observed by Scheiner in 1630, which, although it has been quoted by Dr. David Brewster and other writers on optics, it will be necessary to transcribe.

"The diameter of the circle MQ N, next to the sun, was about 45°, and that of the circle OR P, was about 95° 20'; they were coloured like the primary rainbow, but the red was next the sun, and the other colours in the usual order. The breadths of all the arches were equal to one another, and about a third part less than the diameter of the sun, though I cannot but say that the whitish circle O G P, parallel to the horizon, was rather broader than the rest. The two parhelia, M N, were lively enough, but the other two at O and P were not so brisk. M and N had a purple redness next the sun, and were white in the opposite parts; O and P were all over white. They all differed in their du

while, though it was but faint. The two lateral parhelia, M and N, were seen constantly for three hours together; M was in a languishing state, and died first, after several struggles, but N continued an hour after it at least. Though I did not see the last end of it, yet I am sure it was the only one that accompanied the true sun for a long time, having escaped those clouds and vapours which extinguished the rest. However, it vanished at last, upon the fall of some showers. This phenomenon was observed to last four hours and a half at least; and since it appeared in perfection, when I first saw it, I am persuaded its whole duration might be above five hours.

"The parhelia QR were situated in a vertical plane, passing through the eye at F, and the sun at G, in which vertical plane the arches HRC and O PR either crossed or touched one another. These parhelia were sometimes brighter, sometimes fainter than the rest, but were not so perfect in their shape and white colour. They varied their magnitude and colour according to the different temperature of the sun's light at G, and the matter which receives it at Q and R; and therefore their light and colour were almost always fluctuating, and continued, as it were, in a perpetual conflict. I took particular notice that they appeared almost the first and last of the parhelia, excepting that of N.

"The arches which composed the small halo M N next to the sun, seemed, to the eye, to compose a single circumference, but it was confused, and had unequal breadths; nor did it constantly continue like itself, but was perpetually fluctuating. But, in reality, it consisted of the arches expressed in the figure, as I accurately observed for this very purpose. These arches cut each other in a point at Q, and there they formed a parhelion; the parhelia M and N shining from the common intersections of the inner halo, and the whitish circle O NM P."

Phenomena of the same kind are sometimes seen during the night as the effects of lunar light, and these are called paraselena. The most beautiful appearance of this kind which has been described, was that seen by Hevelius at Dantzic, in 1660. When first observed, the moon was surrounded by a white circle, in which there were two coloured paraselenæ opposite to each other. Another circle of light was afterward formed, the lower line of the circumference

touching the horizon, and inverted coloured arches appeared on the summit of the circles.

Captain Parry several times observed this phenomenon during his polar voyages. At one time he observed the moon to be surrounded with a halo on which three luminous paraselenæ were formed, and on the following night the same appearance was observed, with the addition of stripes of white light, which gave it a resemblance to a cross.

The natural phenomena which result from the action of light are extremely complicated, and their explanation is involved in difficulties. Our ignorance of the agent itself, and the various circumstances under which it is influenced by repellant and attractive forces, prevent us from tracing the cause of many phenomena which are produced by experiment. There is no branch of philosophical knowledge so little studied by those who have not formally devoted themselves to the investigation of the sciences, as the cause and effects of that principle we call light. Surrounded with uncertainties, we fear lest the explanation given in this chapter should be unsatisfactory to the general reader; but we are chiefly anxious that this circumstance may induce a farther investigation of the subject, and not repress an anxiety for a more accurate acquaintance with this interesting, though subtle branch of natural philosophy.

CHAPTER VI.

PHENOMENA DEPENDANT ON THE DISTRIBUTION OF ELECTRICITY.

OUR entire ignorance of that agent which we call electricity, may be urged as some excuse for the vague and unsatisfactory manner in which we are compelled to speak of all the natural phenomena that depend upon it as an agent. But, although we are unacquainted with the cause of phenomena universally attributed to electricity, we have ascertained with great precision its habitudes and relations. There are some philosophers who attribute electric appearances to an exceed

ingly subtile fluid, while others imagine that there are two such fluids ;-some believe electricity to be a property of matter, and others identify it with gravity. But, whatever opinion may be entertained concerning the agent, there is no doubt as to the means by which it may be excited, the conditions it obeys, and the effects it produces.

Supposing electricity to be present in a latent state in all matter, it may be disturbed by five means; friction, chymical action, the contact and disunion of magnetic poles, the unequal circulation of heat through metals, and the muscular action of certain fishes. The electricities derived from these several sources have received different names, but are distinguished from each other by comparative rather than by absolute qualities, all producing the same effects, though not in an equal degree. A few general remarks seem to be necessary for an accurate explanation of the phenomena, to which we must refer.

When electricity is excited upon the surface of a substance, as always happens when two substances, whatever their nature, are rubbed together, it may be transferred, under particular circumstances, from place to place, and even accumulated. But all bodies are not capable of conducting electricity; for, while some absolutely resist its progress, there are others that give it an easy passage. Glass, the resins, and atmospheric air, are not conducting substances; while the metals, water, and aqueous vapour transfer it with great facility. It might, therefore, be expected, that while the one class conducts electricity of the smallest possible intensity, the other can only transmit it when its resisting force is overpowered by the energy of the electric agent.

According to some persons, there are two kinds of electricity, while others acknowledge the existence of but one, though they admit that its conditions may vary. Without attempting to prove or disprove either of these statements, it may be mentioned that there is an evident difference of character, according to the manner and circumstances under which the agent is produced. A substance charged with electricity produced by the excitation of glass, will attract one that is charged with the electricity of resin; but if the two bodies be excited with the electricity of either glass or resin, they will repel each other. Now, if we consider that in one case the agent is in a positive, and in the other in a negative

condition, this fact may be thus generally stated, that bodies in the same electric state repel each other, in opposite states they attract.

The effects produced upon bodies by the passage of accumulated electricity through them, are various and important. When discharged through steel, it frequently induces the magnetic property; when through magnets, it often destroys or disturbs their magnetism. The temperature of metals is raised when they are employed as conductors; and if the size of the metallic substance be proportioned to the intensity of the electric agent, fusion will be effected. Light is produced when electricity passes through elastic fluids, or liquids that have an inferior conducting power. Chymical effects, such as the composition and decomposition of bodies, may, under certain circumstances, result from the conduction of electricity. When the animal body is made the medium of transference, muscular action of a more or less violent character is produced, and instances are on record in which life has been destroyed. By these effects, the presence of accumulated electricity is determined; and when its quantity and intensity are not sufficient to produce them, the phenomena of attraction and repulsion never fail to give evidence of its presence.

IDENTITY OF ELECTRICITY AND THE AGENT THAT PRODUCES

LIGHTNING.

The identity of agents is to be determined by the identity of their effects. Franklin seems to have recognised this principle; for, having observed that many of the effects produced by lightning were in a lesser degree occasioned by accumulated electricity as derived from the machine, he suspected that the agencies might differ in no other respect than in intensity. Among their many points of resemblance, he noticed that their motion from one irregular conductor to another is never in a right line; that they always strike the highest bodies, preferring those that are pointed and good conductors; that they ignite combustible bodies; fuse the metals; affect magnetic polarity; and destroy animal life. This similarity of effect might have been considered by some persons as a sufficient proof of the identity of the cause which produced lightning, and that we call electricity. But Franklin was not satisfied with any thing less than a demon