic attraction vary with the distance? Does the magnetic force vary with the interposition of any substance between the attracting bodies? What is the effect of heat on the magnet? What is the effect of electricity, or the explosion of gun-powder on it? How may the power of a magnet be increased? What is said concerning the comparative powers of great and small magnets?

such a distance, that the ends of the steel bar to be touched, may rest upon

them. Having thus arranged them, as shown in the figure, take the two magnetic bars, d and e, and apply the south end of e, and the north end of d, to the middle of the bar c, eleva. ting their ends, as seen in the figure. Next separate the bars e, and d, by drawing them in opposite directions along the surface of c, still preserving the elevation of their ends; then removing the bars d and e to the distance of a foot or more from the bar c, bring their north and south poles into contact, and then having again placed them on the middle of c, draw them in contrary directions, as before. The same process must be repeated many times, on each side of the bar, c, when it will be found to have acquired a strong and permanent magnetism.

If a bar of iron be placed, for a long period of time, in a north and south direction, or in a perpendicular position, it will often acquire a strong magnetic power. Old tongs, pokers, and fire shovels, almost always possess more or less magnetic virtue, and the same is found to be the case with the iron window bars of ancient houses, whenever they have happened to be placed in the direction of the magnetic line.

A magnetic needle, such as is employed in the mariner's and surveyor's compass may be made by fixing a piece of steel on a board, and then drawing two magnets from the centre towards each end, as directed, at fig. 223. Some magnetic needles, in time, lose their virtue, and require again to be magnetized. This may be done by placing the needle, still suspended on its pivot, between the opposite poles of two magnetic bars. While it is receiving the magnetism, it will be agitated, moving backwards and forwards, as though it were animated but when it has become perfectly magnetized, it will remain quiescent.

Explain fig. 223, and describe the mode of making a magnet. In what positions do bars of iron bebome magnetic spontaneously? How may a needle be magnetized without removing it from its pivot?

The dip, or inclination of the magnetic needle is its devia. tion from its horizontal position, as already mentioned. A piece of steel, or a needle, which will rest on its centre, in a direction parallel to the horizon, before it is magnetized, will afterwards incline one of its ends towards the earth. This property of the magnetic needle, was discovered by a compass maker, who, having finished his needles before they were magnetized, found that immediately afterwards, their north ends inclined towards the earth, so that he was obliged to add small weights to their souty poles, in order to make them balance, as before.

The dip of the magnetic needle is measured by a graduated circle, placed in the vertical position, with the needle suspended by its side. Its inclination from a horizontal line marked across the face of this circle, is the measure of its dip. The circle, as usual, is divided into 360 degrees, and these into minutes and seconds.

The dip of the needle does not vary materially at the same place, but differs in different latitudes, increasing as it is carried towards the north, and diminishing as it is carried towards the south. At London, the dip for many years has varied little from 72 degrees. In the latitude of 80 degrees north, the dip according to the observations of Capt. Parry, was 88 degrees.

Although, in general terms, the magnetic needle is said to point north and south, yet this is very seldom strictly true, there being a variation in its direction, which differs in degree at different times and places. This is called the variation, or declination of the magnetic needle.

This variation is determined at sea, by observing the different points of the compass at which the sun rises, or sets, and comparing them with the true points of the sun's rising or setting, according to astronomical tables. By such observations, it has been ascertained, that the magnetic needle is continually declining alternately to the east, or west, from due north, and that this variation differs in different parts of the world at the same time, and at the same place at different times.

How was the dip of the magnetic needle first discovered? In what manner is the dip measured? What circumstance increases or dimin. ishes the dip of the needle? What is meant by the declination of the magnetic needle? How is this variation determined? What has been ascertained, concerning the variation of the needle at different times and places?