Page images

when moderately heated will support a greater quantity of vapour invisibly, than when it is cold: as it can more easily keep the particles of the suspended fluid at a distance from each other, and prevent them from running together into larger concretions to refract the rays of light and obstruct vision. Hence the moisture carried off from the lungs in respiration is not so visible in warm as in cold air. Hence the dissipation of fogs by the morning sun, the visible wasting of clouds in a hot and serene sky, the sudden overcasting and troubled state of the atmosphere taking place of a pleasant and clear state of the the heavens, only by a stream of cold air condensing the vapours and rendering them visible. Wind also, which is another effect of heat, greatly assists evaporation, by carrying off the saturated particles of air, and bringing fresh particles in contact with the evaporating fluid, to dissolve and carry off more. Hence a wet room is quickly dried by being exposed to the open air. Hence the sudden drying of the roads after rain, by a strong wind. Because of the limited sphere of attraction belonging to each particle of air, it can suspend but a certain quantity of moisture; and therefore when it is saturated, it can receive no more, until it has discharged part of its burden to the neighbouring air. Hence air frequently breathed, at last refuses to carry off the necessary discharge from the lungs, and becomes unfit for respiration. Earth when pulverised and reduced to dust is also attracted and suspended by the air, until it be returned again by coalescing with the drops of descending rain, or dew.

Although air attracts both water and oil separately, yet, because of the natural repulsion between these two fluids, it cannot attract and dissolve them both at

the same time. Hence the suffocating nature of air impregnated with oily particles, in a chandler's shop, or from lamps and the wicks of extinguished candles: the air in this situation not being able to attract and carry off the moisture of the lungs. But when air is saturated with poisonous and pestilential particles collected from putrefying substances, hospitals or gaols, it may in respiration exchange them for the moisture of the lungs, and thereby produce various diseases, by what is called by the chemists, a single elective attraction: the lungs attracting these poisonous particles more forcibly than their own moisture, while the air attracts the moisture of the lungs with greater force than that by which it attracts these noxious particles.

An easy experiment confirms this theory of evaporation on the principles of chemical solution. Place a vessel of water under the receiver of an airpump, and when the air is drawn from the pores of the water, in which it is copiously found, it will bring with it such a quantity of vapour, as is sufficient to obscure the contents of the receiver, until being drawn off with the air, or adhering to the sides of the receiver, it will leave it quite clear and transparent, as before the operation began.


THE next species of attraction to be explained is that of magnetism, which is found to take place only between the loadstone and iron. Its principal properties, so far as they have yet been discovered, are such as these. Every loadstone, or artificial magnet, has two poles opposite to each other, called its north and south

poles; because the magnet, when at liberty to move, will naturally place itself nearly in the direction of the meridian, and always with the same end towards the north point of the heavens. If a magnet be laid on a sheet of clean paper in the midst of a parcel of steel filings, they will soon be found disposed in regular curves terminating in the poles; from whence some have been led to conjecture, that the magnetic virtue, by which it attracts iron, is emitted from one pole and received by the other. Either pole of a magnet will attract iron or steel that has not previously received any magnetic virtue; but when two magnets are brought near together, they will attract or repel each other accordingly as the poles that are presented to each other are of different or of the same denomination. Though if their magnetic virtues be very different, the same poles will attract each other, but with little force, and at a small distance. A magnet exerts its virtue at the distance of several feet, constantly decreasing in strength as the distances increase, but in very different ratios in different magnets. In some the force of attraction seems to be inversely as the squares of the distances; in others as the cubes of the distances; and in others in other proportions. Its virtue is exerted in vacuo, as well as in open air, and without any sensible diminution of its force by the interposition of the hardest bodies, whose pores it easily pervades. This virtue is copiously communicated to iron by the touch, and to no other substance yet known. This iron when properly balanced on its center will, like

Iron, or soft steel will more readily receive the magnetic virtue than hard or tempered steel, but will also more readily part with it, and therefore all artificial magnets, or magnetic needles should be made of the latter.


the loadstone, settle itself nearly in the position of the meridian, turning that end to the north, which was touched with the south pole of the magnet. This touched iron becomes an artificial magnet and will communicate the same polarity to any other small pieces of iron, without any diminution of its own virtue, and therefore answers all the purposes of the loadstone. The polarity of the needle may be reversed, by reversing the manner of touching it; that is, if the north end of the needle be touched with the north pole of the magnet it will then turn to the south: and this may be done as often as we please. A smart stroke of a hammer will also reverse the poles of the needle, when the end of the needle is only struck. And the whole magnetic virtue may be destroyed by fire. This polarity is also given to iron various ways without the touch of the loadstone: such as by a long and hard attrition in the same direction; (hence files and drills are always found to be magnetical;)—by suffering iron bars to stand long in one position;-by conducting a violent shock of lightning or charge of electricity;by heating the iron until it become redhot, and suddenly quenching it in water, holding it at the same time in an angle of about 75 degrees below the northern horizon, and nearly in the direction of the meridian; and lastly, by preparing and polishing two pieces of steel, and arming one of them with a cap of the same: then, if the unarmed piece be held in the magnetic meridian, and the other be drawn along it several times, one way, it will acquire a strong degree of magnetism, and be able to communicate the same to the other piece, when the armour is changed from one to the other.

If a needle be properly balanced on its center, so

[ocr errors]

that it hangs horizontally before it is touched with the magnet, it will now dip below the horizon in an angle of some degrees, towards the north; which seems to be owing to its losing some of its gravity from the south end of the needle; as the whole needle has been found by many experiments to weigh less than before the touch. This is called the dipping needle. The south end of the needle dips below the horizon in south latitudes.

This singular property of the magnet giving polarity to the balanced needle, has been of prodigious advantage to mankind, in the arts of surveying and navigation.

It was said above, that the needle settles nearly in the direction of the meridian of any place. It will however deviate a little from this direction, and this deviation is called the variation of the compass or needle. There is a certain irregular curve line which surrounds the earth towards the north and south poles of the world; on any point of which line the needle points exactly to the north and settles in the meridian. This is called the line of no variation; on the east side of which the needle points to the westward of the meridian, and on the west side of it, it points to the eastward of the true meridian. This variation increases gradually as you recede either eastward or westward from the line of no variation until it comes to its maximum; which is found to be about 40 degrees in some places, after which it gradually decreases again until you arrive at the line of no variation. Did this variation increase or decrease in any regular manner from any known meridian, it would enable the mariner to correct his longitude at sea. Dr. Halley has proposed this use of it; and has, from a number of

« PreviousContinue »