I have said that comets arrive from the depths of s.p.a.ce, rush towards and round the sun, whizzing past the earth with a speed of twenty-six miles a second, on round the sun with a far greater velocity than that, and then rush off again. Now, all the time they are away from the sun they are invisible. It is only as they get near him that they begin to expand and throw off tails and other appendages. The sun"s heat is evidently evaporating them, and driving away a cloud of mist and volatile matter. This is when they can be seen. The comet is most gorgeous when it is near the sun, and as soon as it gets a reasonable distance away from him it is perfectly invisible.
The matter evaporated from the comet by the sun"s heat does not return--it is lost to the comet; and hence, after a few such journeys, its volatile matter gets appreciably diminished, and so old-established periodic comets have no tails to speak of. But the new visitants, coming from the depths of s.p.a.ce for the first time--these have great supplies of volatile matter, and these are they which show the most magnificent tails.
[Ill.u.s.tration: FIG. 101.--Head of Donati"s comet of 1858.]
The tail of a comet is always directed away from the sun as if it were repelled. To this rule there is no exception. It is suggested, and held as most probable, that the tail and sun are similarly electrified, and that the repulsion of the tail is electrical repulsion. Some great force is obviously at work to account for the enormous distance to which the tail is shot in a few hours. The pressure of the sun"s light can do something, and is a force that must not be ignored when small particles are being dealt with. (Cf. _Modern Views of Electricity_, 2nd edition, p. 363.)
Now just think what a.n.a.logies there are between comets and meteors. Both are bodies travelling in orbits round the sun, and both are mostly invisible, but both become visible to us under certain circ.u.mstances.
Meteors become visible when they plunge into the extreme limits of our atmosphere. Comets become visible when they approach the sun. Is it possible that comets are large meteors which dip into the solar atmosphere, and are thus rendered conspicuously luminous? Certainly they do not dip into the actual main atmosphere of the sun, else they would be utterly destroyed; but it is possible that the sun has a faint trace of atmosphere extending far beyond this, and into this perhaps these meteors dip, and glow with the friction. The particles thrown off might be, also by friction, electrified; and the vaporous tail might be thus accounted for.
[Ill.u.s.tration: FIG. 102.--Halley"s Comet.]
Let us make this hypothesis provisionally--that comets are large meteors, or a compact swarm of meteors, which, coming near the sun, find a highly rarefied sort of atmosphere, in which they get heated and partly vaporized, just as ordinary meteorites do when they dip into the atmosphere of the earth. And let us see whether any facts bear out the a.n.a.logy and justify the hypothesis.
I must tell you now the history of three bodies, and you will see that some intimate connection between comets and meteors is proved. The three bodies are known as, first, Encke"s comet; second, Biela"s comet; third, the November swarm of meteors.
Encke"s comet (one of those discovered by Miss Herschel) is an insignificant-looking telescopic comet of small period, the orbit of which was well known, and which was carefully observed at each reappearance after Encke had calculated its...o...b..t. It was the quickest of the comets, returning every 3-1/2 years.
[Ill.u.s.tration: FIG. 103.--Encke"s comet.]
It was found, however, that its period was not quite constant; it kept on getting slightly shorter. The comet, in fact, returned to the sun slightly before its time. Now this effect is exactly what friction against a solar atmosphere would bring about. Every time it pa.s.sed near the sun a little velocity would be rubbed out of it. But the velocity is that which carries it away, hence it would not go quite so far, and therefore would return a little sooner. Any revolving body subject to friction must revolve quicker and quicker, and get nearer and nearer its central body, until, if the process goes on long enough, it must drop upon its surface. This seems the kind of thing happening to Encke"s comet. The effect is very small, and not thoroughly proved; but, so far as it goes, the evidence points to a greatly extended rare solar atmosphere, which rubs some energy out of it at every perihelion pa.s.sage.
[Ill.u.s.tration: FIG. 104.--Biela"s comet as last seen, in two portions.]
Next, Biela"s comet. This also was a well known and carefully observed telescopic comet, with a period of six years. In one of its distant excursions, it was calculated that it must pa.s.s very near Jupiter, and much curiosity was excited as to what would happen to it in consequence of the perturbation it must experience. As I have said, comets are only visible as they approach the sun, and a watch was kept for it about its appointed time. It was late, but it did ultimately arrive.
The singular thing about it, however, was that it was now double. It had apparently separated into two. This was in 1846. It was looked for again in 1852, and this time the components were further separated. Sometimes one was brighter, sometimes the other. Next time it ought to have come round no one could find either portion. The comet seemed to have wholly disappeared. It has never been seen since. It was then recorded and advertised as the missing comet.
But now comes the interesting part of the story. The orbit of this Biela comet was well known, and it was found that on a certain night in 1872 the earth would cross the orbit, and had some chance of encountering the comet. Not a very likely chance, because it need not be in that part of its...o...b..t at the time; but it was suspected not to be far off--if still existent. Well, the night arrived, the earth did cross the orbit, and there was seen, not the comet, but a number of shooting-stars. Not one body, nor yet two, but a mult.i.tude of bodies--in fact, a swarm of meteors. Not a very great swarm, such as sometimes occurs, but still a quite noticeable one; and this shower of meteors is definitely recognized as flying along the track of Biela"s comet. They are known as the Andromedes.
This observation has been generalized. Every cometary orbit is marked by a ring of meteoric stones travelling round it, and whenever a number of shooting-stars are seen quickly one after the other, it is an evidence that we are crossing the track of some comet. But suppose instead of only crossing the track of a comet we were to pa.s.s close to the comet itself, we should then expect to see an extraordinary swarm--a mult.i.tude of shooting-stars. Such phenomena have occurred. The most famous are those known as the November meteors, or Leonids.
This is the third of those bodies whose history I had to tell you.
Professor H.A. Newton, of America, by examining ancient records arrived at the conclusion that the earth pa.s.sed through a certain definite meteor shoal every thirty-three years. He found, in fact, that every thirty-three years an unusual flight of shooting-stars was witnessed in November, the earliest record being 599 A.D. Their last appearance had been in 1833, and he therefore predicted their return in 1866 or 1867.
Sure enough, in November, 1866, they appeared; and many must remember seeing that glorious display. Although their hail was almost continuous, it is estimated that their average distance apart was thirty-five miles!
Their radiant point was and always is in the constellation Leo, and hence their name Leonids.
[Ill.u.s.tration: FIG. 105.--Radiant point perspective. The arrows represent a number of approximately parallel meteor-streaks foreshortened from a common vanishing-point.]
A parallel stream fixed in s.p.a.ce necessarily exhibits a definite aspect with reference to the fixed stars. Its aspect with respect to the earth will be very changeable, because of the rotation and revolution of that body, but its position with respect to constellations will be steady. Hence each meteor swarm, being a steady parallel stream of rushing ma.s.ses, always strikes us from the same point in stellar s.p.a.ce, and by this point (or radiant) it is identified and named.
The paths do not appear to us to be parallel, because of perspective: they seem to radiate and spread in all directions from a fixed centre like spokes, but all these diverging streaks are really parallel lines optically foreshortened by different amounts so as to produce the radiant impression.
The annexed diagram (Fig. 105) clearly ill.u.s.trates the fact that the "radiant" is the vanishing point of a number of parallel lines.
[Ill.u.s.tration: FIG. 106.--Orbit of November meteors.]
This swarm is specially interesting to us from the fact that we cross its...o...b..t every year. Its...o...b..t and the earth"s intersect. Every November we go through it, and hence every November we see a few stragglers of this immense swarm. The swarm itself takes thirty-three years on its revolution round the sun, and hence we only encounter it every thirty-three years.
The swarm is of immense size. In breadth it is such that the earth, flying nineteen miles a second, takes four or five hours to cross it, and this is therefore the time the display lasts. But in length it is far more enormous. The speed with which it travels is twenty-five miles a second, (for its...o...b..t extends as far as Ura.n.u.s, although by no means parabolic), and yet it takes more than a year to pa.s.s. Imagine a procession 200,000 miles broad, every individual rushing along at the rate of twenty-five miles every second, and the whole procession so long that it takes more than a year to pa.s.s. It is like a gigantic shoal of herrings swimming round and round the sun every thirty-three years, and travelling past the earth with that tremendous velocity of twenty-five miles a second. The earth dashes through the swarm and sweeps up myriads. Think of the countless numbers swept up by the whole earth in crossing such a shoal as that! But heaps more remain, and probably the millions which are destroyed every thirty-three years have not yet made any very important difference to the numbers still remaining.
The earth never misses this swarm. Every thirty-three years it is bound to pa.s.s through some part of them, for the shoal is so long that if the head is just missed one November the tail will be encountered next November. This is a plain and obvious result of its enormous length. It may be likened to a two-foot length of sewing silk swimming round and round an oval sixty feet in circ.u.mference. But, you will say, although the numbers are so great that destroying a few millions or so every thirty-three years makes but little difference to them, yet, if this process has been going on from all eternity, they ought to be all swept up. Granted; and no doubt the most ancient swarms have already all or nearly all been swept up.
[Ill.u.s.tration: FIG. 107.--Orbit of November meteors; showing their probable parabolic orbit previous to 126 A.D., and its sudden conversion into an elliptic orbit by the violent perturbation caused by Ura.n.u.s, which at that date occupied the position shown.]
The August meteors, or Perseids, are an example. Every August we cross their path, and we have a small meteoric display radiating from the sword-hand of Perseus, but never specially more in one August than another. It would seem as if the main shoal has disappeared, and nothing is now left but the stragglers; or perhaps it is that the shoal has gradually become uniformly distributed all along the path. Anyhow, these August meteors are reckoned much more ancient members of the solar system than are the November meteors. The November meteors are believed to have entered the solar system in the year 126 A.D.
This may seem an extraordinary statement. It is not final, but it is based on the calculations of Leverrier--confirmed recently by Mr. Adams.
A few moments will suffice to make the grounds of it clear. Leverrier calculated the orbit of the November meteors, and found them to be an oval extending beyond Ura.n.u.s. It was perturbed by the outer planets near which it went, so that in past times it must have moved in a slightly different orbit. Calculating back to their past positions, it was found that in a certain year it must have gone very near to Ura.n.u.s, and that by the perturbation of this planet its path had been completely changed.
Originally it had in all probability been a comet, flying in a parabolic orbit towards the sun like many others. This one, encountering Ura.n.u.s, was pulled to pieces as it were, and its...o...b..t made elliptical as shown in Fig. 107. It was no longer free to escape and go away into the depths of s.p.a.ce: it was enchained and made a member of the solar system. It also ceased to be a comet; it was degraded into a shoal of meteors.
This is believed to be the past history of this splendid swarm. Since its introduction to the solar system it has made 52 revolutions: its next return is due in November, 1899, and I hope that it may occur in the English dusk, and (see Fig. 97) in a cloudless after-midnight sky, as it did in 1866.
NOTES FOR LECTURE XVII
The tide-generating force of one body on another is directly as the ma.s.s of the one body and inversely as the cube of the distance between them.
Hence the moon is more effective in producing terrestrial tides than the sun.
The tidal wave directly produced by the moon in the open ocean is about 5 feet high, that produced by the sun is about 2 feet. Hence the average spring tide is to the average neap as about 7 to 3. The lunar tide varies between apogee and perigee from 43 to 59.
The solar tide varies between aphelion and perihelion from 19 to 21.
Hence the highest spring tide is to the lowest neap as 59 + 21 is to 43 -21, or as 8 to 22.
The semi-synchronous oscillation of the Southern Ocean raises the magnitude of oceanic tides somewhat above these directly generated values.
Oceanic tides are true waves, not currents. Coast tides are currents.
The momentum of the water, when the tidal wave breaks upon a continent and rushes up channels, raises coast tides to a much greater height--in some places up to 50 or 60 feet, or even more.
Early observed connections between moon and tides would be these:--
1st. Spring tides at new and full moon.
2nd. Average interval between tide and tide is half a lunar, not a solar, day--a lunar day being the interval between two successive returns of the moon to the meridian: 24 hours and 50 minutes.
3rd. The tides of a given place at new and full moon occur always at the same time of day whatever the season of the year.
LECTURE XVII
THE TIDES