services to humanity by their investigations, and Halley's
achievements in this respect deserve to be noted. A few years after
he had settled in England, he published an important paper on the
variation of the magnetic compass, for so the departure of the needle
from the true north is termed. This subject had indeed early engaged
his attention, and he continued to feel much interest in it up to the
end of his life. With respect to his labours in this direction, Sir
John Herschel says: "To Halley we owe the first appreciation of the
real complexity of the subject of magnetism. It is wonderful indeed,
and a striking proof of the penetration and sagacity of this
extraordinary man, that with his means of information he should have
been able to draw such conclusions, and to take so large and
comprehensive a view of the subject as he appears to have done." In
1692, Halley explained his theory of terrestrial magnetism, and
begged captains of ships to take observations of the variations of
the compass in all parts of the world, and to communicate them to the
Royal Society, "in order that all the facts may be readily available
to those who are hereafter to complete this difficult and complicated
subject."
The extent to which Halley was in advance of his contemporaries, in
the study of terrestrial magnetism, may be judged from the fact that
the subject was scarcely touched after his time till the year 1811.
The interest which he felt in it was not of a merely theoretical
kind, nor was it one which could be cultivated in an easy-chair. Like
all true investigators, he longed to submit his theory to the 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 or early in 1758. But the attraction of the great planet
would cause delay, so that Halley assigned, for the date of its
re-appearance, either the end of 1758 or the beginning of 1759.
Halley knew that he could not himself live to witness the fulfilment
of his prediction, but he says: "If it should return, according to
our predictions, about the year 1758, impartial posterity will not
refuse to acknowledge that this was first discovered by an
Englishman." This was, indeed, a remarkable prediction of an event
to occur fifty-three years after it had been uttered. The way in
which it was fulfilled forms one of the most striking episodes in the
history of astronomy. The comet was first seen on Christmas Day,
1758, and passed through its nearest point to the sun on March 13th,
1759. Halley had then been lying in his grave for seventeen years,
yet the verification of his prophecy reflects a glory on his name
which will cause it to live for ever in the annals of astronomy. The
comet paid a subsequent visit in 1835, and its next appearance is due
about 1910.
Halley next entered upon a labour which, if less striking to the
imagination than his discoveries with regard to comets, is still of
inestimable value in astronomy. He undertook a series of
investigations with the object of improving our knowledge of the
movements of the planets. This task was practically finished in
1719, though the results of it were not published until after his
death in 1749. In the course of it he was led to investigate closely
the motion of Venus, and thus he came to recognise for the first time
the peculiar importance which attaches to the phenomenon of the
transit of this planet across the sun. Halley saw that the transit,
which was to take place in the year 1761, would afford a favourable
opportunity for determining the distance of the sun, and thus
learning the scale of the solar system. He predicted the
circumstances of the phenomenon with an astonishing degree of
accuracy, considering his means of information, and it is
unquestionably to the exertions of Halley in urging the importance of
the matter upon astronomers that we owe the unexampled degree of
interest taken in the event, and the energy which scientific men
exhibited in observing it. The illustrious astronomer had no hope of
being himself a witness of the event, for it could not happen till
many years after his death. This did not, however, diminish his
anxiety to impress upon those who would then be alive, the importance
of the occurrence, nor did it lead him to neglect anything which
might contribute to the success of the observations. As we now know,
Halley rather over-estimated the value of the transit of Venus, as a
means of determining the solar distance. The fact is that the
circumstances are such that the observation of the time of contact
between the edge of the planet and the edge of the sun cannot be made
with the accuracy which he had expected.
In 1691, Halley became a candidate for the Savilian Professorship of
Astronomy at Oxford. He was not, however, successful, for his
candidature was opposed by Flamsteed, the Astronomer Royal of the
time, and another was appointed. He received some consolation for
this particular disappointment by the fact that, in 1696, owing to
Newton's friendly influence, he was appointed deputy Controller of
the Mint at Chester, an office which he did not retain for long, as
it was abolished two years later. At last, in 1703, he received what
he had before vainly sought, and he was appointed to the Savilian
chair.
His observations of the eclipse of the sun, which occurred in 1715,
added greatly to Halley's reputation. This phenomenon excited
special attention, inasmuch as it was the first total eclipse of the
sun which had been visible in London since the year 1140. Halley
undertook the necessary calculations, and predicted the various
circumstances with a far higher degree of precision than the official
announcement. He himself observed the phenomenon from the Royal
Society's rooms, and he minutely describes the outer atmosphere of
the sun, now known as the corona; without, however, offering an
opinion as to whether it was a solar or a lunar appendage.
At last Halley was called to the dignified office which he of all men
was most competent to fill. On February 9th, 1720, he was appointed
Astronomer Royal in succession to Flamsteed. He found things at the
Royal Observatory in a most unsatisfactory state. Indeed, there were
no instruments, nor anything else that was movable; for such things,
being the property of Flamsteed, had been removed by his widow, and
though Halley attempted to purchase from that lady some of the
instruments which his predecessor had employed, the unhappy personal
differences which had existed between him and Flamsteed, and which,
as we have already seen, prevented his election as Savilian Professor
of Astronomy, proved a bar to the negotiation. Greenwich Observatory
wore a very different appearance in those days, from that which the
modern visitor, who is fortunate enough to gain admission, may now
behold. Not only did Halley find it bereft of instruments, we learn
besides that he had no assistants, and was obliged to transact the
whole business of the establishment single-handed.
In 1721, however, he obtained a grant of 500 pounds from the Board of
Ordnance, and accordingly a transit instrument was erected in the
same year. Some time afterwards he procured an eight-foot quadrant,
and with these instruments, at the age of sixty-four, he commenced a
series of observations on the moon. He intended, if his life was
spared, to continue his observations for a period of eighteen years,
this being, as astronomers know, a very important cycle in connection
with lunar movements. The special object of this vast undertaking
was to improve the theory of the moon's motion, so that it might
serve more accurately to determine longitudes at sea. This
self-imposed task Halley lived to carry to a successful termination,
and the tables deduced from his observations, and published after his
death, were adopted almost universally by astronomers, those of the
French nation being the only exception.
Throughout his life Halley had been singularly free from illness of
every kind, but in 1737 he had a stroke of paralysis. Notwithstanding
this, however, he worked diligently at his telescope till 1739, after
which his health began rapidly to give way. He died on January 14th,
1742, in the eighty-sixth year of his age, retaining his mental
faculties to the end. He was buried in the cemetery of the church of
Lee in Kent, in the same grave as his wife, who had died five years
previously. We are informed by Admiral Smyth that Pond, a later
Astronomer Royal, was afterwards laid in the same tomb.
Halley's disposition seems to have been generous and candid, and
wholly free from anything like jealousy or rancour. In person he was
rather above the middle height, and slight in build; his complexion
was fair, and he is said to have always spoken, as well as acted,
with uncommon sprightliness. In the eloge pronounced upon him at the
Paris Academie Des Sciences, of which Halley had been made a member
in 1719 it was said, "he possessed all the qualifications which were
necessary to please princes who were desirous of instruction, with a
great extent of knowledge and a constant presence of mind; his
answers were ready, and at the same time pertinent, judicious, polite
and sincere."
[PLATE: GREENWICH OBSERVATORY IN HALLEY'S TIME.]
Thus we find that Peter the Great was one of his most ardent
admirers. He consulted the astronomer on matters connected with
shipbuilding, and invited him to his own table. But Halley possessed
nobler qualifications than the capacity of pleasing Princes. He was
able to excite and to retain the love and admiration of his equals.
This was due to the warmth of his attachments, the unselfishness of
his devotion to his friends, and to a vein of gaiety and good-humour
which pervaded all his conversation.
BRADLEY.
James Bradley was descended from an ancient family in the county of
Durham. He was born in 1692 or 1693, at Sherbourne, in
Gloucestershire, and was educated in the Grammar School at
Northleach. From thence he proceeded in due course to Oxford, where
he was admitted a commoner at Balliol College, on March 15th, 1711.
Much of his time, while an undergraduate, was passed in Essex with
his maternal uncle, the Rev. James Pound, who was a well-known man of
science and a diligent observer of the stars. It was doubtless by
intercourse with his uncle that young Bradley became so expert in the
use of astronomical instruments, but the immortal discoveries he
subsequently made show him to have been a born astronomer.
The first exhibition of Bradley's practical skill seems to be
contained in two observations which he made in 1717 and 1718. They
have been published by Halley, whose acuteness had led him to
perceive the extraordinary scientific talents of the young
astronomer. Another illustration of the sagacity which Bradley
manifested, even at the very commencement of his astronomical career,
is contained in a remark of Halley's, who says: "Dr. Pound and his
nephew, Mr. Bradley, did, myself being present, in the last
opposition of the sun and Mars this way demonstrate the extreme
minuteness of the sun's parallax, and that it was not more than
twelve seconds nor less than nine seconds." To make the significance
of this plain, it should be observed that the determination of the
sun's parallax is equivalent to the determination of the distance
from the earth to the sun. At the time of which we are now writing,
this very important unit of celestial measurement was only very
imperfectly known, and the observations of Pound and Bradley may be
interpreted to mean that, from their observations, they had come to
the conclusion that the distance from the earth to the sun must be
more than 94 millions of miles, and less than 125 millions. We now,
of course, know that they were not exactly right, for the true
distance of the sun is about 93 millions of miles. We cannot,
however, but think that it was a very remarkable approach for the
veteran astronomer and his brilliant nephew to make towards the
determination of a magnitude which did not become accurately known
till fifty years later.
Among the earliest parts of astronomical work to which Bradley's
attention was directed, were the eclipses of Jupiter's satellites.
These phenomena are specially attractive inasmuch as they can be so
readily observed, and Bradley found it extremely interesting to
calculate the times at which the eclipses should take place, and then
to compare his observations with the predicted times. From the
success that he met with in this work, and from his other labours,
Bradley's reputation as an astronomer increased so greatly that on
November the 6th, 1718, he was elected a Fellow of the Royal Society.