more those of an amateur than of a professional astronomer, and as it
did not at first seem likely that scientific work would lead to any
permanent provision, it became necessary for the youthful astronomer
to choose a profession. It had been all along intended that he
should enter the Church, though for some reason which is not told us,
he did not take orders as soon as his age would have entitled him to
do so. In 1719, however, the Bishop of Hereford offered Bradley the
Vicarage of Bridstow, near Ross, in Monmouthshire, and on July 25th,
1720, he having then taken priest's orders, was duly instituted in
his vicarage. In the beginning of the next year, Bradley had some
addition to his income from the proceeds of a Welsh living, which,
being a sinecure, he was able to hold with his appointment at
Bridstow. It appears, however, that his clerical occupations were
not very exacting in their demands upon his time, for he was still
able to pay long and often-repeated visits to his uncle at
Wandsworth, who, being himself a clergyman, seems to have received
occasional assistance in his ministerial duties from his astronomical
nephew.
The time, however, soon arrived when Bradley was able to make a
choice between continuing to exercise his profession as a divine, or
devoting himself to a scientific career. The Savilian Professorship
of Astronomy in the University of Oxford became vacant by the death
of Dr. John Keill. The statutes forbade that the Savilian Professor
should also hold a clerical appointment, and Mr. Pound would
certainly have been elected to the professorship had he consented to
surrender his preferments in the Church. But Pound was unwilling to
sacrifice his clerical position, and though two or three other
candidates appeared in the field, yet the talents of Bradley were so
conspicuous that he was duly elected, his willingness to resign the
clerical profession having been first ascertained.
There can be no doubt that, with such influential friends as Bradley
possessed, he would have made great advances had he adhered to his
profession as a divine. Bishop Hoadly, indeed, with other marks of
favour, had already made the astronomer his chaplain. The engrossing
nature of Bradley's interest in astronomy decided him, however, to
sacrifice all other prospects in comparison with the opening afforded
by the Savilian Professorship. It was not that Bradley found himself
devoid of interest in clerical matters, but he felt that the true
scope for such abilities as he possessed would be better found in the
discharge of the scientific duties of the Oxford chair than in the
spiritual charge of a parish. On April the 26th, 1722, Bradley read
his inaugural lecture in that new position on which he was destined
to confer such lustre.
It must, of course, be remembered that in those early days the art of
constructing the astronomical telescope was very imperfectly
understood. The only known method for getting over the peculiar
difficulties presented in the construction of the refracting
telescope, was to have it of the most portentous length. In fact,
Bradley made several of his observations with an instrument of two
hundred and twelve feet focus. In such a case, no tube could be
used, and the object glass was merely fixed at the top of a high
pole. Notwithstanding the inconvenience and awkwardness of such an
instrument, Bradley by its means succeeded in making many careful
measurements. He observed, for example, the transit of Mercury over
the sun's disc, on October 9th, 1723; he also observed the dimensions
of the planet Venus, while a comet which Halley discovered on October
the 9th, 1723, was assiduously observed at Wanstead up to the middle
of the ensuing month. The first of Bradley's remarkable
contributions to the "Philosophical Transactions" relates to this
comet, and the extraordinary amount of work that he went through in
connection therewith may be seen from an examination of his book of
Calculations which is still extant.
The time was now approaching when Bradley was to make the first of
those two great discoveries by which his name has acquired a lustre
that has placed him in the very foremost rank of astronomical
discoverers. As has been often the case in the history of science,
the first of these great successes was attained while he was pursuing
a research intended for a wholly different purpose. It had long been
recognised that as the earth describes a vast orbit, nearly two
hundred million miles in diameter, in its annual journey round the
sun, the apparent places of the stars should alter, to some extent,
in correspondence with the changes in the earth's position. The
nearer the star the greater the shift in its apparent place on the
heavens, which must arise from the fact that it was seen from
different positions in the earth's orbit. It had been pointed out
that these apparent changes in the places of the stars, due to the
movement of the earth, would provide the means of measuring the
distances of the stars. As, however, these distances are enormously
great in comparison with the orbit which the earth describes around
the sun, the attempt to determine the distances of the stars by the
shift in their positions had hitherto proved ineffectual. Bradley
determined to enter on this research once again; he thought that by
using instruments of greater power, and by making measurements of
increased delicacy, he would be able to perceive and to measure
displacements which had proved so small as to elude the skill of the
other astronomers who had previously made efforts in the same
direction. In order to simplify the investigation as much as
possible, Bradley devoted his attention to one particular star, Beta
Draconis, which happened to pass near his zenith. The object of
choosing a star in this position was to avoid the difficulties which
would be introduced by refraction had the star occupied any other
place in the heavens than that directly overhead.
We are still able to identify the very spot on which the telescope
stood which was used in this memorable research. It was erected at
the house then occupied by Molyneux, on the western extremity of Kew
Green. The focal length was 24 feet 3 inches, and the eye-glass was
3 and a half feet above the ground floor. The instrument was first
set up on November 26th, 1725. If there had been any appreciable
disturbance in the place of Beta Draconis in consequence of the
movement of the earth around the sun, the star must appear to have
the smallest latitude when in conjunction with the sun, and the
greatest when in opposition. The star passed the meridian at noon in
December, and its position was particularly noticed by Molyneux on
the third of that month. Any perceptible displacement by
parallax--for so the apparent change in position, due to the earth's
motion, is called--would would have made the star shift towards the
north. Bradley, however, when observing it on the 17th, was
surprised to find that the apparent place of the star, so far from
shifting towards the north, as they had perhaps hoped it would, was
found to lie a little more to the south than when it was observed
before. He took extreme care to be sure that there was no mistake in
his observation, and, true astronomer as he was, he scrutinized with
the utmost minuteness all the circumstances of the adjustment of his
instruments. Still the star went to the south, and it continued so
advancing in the same direction until the following March, by which
time it had moved no less than twenty seconds south from the place
which it occupied when the first observation was made. After a brief
pause, in which no apparent movement was perceptible, the star by the
middle of April appeared to be returning to the north. Early in June
it reached the same distance from the zenith which it had in
December. By September the star was as much as thirty-nine seconds
more to the north than it had been in March, then it returned towards
the south, regaining in December the same situation which it had
occupied twelve months before.
This movement of the star being directly opposite to the movements
which would have been the consequence of parallax, seemed to show
that even if the star had any parallax its effects upon the apparent
place were entirely masked by a much larger motion of a totally
different description. Various attempts were made to account for the
phenomenon, but they were not successful. Bradley accordingly
determined to investigate the whole subject in a more thorough
manner. One of his objects was to try whether the same movements
which he had observed in one star were in any similar degree
possessed by other stars. For this purpose he set up a new
instrument at Wanstead, and there he commenced a most diligent
scrutiny of the apparent places of several stars which passed at
different distances from the zenith. He found in the course of this
research that other stars exhibited movements of a similar
description to those which had already proved so perplexing. For a
long time the cause of these apparent movements seemed a mystery. At
last, however, the explanation of these remarkable phenomena dawned
upon him, and his great discovery was made.
One day when Bradley was out sailing he happened to remark that every
time the boat was laid on a different tack the vane at the top of the
boat's mast shifted a little, as if there had been a slight change in
the direction of the wind. After he had noticed this three or four
times he made a remark to the sailors to the effect that it was very
strange the wind should always happen to change just at the moment
when the boat was going about. The sailors, however, said there had
been no change in the wind, but that the alteration in the vane was
due to the fact that the boat's course had been altered. In fact,
the position of the vane was determined both by the course of the
boat and the direction of the wind, and if either of these were
altered there would be a corresponding change in the direction of the
vane. This meant, of course, that the observer in the boat which was
moving along would feel the wind coming from a point different from
that in which the wind appeared to be blowing when the boat was at
rest, or when it was sailing in some different direction. Bradley's
sagacity saw in this observation the clue to the Difficulty which had
so long troubled him.
It had been discovered before the time of Bradley that the passage of
light through space is not an instantaneous phenomenon. Light
requires time for its journey. Galileo surmised that the sun may
have reached the horizon before we see it there, and it was indeed
sufficiently obvious that a physical action, like the transmission of
light, could hardly take place without requiring some lapse of time.
The speed with which light actually travelled was, however, so rapid
that its determination eluded all the means of experimenting which
were available in those days. The penetration of Roemer had
previously detected irregularities in the observed times of the
eclipses of Jupiter's satellites, which were undoubtedly due to the
interval which light required for stretching across the
interplanetary spaces. Bradley argued that as light can only travel
with a certain speed, it may in a measure be regarded like the wind,
which he noticed in the boat. If the observer were at rest, that is
to say, if the earth were a stationary object, the direction in which
the light actually does come would be different from that in which it
appears to come when the earth is in motion. It is true that the
earth travels but eighteen miles a second, while the velocity with
which light is borne along attains to as much as 180,000 miles a
second. The velocity of light is thus ten thousand times greater
than the speed of the earth. But even though the wind blew ten
thousand times faster than the speed with which the boat was sailing
there would still be some change, though no doubt a very small
change, in the position of the vane when the boat was in progress
from the position it would have if the boat were at rest. It
therefore occurred to this most acute of astronomers that when the
telescope was pointed towards a star so as to place it apparently in
the centre of the field of view, yet it was not generally the true
position of the star. It was not, in fact, the position in which the
star would have been observed had the earth been at rest. Provided
with this suggestion, he explained the apparent movements of the
stars by the principle known as the "aberration of light." Every
circumstance was accounted for as a consequence of the relative
movements of the earth and of the light from the star. This
beautiful discovery not only established in the most forcible manner
the nature of the movement of light; not only did it illustrate the
truth of the Copernican theory which asserted that the earth revolved
around the sun, but it was also of the utmost importance in the
improvement of practical astronomy. Every observer now knows that,
generally speaking, the position which the star appears to have is
not exactly the position in which the star does actually lie. The
observer is, however, able, by the application of the principles
which Bradley so clearly laid down, to apply to an observation the
correction which is necessary to obtain from it the true place in
which the object is actually situated. This memorable achievement at
once conferred on Bradley the highest astronomical fame. He tested
his discovery in every way, but only to confirm its truth in the most
complete manner.
Halley, the Astronomer Royal, died on the 14th, January, 1742, and
Bradley was immediately pointed out as his successor. He was
accordingly appointed Astronomer Royal in February, 1742. On first
taking up his abode at Greenwich he was unable to conduct his
observations owing to the wretched condition in which he found the
instruments. He devoted himself, however, assiduously to their
repair, and his first transit observation is recorded on the 25th
July, 1742. He worked with such energy that on one day it appears
that 255 transit observations were taken by himself alone, and in
September, 1747, he had completed the series of observations which
established his second great discovery, the nutation of the earth's
axis. The way in which he was led to the detection of the nutation
is strikingly illustrative of the extreme care with which Bradley
conducted his observations. He found that in the course of a
twelve-month, when the star had completed the movement which was due
to aberration, it did not return exactly to the same position which
it had previously occupied. At first he thought this must be due to
some instrumental error, but after closer examination and repeated
study of the effect as manifested by many different stars, he came to
the conclusion that its origin must be sought in some quite different
source. The fact is that a certain change takes place in the
apparent position of the stars which is not due to the movement of
the star itself, but is rather to be attributed to changes in the
points from which the star's positions are measured.
We may explain the matter in this way. As the earth is not a sphere,
but has protuberant parts at the equator, the attraction of the moon
exercises on those protuberant parts a pulling effect which
continually changes the direction of the earth's axis, and
consequently the position of the pole must be in a state of incessant
fluctuation. The pole to which the earth's axis points on the sky
is, therefore, slowly changing. At present it happens to lie near
the Pole Star, but it will not always remain there. It describes a
circle around the pole of the Ecliptic, requiring about 25,000 years
for a complete circuit. In the course of its progress the pole will
gradually pass now near one star and now near another, so that many
stars will in the lapse of ages discharge the various functions which
the present Pole Star does for us. In about 12,000 years, for
instance, the pole will have come near the bright star, Vega. This
movement of the pole had been known for ages. But what Bradley
discovered was that the pole, instead of describing an uniform
movement as had been previously supposed, followed a sinuous course
now on one side and now on the other of its mean place. This he
traced to the fluctuations of the moon's orbit, which undergoes a
continuous change in a period of nineteen years. Thus the efficiency
with which the moon acts on the protuberant mass of the earth varies,
and thus the pole is caused to oscillate.
This subtle discovery, if perhaps in some ways less impressive than
Bradley's earlier achievements of the detection of the aberration of
light, is regarded by astronomers as testifying even in a higher
degree to his astonishing care and skill as an observer, and justly
entitles him to a unique place among the astronomers whose
discoveries have been effected by consummate practical skill in the
use of astronomical instruments.
Of Bradley's private or domestic life there is but little to tell. In
1744, soon after he became Astronomer Royal, he married a daughter of
Samuel Peach, of Chalford, in Gloucestershire. There was but one
child, a daughter, who became the wife of her cousin, Rev. Samuel
Peach, rector of Compton, Beauchamp, in Berkshire.
Bradley's last two years of life were clouded by a melancholy
depression of spirits, due to an apprehension that he should survive
his rational faculties. It seems, however, that the ill he dreaded
never came upon him, for he retained his mental powers to the close.
He died on 13th July, 1762, aged seventy, and was buried at
Michinghamton.
WILLIAM HERSCHEL.
William Herschel, one of the greatest astronomers that has ever
lived, was born at Hanover, on the 15th November, 1738. His father,
Isaac Herschel, was a man evidently of considerable ability, whose
life was devoted to the study and practice of music, by which he
earned a somewhat precarious maintenance. He had but few worldly
goods to leave to his children, but he more than compensated for this
by bequeathing to them a splendid inheritance of genius. Touches of
genius were, indeed, liberally scattered among the members of Isaac's
large family, and in the case of his forth child, William, and of a
sister several years younger, it was united with that determined
perseverance and rigid adherence to principle which enabled genius to
fulfil its perfect work.
A faithful chronicler has given us an interesting account of the way
in which Isaac Herschel educated his sons; the narrative is taken
from the recollections of one who, at the time we are speaking of,
was an unnoticed little girl five or six years old. She writes:--
"My brothers were often introduced as solo performers and assistants
in the orchestra at the Court, and I remember that I was frequently
prevented from going to sleep by the lively criticisms on music on
coming from a concert. Often I would keep myself awake that I might
listen to their animating remarks, for it made me so happy to see
them so happy. But generally their conversation would branch out on
philosophical subjects, when my brother William and my father often
argued with such warmth that my mother's interference became
necessary, when the names--Euler, Leibnitz, and Newton--sounded
rather too loud for the repose of her little ones, who had to be at
school by seven in the morning." The child whose reminiscences are
here given became afterwards the famous Caroline Herschel. The
narrative of her life, by Mrs. John Herschel, is a most interesting
book, not only for the account it contains of the remarkable woman
herself, but also because it provides the best picture we have of the
great astronomer to whom Caroline devoted her life.
This modest family circle was, in a measure, dispersed at the
outbreak of the Seven Years' War in 1756. The French proceeded to
invade Hanover, which, it will be remembered, belonged at this time
to the British dominions. Young William Herschel had already
obtained the position of a regular performer in the regimental band
of the Hanoverian Guards, and it was his fortune to obtain some
experience of actual warfare in the disastrous battle of Hastenbeck.
He was not wounded, but he had to spend the night after the battle in
a ditch, and his meditations on the occasion convinced him that
soldiering was not the profession exactly adapted to his tastes. We
need not attempt to conceal the fact that he left his regiment by the
very simple but somewhat risky process of desertion. He had, it
would seem, to adopt disguises to effect his escape. At all events,
by some means he succeeded in eluding detection and reached England
in safety. It is interesting to have learned on good authority that
many years after this offence was committed it was solemnly
forgiven. When Herschel had become the famous astronomer, and as
such visited King George at Windsor, the King at their first meeting
handed to him his pardon for deserting from the army, written out in
due form by his Majesty himself.
It seems that the young musician must have had some difficulty in
providing for his maintenance during the first few years of his abode
in England. It was not until he had reached the age of twenty-two
that he succeeded in obtaining any regular appointment. He was then
made Instructor of Music to the Durham Militia. Shortly afterwards,
his talents being more widely recognised, he was appointed as
organist at the parish church at Halifax, and his prospects in life
now being fairly favourable, and the Seven Years' War being over, he
ventured to pay a visit to Hanover to see his father. We can imagine
the delight with which old Isaac Herschel welcomed his promising son,
as well as his parental pride when a concert was given at which some
of William's compositions were performed. If the father was so
intensely gratified on this occasion, what would his feelings have
been could he have lived to witness his son's future career? But
this pleasure was not to be his, for he died many years before
William became an astronomer.
In 1766, about a couple of years after his return to England from
This visit to his old home, we find that Herschel had received a
further promotion to be organist in the Octagon Chapel, at Bath.
Bath was then, as now, a highly fashionable resort, and many notable
personages patronised the rising musician. Herschel had other points
in his favour besides his professional skill; his appearance was
good, his address was prepossessing, and even his nationality was a
distinct advantage, inasmuch as he was a Hanoverian in the reign of
King George the Third. On Sundays he played the organ, to the great
delight of the congregation, and on week-days he was occupied by
giving lessons to private pupils, and in preparation for public
performances. He thus came to be busily employed, and seems to have
been in the enjoyment of comfortable means.
[PLATE: 7, NEW KING STREET, BATH, WHERE HERSCHEL LIVED.]
From his earliest youth Herschel had been endowed with that
invaluable characteristic, an eager curiosity for knowledge. He was
naturally desirous of perfecting himself in the theory of music, and
thus he was led to study mathematics. When he had once tasted the
charms of mathematics, he saw vast regions of knowledge unfolded
before him, and in this way he was induced to direct his attention to
astronomy. More and more this pursuit seems to have engrossed his
attention, until at last it had become an absorbing passion. Herschel
was, however, still obliged, by the exigency of procuring a
livelihood, to give up the best part of his time to his profession as
a musician; but his heart was eagerly fixed on another science, and
every spare moment was steadily devoted to astronomy. For many
years, however, he continued to labour at his original calling, nor
was it until he had attained middle age and become the most
celebrated astronomer of the time, that he was enabled to concentrate
his attention exclusively on his favourite pursuit.