Part 6

Halley and Newton became life-long friends. Halley had Newton elected to the Royal Society, and when Newton became too poor to pay his quarterly dues, Halley, through his influence with the leading members of the Society, had them remitted. It was Halley who encouraged Newton to put his momentous discovery and elucidation of the forces of gravity into permanent form in his “Principia,” the first volume of which, “De Motu,” was presented to the Royal Society at Halley’s suggestion.

In the proceedings of the Royal Society for December, 1684, there is an entry that “Mr. Halley had lately seen Mr. Newton at Cambridge, who had told him of a curious treatise ‘De Motu,’ which at Mr. Halley’s desire he promised to send to the Society to be entered upon their register. Mr. Halley was desired to put Mr. Newton in mind of his promise for the securing this invention to himself, till such time as he could be at leisure to publish it.”

Early in the following year Newton sent his treatise to the Society, to whom it was read aloud by Halley. This treatise “De Motu” was the germ of the “Principia” and was intended to be a short account of what the greater work was to embrace.

During the next two years Newton was hard at work on his “Principia,” while Halley was equally hard at work on his computations of the Comet of 1682, and on his theory of the orbits and the periodical returns of Comets which grew out of his observations.

On April 21, 1686, Halley read to the Royal Society his own “Discourse Concerning Gravity and its Properties,” in which he stated that his “worthy countryman, Mr. Issac Newton, has an incomparable treatise on Motion almost ready for the press,” and that the law of the inverse square “is the principle on which Mr. Newton has made out all the phenomena of the celestial motions so easily and naturally that its truth is past dispute.”

Shortly afterward Newton sent in the manuscript of his great work. The Society voted “that a letter of thanks be written to Mr. Newton and that the printing of his book be referred to the consideration of the council and that in the meantime the book be put into the hands of Mr. Halley.”

The truth was that the Royal Society, at that time, did not have money enough to print the book. The Society went through the empty form of “ordering” that the book be printed “forthwith,” but no printer was forthcoming until Halley himself undertook the publication of the great work at his own expense.

The delicacy of Halley’s feeling is revealed by his correspondence with Newton, in which he informed Newton that the book had “been ordered to be printed at the Society’s charge.” The preliminary delay about printing he explained to Newton “arose from the President’s attendance on the King, and the absence of the vice-presidents, whom the good weather had drawn out of town.”

Later Newton came to realize how much he owed to Halley in this matter. In his letters to Halley henceforth he always referred to his book as if it had been Halley’s book. When the great work was finished at last Newton wrote to Halley under the date of July 5, 1687: “I have at length brought your book to an end, and hope it will please you.”

The finished work contained a note to this effect: “The inverse law of gravity holds in all the celestial motions, as was discovered also independently by my countrymen Wren, Hooke, and Halley.”

The book was dedicated to the Royal Society, and to it was prefixed a set of Latin hexameters addressed by Halley to the author, ending with the well known line:

“Nec fac est propius mortali attingere divos.”

(“It is not given to a mortal to get in closer touch with the gods.”)

Halley was fifty years old when he made his famous prediction of the return of the Comet of 1682. This was in his “Synopsis of Comet Astronomy,” which ended with these words: “Hence I may venture to foretell that this Comet will return again in the year 1758.”

Besides being an astronomer of the first class, Halley was also a good navigator. In 1698 he was commissioned a captain in the Royal Navy and was put in command of the King’s ship, “The Paramour Pink.” With this vessel he set out on a long cruise to the Pacific for the purpose of making observations on the laws which govern magnetic variations. This task he accomplished in a voyage which lasted two years and extended to the fifty-second degree of southern latitude, when the ice compelled him to turn back. On the return voyage his crew mutinied and his lieutenant sided with the mutineers. Halley quelled the mutiny by sheer force of personality, and returning to England got rid of his lieutenant. The results of his voyage were published in his “General Chart of the Variation of the Compass” in 1701. Immediately afterwards Halley set out on another King’s ship and executed by royal command a careful survey of the tides and coasts of the British Channel, an elaborate chart of which he published in 1702.

Next Halley was sent by the King to Dalmatia, for the purpose of selecting and fortifying the port of Trieste.

On Halley’s return to England, he was made Savilian professor of geometry at Oxford, and received an honorary doctor’s degree. He filled two terms of eight years each as secretary to the Royal Society, and early in 1720 he succeeded Flamsteed as Astronomer Royal.

He died on January 14, 1742, at the age of eighty-five in the full possession of his faculties, the foremost astronomer of the day and a man universally beloved and respected. His gravestone stands at the Greenwich Observatory.

Halley’s works fill several shelves in the library of the Royal Society. His fame is kept green by the periodical return of the wandering star known by his name.

WHAT ARE COMETS?

The modern answer to the question “What are Comets made of?” is this:

Probably the heads are a mixture of solid and gaseous matter. The tails are gaseous—the result of the volatilisation of the solid matter of the heads.

The spectroscope shows that gases appear to be a constituent of all Comets. The spectra of Comets are very similar to those of a Bunsen flame. Recent spectroscopic photographs have revealed the presence of hydrocarbons, nitro-carbons, of cyanogen and of the vapours of sodium, iron and other metals.

The connection between Comets and Meteors implies the presence in Comets of solid matter. A modern theory, voiced by Schiaparelli, is that meteor showers are broken up Comets.

The tails of Comets appear to be composed of luminous gases ejected from the head of the Comet through a solar force held to be “Light Pressure,” which causes these tails to shoot off and disperse into space at the rate of 865,000 miles an hour.

The length of some Comets’ tails has been estimated at 125,000,000 miles, while the Comets’ heads themselves are generally much larger in size than our Earth. Halley’s Comet is more than ten-fold the size of our Earth.

E. W. Maunder, of the Royal Observatory of Greenwich, a modern astronomer, has thus summarized the latest theories of the substance of Comets:

“Though the bulk of comets is huge, they contain extraordinarily little substance. Their heads must contain some solid matter, but it is probably in the form of a loose aggregation of stones enveloped in vaporous material. There is some reason to suppose that Comets are apt to shed some of these stones as they travel along their paths, for the orbits of the meteors that cause some of our greatest ‘star showers’ are coincident with the paths of Comets that have been observed. But it is not only by shedding its loose stones that a Comet diminishes its bulk; it loses also through its tail. As the Comet gets close to the Sun its head becomes heated, and throws off concentric envelopes, much of which consists of matter in an extremely fine station of division.”

The orbits of Comets visible to human eyes are all governed by the Sun. In the words of C. L. Poor: “The attraction of the Sun is to the Comet like the flame to the moth. The Comet flutters for a moment about the Sun, and then swings back into outward space. But not unscathed; like the moth, the Comet has been singed. The fierce light of the Sun has beaten upon it, and spread out its particles and scattered them along its path.”

As a comet swings toward and away from the Sun, it travels at a tremendous rate of speed—over a million miles an hour. The distance covered from one end of the orbit to the other is 3,370,000,000 miles.

The great majority of Comets appear to travel in parabolas, open curves leading from infinite space to and around the Sun, and thence back into infinite space to some other fixed star invisible to us. As a matter of fact, though, the parabolic curves of Comets’ orbits through the gravitational attraction of the planets, whose orbits are crossed by it, may be changed into hyperbolic curves and ellipses by planetary perturbations. Hence the differences in time between the returns of certain Comets, like Halley’s, for instance.

In a general way, it may be said that every Comet comprises a nucleus, an envelope (called the “coma”) surrounding the nucleus and measuring from 20,000 to 1,000,000 miles in diameter, and a long tail which streams behind the nucleus from sixty to a hundred million miles or more.

Astronomers have decided that the nucleus is probably a heap of meteorites varying in size from a grain to masses weighing several tons each; a heap, moreover, so easily sundered that its elements are distributed gradually along the orbit. It follows that every Comet must eventually perish unless it restores its nucleus by collecting stray meteors. That disintegration does occur has been observed time and time again.

For example, Biela’s Comet, which was discovered in 1826, burst into two fragments, which drifted apart a distance of one million miles. Thus it became a twin Comet. Eventually it disappeared as a Comet, and in its stead we see a shoal of meteors whenever we cross its track every six and a half years.

It is possible that the Comets of 1668, 1843, 1880, 1882 and 1887, all travelling in approximately the same path, are fragments of a single large body which was broken up by the gravitational action of other bodies in the system, or through violent encounter with the Sun’s surroundings.

The luminous tail which streams behind the nucleus, which Shakespeare described so beautifully as “crystal tresses,” is startling, to say the least. Despite a length which may exceed a hundred million miles, it is so diaphanously light and subtle that it is difficult to compare it with any earthly fabric. The air that we breathe is a dense blanket in comparison. Several hundred cubic miles of the matter composing that wonderful luminous plume would not outweigh a jarful of air. By reason of its fairy lightness, it is possible for a tail occupying a volume thousands of times greater than the sun to sweep through our solar system without causing any perturbations in planetary movements.

No celestial phenomenon has caused more perplexity than the ghostly sheaf of light we call a Comet’s tail. In a day, in a few hours even, the form of that wonderful gossamer may change. Hence it is that periodic Comets are identified when they return, not by the length and arch of their tails, but by their orbits. These alone are permanent.

When a Comet is first seen in the telescope, it appears as a diminutive filmy patch, often unadorned by any tail. As it travels on toward the Sun, at a speed compared with which a modern rifle bullet would seem to crawl, violent eruptions occur in the nucleus.

The ejected matter is bent back to form the cloak called the “coma.” With a nearer approach to the sun, the tail begins to sprout, increasing in size and brightness as it proceeds. Evidently there is some connection between the Sun and the tail, something akin to cause and effect.

When the Comet rushes on toward the Sun, invariably the tail drifts behind the nucleus like the smoke from a locomotive. But when the Comet swings around the Sun and travels away from it, a startling change takes place. The tail no longer trails behind, but projects in front as if some mighty solar wind were blowing it in advance of the head.

This phenomenon has long been an astronomical riddle. Here was a kind of matter that refused to obey the laws of gravitation and yield to the enormous pull of the Sun.

It was thought for a time that the tail was flung away from the Sun by stupendous repelling electrical forces. That electricity plays its part in the formation of the fairy plume is conceivable, and even probable; but recently the physicist has discovered a new source of repellent energy which very plausibly explains the mystery of a Comet’s tail.

This new source of energy is nothing less than the pressure or push of the Sun’s light. Solar gravitation is a force more powerful than we can realize. If it were possible for us to live on the Sun, we would find ourselves pulled down so violently that our body would weigh two tons. Our clothing alone would weigh more than one hundred pounds. Running would be a very difficult athletic feat. Light-pressure must indeed be powerful if it can conquer so relentless a force.

Because we have never seen objects torn from our hands by the pressure of light, it may be inferred that this newly discovered force affects only bodies that are invisibly small. With the aid of instruments that feel what our hands can never feel and see what our eyes can never see, the modern physicist has critically analyzed the radiation that beats upon the earth from the distant Sun.

Light really does sway infinitely small particles, as was first experimentally proved by the Russian Lebedev. Two American astronomers, Nichols and Hull, improved upon his method. They cast the solar effulgence into mighty mathematical scales and found that the earth sustains a light-load of no less than 75,000 tons.

Most city-bred people are familiar with the so-called “Sun Motors”—little mills with black and white wings, enclosed in airtight vessels, which spin around in “perpetual motion” under the effect of “Sun Pressure.”

It remained for the broad mind of a Swedish physicist, Svante Arrhenius, to apply the principle of light-pressure cosmically. He explained, very simply, that because a Comet’s tail is composed of a very fine dust it can easily be driven away from the Sun by radiation pressure.

To understand how it is possible for so immaterial a thing as a sunbeam to produce so huge an effect, we have only to take a very simple example.

Assume that you have before you a block of wood weighing one pound. The block exposes a certain amount of surface to the Sun’s light. Saw the block in half, and you increase the amount of that surface. Divide each half again into half, and the exposed surface is further augmented. If this process of subdivision is carried on far enough, the block will be reduced to sawdust.

The entire mass of sawdust still weighs one pound; but its surface has been vastly enlarged. Indeed, the particles of sawdust, individually considered, may be said to consist of much surface and very little weight. If it were possible to take each granule of visible sawdust and subdivide it into invisible particles, a point would be reached where the pressure of light would exactly counterbalance the pull of gravitation, so that the particles would remain suspended in space, perfectly balanced in the scale of opposing cosmic forces.

Finally, if the subdivision be continued beyond this critical point, the particles will be wrenched away from the grip of gravitation and hurled out into space by the pressure of light.

So much has been discovered about the particles that compose a Comet’s tail that the more progressive scientists of our day have accepted this ingenious theory. Thus it has been decided by them that the delicate tresses of a Comet are to a large extent composed of fine particles of dust and soot.

Before we can completely accept the view that light-pressure forms this train of soot we must ascertain whether the pressure of light is capable of accounting for the flash-like rapidity with which a Comet’s tail changes.

A Comet may throw out a tail sixty million miles long in two days. Is it actually possible for light-pressure to accomplish that astonishing feat? Arrhenius has computed that 865,000 miles an hour is the speed of a light-flung particle of one-half the critical diameter. Because they are only one-eighteenth as large as this particle of critical diameter, the dust grains in a Comet’s tail would be propelled over the same 865,000 miles in less than four minutes. It follows that the solar radiation is amply strong enough to toss out a tail of sixty million miles in two days.

Photography in the hands of Prof. E. E. Barnard, of the Yerkes Observatory, has revealed some extraordinary changes in Comets’ tails, changes which are not apparent to the eye and which cannot be explained by light-pressure or by solar electrical forces. He has collected a formidable mass of photographic evidence which seems to show that there are other influences at work besides the Sun’s radiation, and that these influences manifest themselves in distorting and breaking a Comet’s tail. In some Comets of recent years, streams of matter have been shot out in large angles to the main direction of the tail without being at all bent by the pressure of light. In Morehouse’s Comet of 1908, tails were repeatedly formed and discarded to drift bodily out into space and melt away. Sometimes the photographic plate has shown the tail twisted like a corkscrew and sometimes it has revealed masses of matter at some distance from the head, where apparently no supply had reached it. At one time the entire tail of Morehouse’s Comet was thrown violently forward, a peculiarity so utterly opposed to the laws of gravitation that Professor Barnard suspects some unknown force at work in planetary space besides a force which undoubtedly resides in the Comet itself. If Halley’s Comet serves no other purpose than to throw light upon this mystery, its return will more than repay astronomers for all their observatory vigils.

From the fact that the matter is ejected from the head to form the tail, it would follow that, unless it has the means of rejuvenating itself, a comet must eventually be disintegrated. Instances of this fragmentation and, eventual disappearance of a Comet are not wanting in astronomical annals. It has been stated previously that when Biela’s Comet appeared in 1846 it became distorted and elongated, that it eventually split up into two separate bodies, that in 1852 it again appeared in its double form, and that it has since disappeared.

In a way, Comets may be said to bleed to death. At each return of Halley’s Comet, future astronomers will find it less brilliant than it was seventy-six or seventy-seven years before. Some time there will be no Halley’s Comet left, and the most famous Comet of its kind will be reduced to a shoal of meteors varying in weight from a few ounces to several tons and faithfully pursuing the orbit which their parent traced and retraced century after century.

THE PERIL OF THE COMET

It was Edmund Halley who first revealed a source of danger from Comets, of which even medieval superstition had never dreamed.

While he was patiently plotting out the orbit of the Comet of 1680, which had inspired no little dismay among his contemporaries, Halley found that the Earth’s orbit had been approached by the Comet within four thousand miles—half the diameter of the Earth.

If the Earth had been struck by that fiery wanderer?

None had ever thought of the possibility.

Halley began to do some mathematical figuring, and decided that, if a Comet’s mass were comparable with that of the Earth, our year would have been changed in length because the Earth’s orbit would have been altered. He also speculated what would happen to the Earth, and reached this conclusion:

“If so large a body with so rapid a motion were to strike the Earth—a thing by no means impossible—the shock might reduce this beautiful world to its original chaos.”

Halley even thought it probable that the Earth had actually been struck by a Comet at some remote period, struck obliquely, moreover, so that the axis of rotation had been changed. Thus he was led to infer that possibly the North Pole had once been at a point near Hudson’s Bay, and that the rigour of North America’s climate might thus be accounted for.

The seed which was thus sown by Halley has borne fruit. In Halley’s own time, learned men were brooding over the ultimate destruction of the Earth by collision with a Comet.

Dr. Whiston, who succeeded Newton at Cambridge in the Lucasian chair of mathematics, was sure that a Comet caused the Deluge, and went so far as to prophesy that a Comet, as it passed us on its outward course from the Sun, would ultimately bring about a “General Conflagration,” and thus envelope the Earth in flames.

One century after Halley, the French astronomer Laplace, whose mathematical attainments were surpassed only by those of Newton, applied his brilliant mind to the possibility of a collision with a Comet, and arrived at this conclusion:

“The seas would abandon their ancient beds and rush towards the new equator, drowning in one universal deluge the greater part of the human race.... We see, then, in effect, why the ocean has receded from the high lands upon which we find incontestable marks of its sojourn; we see how the animals and plants of the south have been able to exist in the climate of the north, where their remains and imprints have been discovered.”

The famous French mathematician Lalande showed that if a Comet as heavy as the Earth were to come within six times the distance of the Moon, it would exert such a powerful attraction upon the waters of the globe as to pull up a tidal wave 13,000 feet above the ordinary sea-level and inundate the continents Every European mountain would be submerged except Mt. Blanc, and only the inhabitants of the Rockies, the Andes and the Himalayas would escape death.

Since Lalande’s day there has been more than one Comet “scare.” One of these startled Europe in 1832. On October 29th of that year, Biela’s Comet crossed the Earth’s orbit. The announcement was received with stupefaction. It was only when Arago soothingly pointed out that the Earth would not reach the exact point where the Comet had intersected the Earth’s orbit until November 30, at which time the Comet would be 50,000,000 miles away, that the popular excitement subsided. A similar alarm seized the world in 1857. Some prophet declared that on June 13 the world would collide with a certain periodic Comet having a period of revolution of three centuries. It is related that the churches and confessionals were crowded for days. Still another prediction, made in 1872 by Plantamour, the distinguished director of the Geneva Observatory, set Europe in a ferment. His calculations were based on errors, which were pointed out by other astronomers, and the public mind was quieted.

Although more than two centuries have passed since Halley was in his prime, the possibility of a collision with some vagabond star still haunts the mind of the astronomer.