Great Astronomers - Robert Stawell Ball (the little red hen ebook .TXT) 📗

test of experiment, and for that purpose Halley

determined to observe the magnetic variation for himself. He

procured from King William III. the command of a vessel called the

“Paramour Pink,” with which he started for the South Seas in 1694.

This particular enterprise was not, however, successful; for, on

crossing the line, some of his men fell sick and one of his

lieutenants mutinied, so that he was obliged to return the

following year with his mission unaccomplished. The government

cashiered the lieutenant, and Halley having procured a second

smaller vessel to accompany the “Paramour Pink,” started once more

in September, 1699. He traversed the Atlantic to the 52nd degree

of southern latitude, beyond which his further advance was

stopped. “In these latitudes,” he writes to say, “we fell in with

great islands of ice of so incredible height and magnitude, that I

scarce dare write my thoughts of it.”

 

On his return in 1700, Halley published a general chart, showing

the variation of the compass at the different places which he had

visited. On these charts he set down lines connecting those

localities at which the magnetic variation was identical. He

thus set an example of the graphic representation of large masses

of complex facts, in such a manner as to appeal at once to the

eye, a method of which we make many applications in the present

day.

 

But probably the greatest service which Halley ever rendered to

human knowledge was the share in which he took in bringing

Newton’s “Principia” before the world. In fact, as Dr. Glaisher,

writing in 1888, has truly remarked, “but for Halley the

‘Principia’ would not have existed.”

 

It was a visit from Halley in the year 1684 which seems to have

first suggested to Newton the idea of publishing the results of

his investigations on gravitation. Halley, and other scientific

contemporaries, had no doubt some faint glimmering of the great

truth which only Newton’s genius was able fully to reveal. Halley

had indeed shown how, on the assumptions that the planets move in

circular orbits round the sun, and that the squares of their

periodic times are proportional to the cubes of their mean

distances, it may be proved that the force acting on each planet

must vary inversely as the square of its distance from the sun.

Since, however, each of the planets actually moves in an ellipse,

and therefore, at continually varying distances from the sun, it

becomes a much more difficult matter to account mathematically for

the body’s motions on the supposition that the attractive force

varies inversely as the square of the distance. This was the

question with which Halley found himself confronted, but which his

mathematical abilities were not adequate to solve. It would seem

that both Hooke and Sir Christopher Wren were interested in

the same problem; in fact, the former claimed to have arrived

at a solution, but declined to make known his results, giving as

an excuse his desire that others having tried and failed might

learn to value his achievements all the more. Halley, however,

confessed that his attempts at the solution were unsuccessful, and

Wren, in order to encourage the other two philosophers to pursue

the inquiry, offered to present a book of forty shillings value to

either of them who should in the space of two months bring him a

convincing proof of it. Such was the value which Sir Christopher

set on the Law of Gravitation, upon which the whole fabric of

modern astronomy may be said to stand.

 

Finding himself unequal to the task, Halley went down to Cambridge

to see Newton on the subject, and was delighted to learn that the

great mathematician had already completed the investigation. He

showed Halley that the motions of all the planets could be

completely accounted for on the hypothesis of a force of

attraction directed towards the sun, which varies inversely as the

square of the distance from that body.

 

Halley had the genius to perceive the tremendous importance of

Newton’s researches, and he ceased not to urge upon the recluse

man of science the necessity for giving his new discoveries

publication. He paid another visit to Cambridge with the object

of learning more with regard to the mathematical methods which

had already conducted Newton to such sublime truths, and he again

encouraged the latter both to pursue his investigations, and to

give some account of them to the world. In December of the same

year Halley had the gratification of announcing to the Royal

Society that Newton had promised to send that body a paper

containing his researches on Gravitation.

 

It seems that at this epoch the finances of the Royal Society

were at a very low ebb. This impecuniosity was due to the fact

that a book by Willoughby, entitled “De Historia Piscium,” had

been recently printed by the society at great expense. In fact,

the coffers were so low that they had some difficulty in paying

the salaries of their permanent officials. It appears that the

public did not care about the history of fishes, or at all events

the volume did not meet with the ready demand which was expected

for it. Indeed, it has been recorded that when Halley had

undertaken to measure the length of a degree of the earth’s

surface, at the request of the Royal Society, it was ordered that

his expenses be defrayed either in 50 pounds sterling, or in fifty

books of fishes. Thus it happened that On June 2nd, the Council, after

due consideration of ways and means in connection with the issue

of the Principia,” ordered that Halley should undertake the

business of looking after the book and printing it at his own

charge,” which he engaged to do.

 

It was, as we have elsewhere mentioned, characteristic of Newton

that he detested controversies, and he was, in fact, inclined to

suppress the third book of the “Principia” altogether rather than

have any conflict with Hooke with respect to the discoveries there

enunciated. He also thought of changing the name of the work to

De Motu Corporum Libri Duo, but upon second thoughts, he

retained the original title, remarking, as he wrote to Halley, “It

will help the sale of the book, which I ought not to diminish, now

it is yours,” a sentence which shows conclusively, if further

proof were necessary, that Halley had assumed the responsibility

of its publication.

 

Halley spared no pains in pushing forward the publication of

his illustrious friend’s great work, so that in the same year he

was in a position to present a complete copy to King James II.,

with a proper discourse of his own. Halley also wrote a set of

Latin hexameters in praise of Newton’s genius, which he printed at

the beginning of the work. The last line of this specimen of

Halley’s poetic muse may be thus rendered: Nor mortals nearer may

approach the gods.”

 

The intimate friendship between the two greatest astronomers of

the time continued without interruption till the death of Newton.

It has, indeed, been alleged that some serious cause of

estrangement arose between them. There is, however, no

satisfactory ground for this statement; indeed, it may be regarded

as effectually disposed of by the fact that, in the year 1727,

Halley took up the defence of his friend, and wrote two learned

papers in support of Newton’s “System of Chronology,” which had

been seriously attacked by a certain ecclesiastic. It is quite

evident to any one who has studied these papers that Halley’s

friendship for Newton was as ardent as ever.

 

The generous zeal with which Halley adopted and defended the

doctrines of Newton with regard to the movements of the celestial

bodies was presently rewarded by a brilliant discovery, which has

more than any of his other researches rendered his name a familiar

one to astronomers. Newton, having explained the movement of the

planets, was naturally led to turn his attention to comets. He

perceived that their journeyings could be completely accounted for

as consequences of the attraction of the sun, and he laid down the

principles by which the orbit of a comet could be determined,

provided that observations of its positions were obtained at three

different dates. The importance of these principles was by no

one more quickly recognised than by Halley, who saw at once that

it provided the means of detecting something like order in the

movements of these strange wanderers. The doctrine of Gravitation

seemed to show that just as the planets revolved around the sun in

ellipses, so also must the comets. The orbit, however, in the

case of the comet, is so extremely elongated that the very small

part of the elliptic path within which the comet is both near

enough and bright enough to be seen from the earth, is

indistinguishable from a parabola. Applying these principles,

Halley thought it would be instructive to study the movements of

certain bright comets, concerning which reliable observations

could be obtained. At the expense of much labour, he laid down

the paths pursued by twenty-four of these bodies, which had

appeared between the years 1337 and 1698. Amongst them he noticed

three, which followed tracks so closely resembling each other,

that he was led to conclude the so called three comets could only

have been three different appearances of the same body. The first

of these occurred in 1531, the second was seen by Kepler in 1607,

and the third by Halley himself in 1682. These dates suggested

that the observed phenomena might be due to the successive returns

of one and the same comet after intervals of seventy-five or

seventy-six years. On the further examination of ancient records,

Halley found that a comet had been seen in the year 1456, a date,

it will be observed, seventy-five years before 1531. Another had

been observed seventy-six years earlier than 1456, viz., in 1380,

and another seventy-five years before that, in 1305.

 

As Halley thus found that a comet had been recorded on several

occasions at intervals of seventy-five or seventy-six years, he

was led to the conclusion that these several apparitions related

to one and the same object, which was an obedient vassal of the

sun, performing an eccentric journey round that luminary in a

period of seventy-five or seventy-six years. To realise the

importance of this discovery, it should be remembered that before

Halley’s time a comet, if not regarded merely as a sign of divine

displeasure, or as an omen of intending disaster, had at least

been regarded as a chance visitor to the solar system, arriving no

one knew whence, and going no one knew whither.

 

A supreme test remained to be applied to Halley’s theory. The

question arose as to the date at which this comet would be seen

again. We must observe that the question was complicated by the

fact that the body, in the course of its voyage around the sun,

was exposed to the incessant disturbing action produced by the

attraction of the several planets. The comet therefore, does not

describe a simple ellipse as it would do if the attraction of the

sun were the only force by which its movement were controlled.

Each of the planets solicits the comet to depart from its track,

and though the amount of these attractions may be insignificant

in comparison with the supreme controlling force of the sun, yet

the departure from the ellipse is quite sufficient to produce

appreciable irregularities in the comet’s movement. At the time

when Halley lived, no means existed of calculating with precision

the effect of the disturbance a comet might experience from the

action of the different planets. Halley exhibited his usual

astronomical sagacity in deciding that Jupiter would retard the

return of the comet to some extent. Had it not been for this

disturbance the comet would apparently have been due in 1757