Great Astronomers by Robert Stawell Ball (best ereader for students .txt) 📖
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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
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