HALLEY AND HIS SUCCESSORS
There is no need to give the lives of the succeeding Astronomers
Royal so fully as that of Flamsteed. Not that they were inferior
men to him; on the contrary, there can be little doubt that
we ought to reckon some of them as his superiors, but, in the
case of several, their best work was done apart from Greenwich
Observatory, and before they came to it.
This was particularly the case with EDMUND HALLEY. Born on October
29, 1656, he was ten years the junior of Flamsteed. Like Flamsteed,
he came of a Derbyshire family, though he was born at Haggerston,
in the parish of St. Leonard's, Shoreditch. He was educated at St.
Paul's School, where he made very rapid progress, and already showed
the bent of his mind. He learnt to make dials; he made himself so
thoroughly acquainted with the heavens that it is said, 'If a star
were displaced in the globe he would presently find it out,' and
he observed the changes in the direction of the mariner's compass.
In 1673 he went to Queen's College, Oxford, where he observed a
sunspot in July and August, 1676, and an occultation of Mars. This
was not his first astronomical observation, as, in June, 1675,
he had observed an eclipse of the moon from his father's house in
Winchester Street.
[Illustration: EDMUND HALLEY.
(_From an old print._)]
A much wider scheme of work than such merely casual observations
now entered his mind, possibly suggested to him by Flamsteed's
appointment to the direction of the new Royal Observatory. This was
to make a catalogue of the southern stars. Tycho's places for the
northern stars were defective enough, but there was no catalogue at
all of stars below the horizon of Tycho's observatory. Here, then,
was a field entirely unworked, and young Halley was so eager to
enter upon it that he would not wait at Oxford to obtain his degree,
but was anxious to start at once for the southern hemisphere.
His father, who was wealthy and proud of his gifted son, strongly
supported him in his project. The station he selected was St.
Helena, an unfortunate choice, as the skies there were almost
always more or less clouded, and rain was frequent during his stay.
However, he remained there a year and a half, and succeeded in
making a catalogue of 341 stars. This catalogue was finally reduced
by Sharp, and included in the third volume of Flamsteed's _Historia
Coelestis_.
In 1678 he was elected Fellow of the Royal Society, and the
following year he was chosen to represent that society in a
discussion with Hevelius. The question at issue was as to whether
more accurate observations of the place of a star could be obtained
by the use of sights without optical assistance, or by the use of
a telescope. The next year he visited the Paris Observatory, and,
later in the same tour, the principal cities of the Continent.
Not long after his return from this tour, Halley was led to that
undertaking for which we owe him the greatest debt of gratitude, and
which must be regarded as his greatest achievement.
Some fifty years before, the great Kepler had brought out the third
of his well-known laws of planetary motion. These laws stated
that the planets move round the sun in ellipses, of which the sun
occupies one of the foci; that the straight line joining any planet
with the sun moves over equal areas of space in equal periods of
time; and, lastly, that the squares of the times in which the
several planets complete a revolution round the sun are proportional
to the cubes of their mean distances from it. These three laws were
deduced from actual examination of the movements of the planets.
Kepler did not work out any underlying cause of which these three
laws were the consequence.
But the desire to find such an underlying cause was keen amongst
astronomers, and had given rise to many researches. Amongst those
at work on the subject was Halley himself. He had seen, and been
able to prove, that if the planets moved in circles round the sun,
with the sun in the centre, then the law of the relation between the
times of revolution and the distances of the planets would show that
the attractive force of the sun varied inversely as the square of
the distance. The actual case, however, of motion in an ellipse was
too hard for him, and he could not deal with it. Halley therefore
went up to Cambridge to consult Newton, and, to his wonder and
delight, found that the latter had already completely solved the
problem, and had proved that Kepler's three laws of planetary motion
were summed up in one, namely, that the sun attracted the planets to
it with a force inversely proportional to the square of the distance.
Halley was most enthusiastic over this great discovery, and he at
once strongly urged Newton to publish it. Newton's unwillingness to
do so was great, but at length Halley overcame his reluctance; and
the Royal Society not being able at the time to afford the expense,
Halley took the charges upon himself, although his own resources had
been recently seriously damaged by the death of his father.
The publication of Newton's _Principia_, which, but for him, might
never have seen the light, and most certainly would have been long
delayed, is Halley's highest claim to our gratitude. But, apart
from this, his record of scientific achievement is indeed a noble
one. Always, from boyhood, he had taken a great interest in the
behaviour of the magnetic compass, and he now followed up the study
of its variations with the greatest energy. For this purpose it was
necessary that he should travel, in view of the great importance of
the subject to navigation. King William III. gave him a captain's
commission in the Royal Navy--a curious and interesting illustration
of the close connection between astronomy and the welfare of our
navy--and placed him in command of a 'pink,' that is to say, a small
vessel with pointed stern, named the Paramour, in which he proceeded
to the southern ocean. His first voyage was unfortunate, but the
Paramour was recommissioned in 1699, and he sailed in it as far as
south latitude 52°.
In 1701 and the succeeding year he made further voyages in the
Paramour, surveying the tides and coasts of the British Channel
and of the Adriatic, and helping in the fortification of Trieste.
He became Savilian Professor of Geometry at Oxford in 1703, having
failed twelve years previously to secure the Savilian Professorship
of Astronomy, mainly through the opposition of Flamsteed, who had
already formed a strong prejudice against him, which some writers
have traced to Halley's detection of several errors in one of
Flamsteed's tide-tables, others to Halley's supposed materialistic
views. Probably the difference was innate in the two men. There was
likely to be but little sympathy between the strong, masterful man
of action and society and the secluded, self-conscious, suffering
invalid. At any rate, in the contest between Newton and Flamsteed,
which has been already described, Halley took warmly the side of the
former, and was appointed to edit the publication of Flamsteed's
results, and, on the death of the latter, to succeed him at the
Royal Observatory.
The condition of things at Greenwich when Halley succeeded to
the post of Astronomer Royal in 1720 was most discouraging. The
instruments there had all belonged to Flamsteed, and therefore,
most naturally, had been removed by his widow. The Observatory
had practically to be begun _de novo_, and Halley had now almost
attained the age at which in the present day an Astronomer
Royal would have to retire. More fortunate, however, than his
predecessor, he was able to get a grant for instruments, and he
equipped the Observatory as well as the resources of the time
permitted, and his transit instrument and great eight-foot quadrant
still hang upon the Observatory walls.
As Astronomer Royal his great work was the systematic observation
of the positions of the moon through an entire _saros_. As is well
known, a period of eighteen years and ten or eleven days brings the
sun and moon very nearly into the same positions relatively to the
earth which they occupied at the commencement of the period. This
period was well known to the ancient Chaldeans, who gave it its
name, since they had noticed that eclipses of the sun or eclipses of
the moon recurred at intervals of the above length. It was Halley's
desire to obtain such a set of observations of the moon through an
entire _saros_ period as to be able to deduce therefrom an improved
set of tables of the moon's motion. It was an ambitious scheme for
a man so much over sixty to undertake, nevertheless he carried it
through successfully.
His desire to complete this scheme, and to found upon it improved
lunar tables, hindered him from publishing his observations, for
he feared that others might make use of them before he was in a
position to complete his work himself. This omission to publish
troubled Newton, who, as President of the Royal Society--the
Greenwich Board of Visitors having lapsed at Queen Anne's
death--drew attention at a meeting of the Royal Society, March 2,
1727, to Halley's disobedience of the order issued under Queen
Anne, for the prompt communication of the Observatory results. That
Newton should thus have put public pressure upon Halley, the man to
whom he was so much indebted, and with whom there was so close an
affection, is sufficient proof that his similar attitude towards
Flamsteed was one of principle and not of arbitrariness. Halley, on
his side, stood firm, as Flamsteed had done, urging the danger that,
by publishing before he had completed his task, he might give an
opportunity to others to forestall his results. It is said--probably
without sufficient ground--that this refusal broke Newton's heart
and caused his death. Certainly Halley's writings in that very year
show his reverence and affection for Newton to have been as keen and
lively as ever.
Halley's work at the Observatory went on smoothly, on the lines he
had laid down for himself, for ten years after Newton's death; but
in 1737 he had a stroke of paralysis, and his health, which had
been remarkably robust up to that time, began to give way. He died
January 14, 1742, and was buried in the cemetery of Lee Church.
As an astronomer, his services to the science rank higher than those
of his predecessor; but as Astronomer Royal, as director, that is to
say, of Greenwich Observatory, he by no means accomplished as much
as Flamsteed had done. Professor Grant, in his _History of Physical
Astronomy_, says that he seems to have undervalued those habits of
minute attention which are indispensable to the attainment of a high
degree of excellence in the practice of astronomical observation. He
was far from being sufficiently careful as to the adjustment of his
instruments, the going of his clocks, or the recording of his own
observations. The important feature of his administration was that
under him the Observatory was first supplied with instruments which
belonged to it.
[Illustration: HALLEY'S QUADRANT.
(_From an old print._)]
His astronomical work apart from the Observatory was of the first
importance. He practically inaugurated the study of terrestrial
magnetism, and his map giving the results of his observations during
his voyage in the Paramour introduced a new and most useful style
of recording observations. He joined together by smooth curves
places of equal variation, the result being that the chart shows at
a glance, not merely the general course of the variation over the
earth's surface, but its value at any spot within the limits of the
chart.
Another work which has justly made his name immortal was the
prediction of the return of the comet which is called by his name,
to which reference will be made later. Another great scheme, and
one destined to bear much fruit, was the working out of a plan to
determine the distance of the sun by observations of the transit of
Venus.
Of attractive appearance, pleasing manners, and ready wit, loyal,
generous, and free from self-seeking, he probably was one of the
most personally engaging men who ever held the office.
The salary of the Astronomer Royal remained under Halley at the same
inadequate rate which it had done under Flamsteed--£100, without
provision for an assistant. But in 1729 Queen Caroline, learning
that Halley had actually had a captain's commission in the Royal
Navy, secured for him a post-captain's pay.
[Illustration: JAMES BRADLEY.
(_From the painting by Hudson._)]
Halley's work is represented at the Observatory by two of his
instruments which are still preserved there, and which hang on the
west wall of the present transit room: the Iron Quadrant afterwards
made famous by the observations of Bradley, and 'Halley's Transit,'
the first of the great series of instruments upon which the fame of
Greenwich chiefly rests. This transit instrument seems to have been
set up in a small room at the west end of what is now known as the
North Terrace. His quadrant was mounted on the pier which is now the
base of the pier of the astrographic telescope. This pier was the
first extension which the Observatory received from the original
building.
On the breakdown of his health Halley nominated as his successor,
James Bradley; indeed, it is stated that he offered to resign
in his favour. He had known him then for over twenty years, and
that keen and generous appreciation of merit in others which was
characteristic of Halley had led him very early to recognize
Bradley's singular ability.
* * * * *
JAMES BRADLEY was born in 1692 or 1693, of an old North of England
family. His birthplace was Sherbourne, in Gloucestershire, and he
was educated at North Leach Grammar School and at Baliol College,
Oxford. During the years of his undergraduateship he resided much
with his uncle, the Rev. James Pound, Rector of Wanstead, Essex, an
ardent amateur astronomer, a frequent visitor at the Observatory
in Flamsteed's time, and one of the most accurate observers in the
country. From him, no doubt, he derived his love of the science,
and possibly some of his skill in observation.
Bradley's earliest observations seem to have been devoted to the
phenomena of Jupiter's satellites and to the measures of double
stars. The accuracy with which he followed up the first drew the
attention of Halley, and so began a friendship which lasted through
life. His observations of double stars, particularly of Castor, only
just failed to show him the orbital movement of the pair, because
his attention was drawn to other subjects before it had become
sufficiently obvious.
In 1719 Bradley and his uncle made an attempt to determine the
distance of the sun through observations of Mars when in opposition,
observations which were so accurate that they sufficed to show that
the distance of the sun could not be greater than 125 millions
of miles, nor less than about 94 millions. The lower limit which
they thus found has proved to be almost exactly correct, our best
modern determinations giving it as 93 millions. The instrument with
which the observations were made was a novel one, being 'moved by a
machine that made it to keep pace with the stars;' in other words,
it was the first, or nearly the first, example of what we should now
call a clock-driven equatorial.
That same year he was offered the Vicarage of Bridstow, near Ross,
in Monmouthshire, where, having by that time taken priest's orders,
he was duly installed, July, 1720. To this was added the sinecure
Rectory of Llandewi-Velgry; but he held both livings only a very
short time. In 1721 the death of Dr. John Keill rendered vacant the
Savilian Professorship of Astronomy at Oxford, for which Bradley
became a candidate, and was duly elected, and resigned his livings
in consequence.
It was whilst he was Savilian Professor that Bradley made that
great discovery which will always be associated with his name.
Though professor at Oxford, he had continued to assist his uncle,
Mr. Pound, at his observations at Wanstead, and after the death of
the latter he still lived there as much as possible, and continued
his astronomical work. But in 1725 he was invited by Mr. Samuel
Molyneux, who had set up a twenty-four-foot telescope made by
Graham as a zenith tube at his house on Kew Green, to verify some
observations which he was making. These were of the star Gamma
Draconis, a star which passes through the zenith of London, and
which, therefore, had been much observed both by Flamsteed and
Hooke, inasmuch as by fixing a telescope in an absolutely vertical
position--a position which could be easily verified--it was easy to
ascertain if there was any minute change in the apparent position
of the star. Dr. Hooke had declared that there was such a change,
a change due to the motion of the earth in its orbit, which would
prove that the star was not an infinite distance from the earth, the
seeming change of its place in the sky corresponding to the change
in the place of the earth from which the observer was viewing it.
Bradley found at once that there was such a change--a marked one. It
amounted to as much as 1´´ of arc in three days; but it was not in
the direction in which the parallax of the star would have moved
it, but in the opposite. Whether, therefore, the star was near
enough to show any parallax or not, some other cause was giving rise
to an apparent displacement of the star, which entirely masked and
overcame the effect of parallax.
So far, Bradley had but come to the same point which Flamsteed
had reached. Flamsteed had detected precisely the same apparent
displacement of stars, and, like Hooke, had ascribed it to
parallax. Cassini had shown that this could not be the case, as
the displacement was in the wrong direction; and there the matter
had rested. Bradley now set to follow the question up. Other stars
beside Gamma Draconis were found to show a displacement of the same
general character, but the amount varied with their distance from
the plane of the ecliptic, the earth's orbit. The first explanation
suggested was that the axis of the earth, which moves very nearly
parallel to itself as the earth moves round the sun, underwent a
slight regular 'wobble' in the course of a year. To check this,
a star was observed on the opposite side of the pole from Gamma
Draconis; then Bradley investigated as to whether refraction might
explain the difficulty, but again without success. He now was
most keenly interested in the problem, and he purchased a zenith
telescope of his own, made, like that of Molyneux, by Graham,
and mounted it in his aunt's house at Wanstead, and observed
continuously with it. The solution of the problem came at last to
him as he was boating on the Thames. Watching a vane at the top of
the mast, he saw with surprise that it shifted its direction every
time that the boat was put about. Remarking to the boatmen that it
was very odd that the wind should change just at the same moment
that there was a shift in the boat's course, they replied that there
was no change in the wind at all, and that the apparent change of
the vane was simply due to the change of direction of the motion of
the boat.
[Illustration: GRAHAM'S ZENITH SECTOR.
(_From an old print._)]
This supplied Bradley with a key to the solution of the mystery
that had troubled him so long. It had been discovered long before
this that light does not travel instantaneously from place to
place, but takes an appreciable time to pass from one member of
the solar system to another. This had been discovered by Römer
from observations of the satellites of Jupiter. He had noted that
the eclipses of the satellites always fell late of the computed
time, when Jupiter was at his greatest distance from the earth;
and Bradley's own work in the observation of those satellites had
brought the fact most intimately under his own acquaintance. The
result of the boating incident taught him, then, that he might look
upon light as analogous to the wind blowing on the boat. As the
wind, so long as it was steady, would seem to blow from one fixed
quarter so long as the boat was also in rest, but as it seemed
to shift its direction when the boat was moving and changed its
direction, so he saw that the light coming from a particular star
must seem to slightly change the direction in which it came, or, in
other words, the apparent position of the star, to correspond with
the movement of the earth in its orbit round the sun.
This was the celebrated discovery of the Aberration of Light,
a triumph of exact observation and of clear insight. As to the
exactness of Bradley's observations, it is sufficient to say that
his determination of the value of the 'Constant of Aberration' gave
it as 20·39´´; the value adopted to-day is 20·47´´.
On the death of Halley, in 1742, Bradley was appointed to succeed
him. He found the Observatory in as utterly disheartening a
condition as his predecessors had done. As already mentioned, Halley
had not the same qualifications as an observer that Flamsteed
had. He was, further, an old man when appointed to the post, he
had no assistant provided for him, and the last five years of his
life his health and strength had entirely given way. Under these
circumstances, it was no wonder that Bradley found the instruments
of the Observatory in a deplorable state. Nevertheless, he set to
work most energetically, and in the year of his appointment he
made 1500 observations in the last five months of the year. He was
particularly earnest in examining the condition and the errors of
his instruments; and as their defects became known to him, he was
more and more anxious for a better equipment. He moved the Royal
Society, therefore, to apply on his behalf for the instruments he
required; and a petition from that body, in 1748, obtained what
in those days must be considered the generous grant of £1000,
the proceeds of the sale of old Admiralty stores. The principal
instruments purchased therewith were a mural quadrant and a transit
instrument, both eight feet in focal length, still preserved on the
walls of the transit-room. It is interesting also to note that,
following in the steps of Halley, and forecasting, as it were, the
magnetic observatory which Airy would found, he devoted £20 of the
grant to purchasing magnetic instruments.
Meantime he had continued his observations on aberration, and had
discovered that the aberration theory was not sufficient entirely
to account for the apparent changes in places of stars which he had
discovered. A second cause was at work, a movement of the earth's
axis, a 'wobble' in its inclination, technically known as Nutation,
which is due to the action of the moon, and goes through its course
in a period of nineteen years.
Beside these two great discoveries of aberration and nutation,
Bradley's reputation rests upon his magnificent observations of the
places of more than three thousand stars. This part of his work was
done with such thoroughness, that the star-places deduced from them
form the basis of most of our knowledge as to the actual movements
of individual stars. In particular, he was careful to investigate
and to correct for the errors of his instrument, and to determine
the laws of refraction, introducing corrections for changes in the
readings of thermometer and barometer. His tables of refraction
were used, indeed, for seventy years after his death. Of his other
labours it may be sufficient to refer to his determination of the
longitudes of Lisbon and of New York, and to his effort to ascertain
the parallax of the sun and moon, in combination with La Caille, who
was observing at the Cape of Good Hope.
As Astronomer Royal, Bradley's great achievement was the high
standard to which he raised the practical work of observation. From
his day onwards, also, there was always at least one assistant.
His first assistant was his own nephew, John Bradley, who received
the munificent salary of ten shillings a week. Still, this was
not out of proportion to the then salary of the Astronomer Royal,
which practically amounted only to £90. However, in 1752, Bradley
was awarded a Crown pension of £250 a year. He refused the living
of Greenwich, which was offered him in order to increase his
emoluments, on the ground that he could not suitably fulfil the
double office. Bradley's later assistants were Charles Mason and
Charles Green.
Bradley's last work was the preparation for the observations of the
transit of Venus of 1761, according to the lines laid down by his
predecessor, Halley. His health gave way, and he became subject to
melancholia, so that the actual observations were taken by the Rev.
Nathaniel Bliss, who succeeded him in his office after his death, in
1762. He was buried at Minchinhampton.
So far as we know Bradley's character, he seems to have been a
gentle, modest, unassuming man, entirely free from self-seeking,
and indifferent to personal gain. He was in many ways an ideal
astronomer, exact, methodical, and conscientious to the last degree.
His skill as an observer was his chief characteristic; and though
his abilities were not equal as a mathematician or a mechanician,
yet, on the one hand, he had a very clear insight into the meaning
of his observations, and, on the other, he was skilful enough to
himself adjust, repair, and improve his instruments.
Of Bradley's instruments, there are still preserved his famous
twelve-and-a-half-foot zenith sector, with which he made his two
great discoveries; his brass quadrant, which in 1750 he substituted
for Halley's iron quadrant; his transit instrument, and equatorial
sector. Bradley added to the buildings of the Observatory that
portion which is now represented by the upper and lower computing
rooms, and the chronometer room, which adjoins the latter. This
room--the chronometer room--was his transit room, and the position
of the shutters is still marked by the window in the roof.
* * * * *
The Rev. NATHANIEL BLISS, who succeeded Bradley, only held the
office for a couple of years, and during that time was much at
Oxford. He, therefore, has left no special mark behind him as
Astronomer Royal.
He was born November 28, 1700. His father, like himself, Nathaniel
Bliss, was a gentleman, of Bisley, Gloucestershire.
[Illustration: NATHANIEL BLISS.
(_From an engraving on an old pewter flagon._)]
Bliss graduated at Pembroke College, Oxford, as B.A. in 1720,
and M.A. in 1723. He became the Rector of St. Ebb's, Oxford, in
1736, and on Halley's death succeeded him as Savilian Professor of
Geometry. He supplied Bradley with his observations of Jupiter's
satellites, and from time to time, at his request, rendered him some
assistance at the Royal Observatory. This was particularly the case,
as has been already mentioned, with respect to the transit of
Venus of 1761, the observations of which were carried out by Bliss,
owing to Bradley's ill-health. It was natural, therefore, that on
Bradley's death he should succeed to the vacant post; but he held
it too short a time to do any distinctive work. Such observations
as he made seem to have been entirely in continuation of Bradley's.
He took a great interest, however, in the improvement of clocks, a
department in which so much was being done at this time by Graham,
Ellicott, and others.
* * * * *
NEVIL MASKELYNE, the fifth Astronomer Royal, was, like Bliss, a
close friend of Bradley's. He was the third son of a wealthy country
gentleman, Edmund Maskelyne, of Purton, in Wiltshire. Maskelyne was
born in London, October 6, 1732, and was educated at Westminster
School. Thence he proceeded to Cambridge, where he graduated seventh
Wrangler in 1754. He was ordained to the curacy of Barnet in 1755,
and, twenty years later, was presented by his nephew, Lord Clive, to
the living of Shrawardine, in Shropshire. In 1782 he was presented
by his college to the Rectory of North Runcton, Norfolk.
The event which turned his thoughts in the direction of astronomy
was the solar eclipse of July 25, 1748; and about the time that he
was appointed to the curacy of Barnet he became acquainted with
Bradley, then the Astronomer Royal, to whom he gave great assistance
in the preparation of his table of refractions.
Like Halley before him, he made an astronomical expedition to the
island of St. Helena. This was for the special purpose of observing
the transit of Venus of June 6, 1761, Bradley having induced the
Royal Society to send him out for that purpose. Here he stayed
ten months, and made many observations. But though the transit
of Venus was his special object, it was not the chief result of
the expedition: not because clouds hindered his observations, but
because the voyage gave him the especial bent of his life.
Halley had actually held a captain's commission in the Royal Navy,
and commanded a ship; Maskelyne, more than any of the Astronomers
Royal before or since, made the improvement of the practical
business of navigation his chief aim. None of all the incumbents of
the office kept its original charter--'To find the so much desired
Longitude at Sea, for the perfecting the Art of Navigation,' so
closely before him.
The solution of the problem was at hand at this time--its solution
in two different ways. On the one hand, the offer by the Government
of a reward of £20,000 for a clock or watch which should go so
perfectly at sea, notwithstanding the tossing of the ship and the
wide changes of temperature to which it might be exposed, that the
navigator might at any moment learn the true Greenwich time from
it, had brought out the invention of Harrison's time-keeper; on the
other hand, the great improvement that had now taken place in the
computation of tables of the moon's motion, and the more accurate
star-catalogues now procurable, had made the method of 'lunars,'
suggested a hundred and thirty years before by the Frenchman, Morin,
and others, a practicable one.
[Illustration: NEVIL MASKELYNE.]
In principle, the method of finding the longitude from 'lunars,'
that is to say, from measurements of the distances between the moon
and certain stars, is an exceedingly simple one. In actual practice,
it involves a very toilsome calculation, beside exact and careful
observation. The principle, as already mentioned, is simply this:
The moon travels round the sky, making a complete circuit of the
heavens in between twenty-seven and twenty-eight days. It thus
moves amongst the stars, roughly speaking, its own diameter, in
about an hour. When once its movements were sufficiently well known
to be exactly predicted, almanacs could be drawn up in which the
Greenwich time of its reaching any definite point of the sky could
be predicted long beforehand; or, what comes to the same thing,
its distances from a number of suitable stars could be given for
definite intervals of Greenwich time. It is only necessary, then, to
measure the distances between the moon and some of these stars, and
by comparing them with the distances given in the almanac, the exact
time at Greenwich can be inferred. As has been already pointed out,
the determination of the latitude of the ship and of the local time
at any place where the ship is, is not by any means so difficult
a matter; but the local time being known and the Greenwich time,
the difference between these gives the longitude; and the latitude
having been also ascertained, the exact position of the ship is
known.
There are, of course, difficulties in the way of working out this
method. One is, that whilst it takes the sun but twenty-four hours
to move round the sky from one noon to the next, and consequently
its movements, from which the local time is inferred, are fairly
rapid, the moon takes nearly twenty-eight days to move amongst the
stars from the neighbourhood of one particular star round to that
particular star again. Consequently, it is much easier to determine
the local time with a given degree of exactness than the Greenwich
time; it is something like the difference of reading a clock from
both hands and from the hour hand alone.
There are other difficulties in the case which make the computation
a long and laborious one, and difficult in that sense; but they do
not otherwise affect its practicability.
During this voyage to St. Helena, both when outward bound and when
returning, Maskelyne gave the method of 'lunars' a very thorough
testing, and convinced himself that it was capable of giving the
information required. For by this time the improvement of the
sextant, or quadrant as it then was, by the introduction of a second
mirror, by Hadley, had rendered the actual observation at sea of
lunar distances, and of altitudes generally, a much more exact
operation.
This conclusion he put at once to practical effect, and, in 1763,
he published the _British Mariner's Guide_, a handbook for the
determination of the longitude at sea by the method of lunars.
At the same time, the other method, that by the time-keeper or
chronometer, was practically tested by him. The time-keeper
constructed by John Harrison had been tested by a voyage to Jamaica
in 1761, and now, in 1763, another time-keeper was tested in a
voyage to Barbadoes. Charles Green, the assistant at Greenwich
Observatory, was sent in charge of the chronometer, and Maskelyne
went with him to test its performance, in the capacity of chaplain
to his Majesty's ship Louisa.
[Illustration: HADLEY'S QUADRANT.
(_From an old print._)]
The position which Maskelyne had already won for himself as a
practical astronomer, and the intimate relations into which he
had entered with Bradley and Bliss, made his appointment to the
Astronomer Royalship, on the death of the latter, most suitable.
At once he bent his mind to the completion of the revolution
in nautical astronomy which his _British Mariner's Guide_ had
inaugurated, and in the year after his appointment he published
the first number of the _Nautical Almanac_, together with a volume
entitled, _Tables Requisite to be Used with the Nautical Ephemeris_,
the value of which was so instantly appreciated, that 10,000 copies
were sold at once.
The _Nautical Almanac_ was Maskelyne's greatest work, and it must
be remembered that he carried it on from this time up to the day of
his death--truly a formidable addition to the routine labours of an
Astronomer Royal who had but a single assistant on his staff. The
_Nautical Almanac_ was, however, in the main not computed at the
Observatory; the calculations were effected by computers living in
different parts of the country, the work being done in duplicate, on
the principle which Flamsteed had inaugurated in the preparation of
his _Historia Coelestis_.
Maskelyne's next service to science was almost as important.
He arranged that the regular and systematic publication of the
observations made at Greenwich should be a distinct part of the
duties of an Astronomer Royal, and he procured an arrangement
by which a special fund was set apart by the Royal Society for
printing them. His observations covering the years 1776 to 1811
fill four large folio volumes, and though, as already stated, he
had but one assistant, they are 90,000 in number. Thus it was
Maskelyne who first rendered effective the design which Charles
II. had in the establishment of the Observatory. Flamsteed and
Halley had been too jealous of their own observations to publish;
Bradley's observations--though he himself was entirely free from
this jealousy--were made, after his death, the subject of litigation
by his heirs and representatives, who claimed an absolute property
in them, a claim which the Government finally allowed. None of the
three, however much their work ultimately tended to the improvement
of the art of navigation, made that their first object. Whereas
Maskelyne set this most eminently practical object in the forefront,
and so gave to the Royal Observatory, which under his predecessors
somewhat resembled a private observatory, its distinctive
characteristics of a public institution.
It fell to Maskelyne to have to advise the Government as to the
assignment of their great reward of £20,000 for the discovery of
the longitude at sea. Maskelyne, while reporting favourably of the
behaviour of Harrison's time-keeper, considered that the method
of 'lunars' was far too important to be ignored, and he therefore
recommended that half the sum should be given to Harrison for his
watch, whilst the other half was awarded for the lunar tables
which Mayer, before his death, had sent to the Board of Longitude.
This decision, though there can be no doubt it was the right one,
led to much dissatisfaction on the part of Harrison, who urged
his claim for the whole grant very vigorously; and eventually
the whole £20,000 was paid him. The whole question of rewards to
chronometer-makers must have been one which caused Maskelyne much
vexation. He was made the subject of a bitter and most voluminous
attack by Thomas Mudge, for having preferred the work of Arnold and
Earnshaw to his own.
Otherwise his reign at the Observatory seems to have been a
singularly peaceful one, and there is little to record about it
beyond the patient prosecution, year by year, of an immense amount
of sober, practical work. To Maskelyne, however, we owe the practice
of taking a transit of a star over five wires instead of over one,
and he provided the transit instrument with a sliding eye-piece, to
get over the difficulty of the displacement which might ensue if the
star were observed askew when out of the centre of the field. To
Maskelyne, too, we owe in a pre-eminent degree the orderly form of
recording, reducing, and printing the observations. Much of the work
in this direction which is generally ascribed to Airy was really
due to Maskelyne. Indeed, without a wonderful gift of organization,
it would have been impossible to plan and to carry the _Nautical
Almanac_.
Beside the editing of various works intended for use in nautical
astronomy or in general computation, the chief events of his long
reign at Greenwich were the transit of Venus in 1769, which he
himself observed, and for which he issued instructions in the
_Nautical Almanac_; and his expedition in 1774 to Scotland, where he
measured the deviation of the plumb-line from the vertical caused by
the attraction of the mountain Schiehallion, deducing therefrom the
mean density of the earth to be four and a half times that of water.
[Illustration: JOHN POND.
(_From an old engraving._)]
He died at the Observatory, February 9, 1811, aged 79, leaving but
one child, a daughter, who married Mr. Anthony Mervin Story,
to whom she brought the family estates in Wiltshire, inherited by
Maskelyne on the deaths of his elder brothers, and, in consequence,
Mr. Story added the name of Maskelyne to his own.
Maskelyne's character and policy as Astronomer Royal have been
sufficiently dwelt upon. His private character was mild, amiable,
and generous. 'Every astronomer, every man of learning, found in him
a brother;' and, in particular, when the French Revolution drove
some French astronomers to this country to find a refuge, they
received from the Astronomer Royal the kindest reception and most
delicate assistance.
Maskelyne added no instrument to the Observatory during his reign,
though he improved Bradley's transit materially. He designed the
mural circle, but it was not completed until after his death. His
additions to the Observatory buildings consisted of three new rooms
in the Astronomer Royal's house, and the present transit circle room.
* * * * *
JOHN POND was recommended by Maskelyne as his successor at
Greenwich. At the time of his succession he was forty-four years of
age, having been born in 1767. He was educated at Trinity College,
Cambridge, and then spent some considerable time travelling in
the south of Europe and Egypt. On his return home he settled at
Westbury, where he erected an altazimuth by Troughton, with a
two-and-a-half-foot circle. A born observer, his observations of the
declinations of some of the principal fixed stars showed that the
instrument which Maskelyne was using at Greenwich--the quadrant by
Bird--could no longer be trusted. Maskelyne, in consequence, ordered
a six-foot mural circle from Troughton, but did not live to see it
installed, and in 1816 this was supplemented by Troughton's transit
instrument of five inches aperture and ten feet focal length.
The introduction of these two important instruments, and of other
new instruments, together with new methods of observation, form one
of the chief characteristics of Pond's administration. Under this
head must be specially mentioned the introduction of the mercury
trough, both for determining the position of the vertical, and for
obtaining a check upon the flexure of the mural circle in different
positions; and the use in combination of a pair of mural circles for
determining the declinations of stars.
Another characteristic of his reign was that under him there was the
first attempt to give the Astronomer Royal a salary somewhat higher
than that of a mechanic, and to support him with an adequate staff
of assistants. His salary was fixed at £600 a year, and the single
assistant of Maskelyne was increased to six.
This multiplication of assistants was for the purpose of multiplying
observations, for Pond was the first astronomer to recognize the
importance of greatly increasing the number of all observations upon
which the fundamental data of astronomy were to be based.
In 1833 he finished his standard catalogue of 1113 stars, at that
time the fullest of any catalogue prepared on the same scale of
accuracy. 'It is not too much to say,' was the verdict of the Royal
Astronomical Society, 'that meridian sidereal observation owes more
to him than to all his countrymen put together since the time of
Bradley.'
A yet higher testimony to the exactness of his work is given by his
successor, Airy.
'The points upon which, in my opinion, Mr. Pond's claims to
the gratitude of astronomers are founded, are principally the
following. _First_ and chief, the accuracy which he introduced
into all the principal observations. This is a thing which,
from its nature, it is extremely difficult to estimate now, so
long after the change has been made; and I can only say that,
so far as I can ascertain from books, the change is one of very
great extent; for certainty and accuracy, astronomy is quite a
different thing from what it was, and this is mainly due to Mr.
Pond.'
The same authority eulogizes him further for his laborious working
out of every conceivable cause or indication of error in his
declination instruments, for the system which he introduced in
the observation of transits, for the thoroughness with which he
determined all his fundamental data, and for the regularity which he
infused into the Greenwich observations.
One result of this great increase of accuracy was that Pond was able
at once authoritatively to discard the erroneous stellar parallaxes
that had been announced by Brinkley, Royal Astronomer for Ireland.
But Pond's administration was open, in several particulars, to
serious censure, and the Board of Visitors, which had been for
many years but a committee of the Royal Society, but which had
recently been reconstituted, proved its value and efficiency by
the remonstrances which it addressed to him, and which eventually
brought about his resignation. His personal skill and insight as an
observer were of the highest order; but either from lack of interest
or failing health, he absented himself almost entirely from the
Observatory in later years, visiting it only every ninth or tenth
day. He had caused the staff of assistants to be increased from one
to six, but had stipulated that the men supplied to him should be
'drudges.' His minute on the subject ran--
'I want indefatigable, hard-working, and, above all, obedient
drudges (for so I must call them, although they are drudges
of a superior order), men who will be contented to pass half
their day in using their hands and eyes in the mechanical act
of observing, and the remainder of it in the dull process of
calculation.'
This was a fatal mistake, and one which it is very hard to
understand how any one with a real interest in the science could
have made. Men who had the spirit of 'drudges,' to whom observation
was a mere 'mechanical act,' and calculation a 'dull process,' were
not likely to maintain the honour of the Observatory, particularly
under an absentee Astronomer Royal. Pond tried to overcome the
difficulty by devising rules for their guidance of iron rigidity.
The result was that after his resignation, in 1835, the First Lord
and the Secretary of the Admiralty expressed their feeling to Airy,
Pond's successor, 'that the Observatory had fallen into such a state
of disrepute that the whole establishment should be cleared out.' A
further evil was the excessive development of chronometer business,
so as practically to swamp the real work of the Observatory, whilst
the prices paid for the chronometers at this time were often much
larger than would have been the case under a more business-like
administration.
With all his merits, therefore, as an observer, the administration
of Pond was, in some respects, the least satisfactory of all that
the Observatory has known, and he alone of all the Astronomers Royal
retired under pressure. He did not long survive his resignation,
dying in September, 1836. He was buried by the side of Halley, in
the churchyard at Lee.
Of Pond's instruments, the Observatory retains the fine transit
instrument which was constructed by Troughton at his direction, and
the mural circle, designed by Maskelyne, but which Pond was the
first to use. Both of these have, of course, long been obsolete, and
now hang on the walls of the transit room. The small equatorial,
called, after its donor, the Shuckburgh equatorial, was also added
in Pond's day, and though practically never used, still remains
mounted in its special dome.