The repellent force, which triumphs so signally over the pull of the sun on these little solid, liquid, and Electrical gaseous emanations, is supposed to be electrical.

In

any physical laboratory may be seen pith-balls and light strips of paper, which are lifted by electrical forces in opposition to the force of gravity. In a similar way the lightest portions of a comet may be driven off by an electrical repulsion originating in the sun, while the heavier portions are dominated by his attraction.

The spectroscope certifies to the presence of a few known elements in comets. The predominant gases seem to be hydro-carbons, which are compounds of hydrogen and carbon. Sodium and iron have been certainly identified, and magnesium and calcium are thought to be present. What happens to these different materials as they are being driven off by the electrical repulsion? Manifestly the lightest elements attain the greatest velocity; moderately heavy ones move with less velocity, and the heaviest with still less. These motions, combined with the orbital motions of comets, cause various degrees of curvature in their tails.

forces.

Different materials.

There are three special types of tails. Tails of the first type are nearly straight and point almost directly away Types of tails. from the sun. They are believed to be composed largely of hydrogen. The majority of the trains belong to the second type, and are gracefully curved; here the repulsive force has less effect than before, as the particles on which it acts are heavier. Tails of this type are composed of hydro-carbons. The third type of tail is uncommon; it, too, is plume-like, but it curves very sharply at the comet's head, and trails behind the nucleus as the latter moves swiftly in its appointed path. Iron vapor is thought to be present in such tails.

The appearances of the three types are aptly repre

The smoke of a locomotive.

Anomalous tails.

If

sented by the smoke which issues from a freight engine
moving in a quiet atmosphere at a moderate speed.
the steam pressure is very high, the puffs of smoke go
nearly straight up; if the pressure is only moderate, the
stream of smoke forms a curving plume; when the
steam is nearly shut off the smoke trails lazily behind the
smoke-stack.

Some comets exhibit more than one type of tail; even so strange a phenomenon as a tail pointing directly toward the sun has been observed. Wonderful changes have been noticed, as in the case of Swift's bright comet of 1892. On April 4 its tail was straight and twenty degrees in length, but consisted of two distinct branches lying close together. On the next night a third tail was seen between the other two; each of the three appeared to be composed of several, so that the whole looked like a fan partially opened. Within twenty-four hours more one tail vanished, and the other two joined their boundaries. One of these then grew bright at the expense of the other, and finally split up into half a dozen branches. These are the most noteworthy of the changes which took place in five days.

The particles which are driven off into the tails are Fate of comets. lost. A periodic comet, i. e., one which moves in an ellipse and returns at stated intervals, loses some of its substance at each perihelion passage, and must be wasted away in time.

Companions.

Comets are sometimes accompanied by smaller companions. In 1889 one was seen which had no less than four of these attendants; two of them were very faint, and did not last long; for a while the other two were veritable twins, and bore a striking resemblance to the main comet. Like foolish children they cut the maternal apron strings and began to move away; this move sealed

FIG. 126.-PHOTOGRAPHS OF SWIFT'S BRIGHT COMET OF 1892.

Changes of

brightness.

Superstitious

terror.

Collisions.

the fate of one of them, which soon faded into invisibility. The other made a brave show for a time, but came back in a few weeks with a swelled head and no tail. The moral is obvious.

Though the brightness of a comet generally changes with considerable regularity as its distances from the sun and earth vary, there are often anomalous variations, which are best explained by electrical discharges between the small masses of which it is made up. The existence of such discharges is not merely conjectured. During the past few years spectroscopic observations of comets have gone hand in hand with laboratory experiments upon gases confined in Geissler tubes, and lit up by electric discharges. A mass of evidence has thus been accumulated which cannot be set aside; unfortunately it is too technical to be reproduced here.* Suffice it to say that the coincidences between electrical appearances produced in the laboratory and those observed in the spectra of comets are very complete.

It is well known that in past centuries comets were objects of superstitious terror, not only to the ignorant, but even to the higher classes of society. The comet of 1528 is thus described by Ambrose Paré :

This comet was so horrible, so frightful, and it produced such great terror in the vulgar that some died of fear and others fell sick. It appeared to be of excessive length, and was of the color of blood. At the summit of it was seen the figure of a bent arm, holding in its hand a great sword, as if about to strike. At the end of the point there were three stars. On both sides of the rays of this comet were seen a great number of axes, knives, blood-colored swords, among which were a great number of hideous human faces, with beards and bristling hair.

Though some unaccountable superstitions still survive * See Scheiner's "Astronomical Spectroscopy," pages 207-22.

among fairly educated members of enlightened communities, very few of them are connected with comets. But there is apprehension in many quarters concerning the results of a collision between a comet and the earth. The fear is that the great heat generated by the impact would blast the earth's surface as effectually as if it were tossed into a gigantic furnace, and would dissolve all its inhabitants in the twinkling of an eye. It No great appears from what we have learned of the constitution of danger. comets that nothing of the sort is to be feared. Astronomers would be delighted if any ordinary comet should run into the earth, for there would be a shower of falling stars most beautiful to behold. A very large comet might make more trouble; for such an one probably contains a good supply of metallic masses, which would come through the air without being consumed. Fortunately they would not be close together, for stars have been seen shining with undiminished splendor through the nuclei of large comets; a city as large as Chicago might catch only a few of the celestial missiles. Some of them might be as large as houses and cause decided havoc where they struck. The celestial spaces are so vast in comparison with the bodies which traverse them that there is little danger to be apprehended from comets.

In November, 1892, there was a comet scare, caused a comet scare. by the apprehension that Biela's comet* was about to dash against our planet. The fright inspired in certain localities is evidenced by the following press-dispatch from Atlanta, Georgia :

The fear which took possession of many citizens has not yet abated. The general expectation hereabouts was that the comet would be heard from on Saturday night. As one result the confessionals of the two Catholic churches were crowded

* Holmes's faint comet was erroneously thought to be a return of Biela's.