BRADLEY'S DISCOVERIES OF THE ABERRATION OF LIGHT AND OF THE NUTATION OF
THE EARTH'S AXIS
[Sidenote: Biographical method adopted.]
In examining different types of astronomical discovery, we shall find
certain advantages in varying to some extent the method of presentation.
In the two previous chapters our opportunities for learning anything of
the life and character of those who made the discoveries have been slight;
but I propose to adopt a more directly biographical method in dealing with
Bradley's discoveries, which are so bound up with the simple earnestness
of his character that we could scarcely appreciate their essential
features properly without some biographical study. But the record of his
life apart from his astronomical work is not in any way sensational;
indeed it is singularly devoid of incident. He had not even a scientific
quarrel. There was scarcely a man of science of that period who had not at
least one violent quarrel with some one, save only Bradley, whose gentle
nature seems to have kept him clear of them all. Judged by ordinary
standards his life was uneventful: and yet it may be doubted whether, to
him who lived it, that life contained one dull moment. Incident came for
him in his scientific work: in the preparation of apparatus, the making of
observations, above all in the hard-thinking which he did to get at the
clue which would explain them; and after reviewing his biography,[2] I
think we shall be inclined to admit that if ever there was a happy life,
albeit one of unremitting toil, it was that of James Bradley.
[Sidenote: Bradley's birth and early life.]
[Sidenote: Brief clerical career.]
He was born at Sherbourn, in Gloucestershire, in 1693. We know little of
his boyhood except that he went to the Grammar School at Northleach, and
that the memory of this fact was preserved at the school in 1832 when
Rigaud was writing his memoir. [The school is at present shut up for want
of funds to carry it on; and all inquiries I have made have failed to
elicit any trace of this memory.] Similarly we know little of his
undergraduate days at Oxford, except that he entered as a commoner at
Balliol in 1710, took his B.A. in the regular course in 1714, and his M.A.
in 1717. As a career he chose the Church, being ordained in 1719, and
presented to the vicarage of Bridstow in Monmouthshire; but he only
discharged the duties of vicar for a couple of years, for in 1721 he
returned to Oxford as Professor of Astronomy, an appointment which
involved the resignation of his livings; and so slight was this
interruption to his career as an astronomer that we may almost disregard
it, and consider him as an astronomer from the first. But to guard against
a possible misconception, let me say that Bradley entered on a clerical
career in a thoroughly earnest spirit; to do otherwise would have been
quite foreign to his nature. When vicar of Bridstow he discharged his
duties faithfully towards that tiny parish, and moreover was so active in
his uncle's parish of Wansted that he left the reputation of having been
curate there, although he held no actual appointment. And thirty years
later, when he was Astronomer Royal and resident at Greenwich, and when
the valuable vicarage of Greenwich was offered to him by the Chancellor of
the Exchequer, he honourably refused the preferment, "because the duty of
a pastor was incompatible with his other studies and necessary
engagements."
[Sidenote: Learnt astronomy _not_ at Oxford, but from his uncle, James
Pound.]
[Sidenote: Pound a first-rate observer.]
But now let us turn to Bradley's astronomical education. I must admit,
with deep regret, that we cannot allow any of the credit of it to Oxford.
There was a great astronomer in Oxford when Bradley was an undergraduate,
for Edmund Halley had been appointed Savilian Professor of Geometry in
1703, and had immediately set to work to compute the orbits of comets,
which led to his immortal discovery that some of these bodies return to us
again and again, especially the one which bears his name--Halley's
Comet--and returns every seventy-five years, being next expected about
1910. But there is no record that Bradley came under Halley's teaching or
influence as an undergraduate. In later years the two men knew each other
well, and it was Halley's one desire towards the close of his life that
Bradley should succeed him as Astronomer Royal at Greenwich; a desire
which was fulfilled in rather melancholy fashion, for Halley died without
any assurance that his wish would be gratified. But Bradley got no
astronomical teaching at Oxford either from Halley or others. The art of
astronomical observation he learnt from his maternal uncle, the Rev. James
Pound, Rector of Wansted, in Essex. He is the man to whom we owe Bradley's
training and the great discoveries which came out of it. He was, I am glad
to say, an Oxford man too; very much an Oxford man; for he seems to have
spent some thirteen years there migrating from one Hall to another. His
record indeed was such as good tutors of colleges frown upon; for it was
seven years before he managed to take a degree at all; and he could not
settle to anything. After ten years at Oxford he thought he would try
medicine; after three years more he gave it up and went out in 1700 as
chaplain to the East Indies. But he seems to have been a thoroughly
lovable man, for news was brought of him four years later that he had a
mind to come home, but was dissuaded by the Governor saying that "if Dr.
Pound goes, I and the rest of the Company will not stay behind." Soon
afterwards the settlement was attacked in an insurrection, and Pound was
one of the few who escaped with his life, losing however all the property
he had gradually acquired. He returned to England in 1706, and was
presented to the living of Wansted; married twice, and ended his days in
peace and fair prosperity in 1724. Such are briefly the facts about
Bradley's uncle, James Pound; but the most important of all remains to be
told--that somehow or other he had learnt to make first-rate astronomical
observations, how or when is not recorded; but in 1719 he was already so
skilled that Sir Isaac Newton made him a present of fifty guineas for some
observations; and repeated the gift in the following year; and even three
years before this we find Halley writing to ask for certain observations
from Mr. Pound.
[Sidenote: Bradley worked with him.]
With this excellent man Bradley used frequently to stay. To his nephew he
seems to have been more like a father than an uncle. When his nephew had
smallpox in 1717, he nursed him through it; and he supplemented from his
own pocket the scanty allowance which was all that Bradley's own father
could afford. But what concerns us most is that he fostered, if he did not
actually implant, a love of astronomical observation in his nephew. The
two worked together, entering their observations one after the other on
the same paper; and it was to the pair of them together, rather than to
the uncle alone, that Newton made his princely presents, and Halley wrote
for help in his observations. There seems to be no doubt that the uncle
and nephew were about this time the best astronomical observers in the
world. There was no rivalry between them, and therefore there is no need
to discuss whether the partnership was one of equal merit on both sides;
but it is interesting to note that it probably was. The ability of Pound
was undoubted; many were keenly desirous that he, and not his nephew,
should be elected to the Oxford Chair in 1721, but he felt unequal to the
duties at his advanced age. On the other hand, when Bradley lost his
uncle's help, there was no trace of faltering in his steps to betray
previous dependence on a supporting or guiding hand. He walked erect and
firm, and trod paths where even his uncle might not have been able to
follow.
[Sidenote: The work done by Pound and Bradley.]
[Sidenote: Use of very long telescopes.]
[Sidenote: Reason for great length.]
A few instances will suffice to show the kind of observations made by this
notable firm of Pound and Bradley. They observed the positions of the
fixed stars and nebulae: these being generally the results required by
Halley and Newton. They also observed the places of the planets among the
stars, and especially the planet Mars, and determined its distance from
the Earth by the method of parallax, thus anticipating the modern standard
method of finding the Sun's distance; and though with their imperfect
instruments they did not obtain a greater accuracy than 1 in 10, still
this was a great advance on what had been done before, and excited the
wonder and admiration of Halley. They also paid some attention to double
stars, and did a great deal of work on Jupiter's satellites. We might
profitably linger over the records of these early years, which are full of
interest, but we must press on to the time of the great discoveries, and
we will dismiss them with brief illustrations of three points: Bradley's
assiduity, his skill in calculation, and his wonderful skill in the
management of instruments. Of his assiduity an example is afforded by his
calculations of the orbits of two comets which are still extant. One of
them fills thirty-two pages of foolscap, and the other sixty; and it must
be remembered that the calculations themselves were quite novel at that
time. Of his _skill_ in calculation, apart from his assiduity, we have a
proof in a paper communicated to the Royal Society rather later (1726),
where he determines the longitudes of Lisbon and New York from the
eclipses of Jupiter's satellites, using observations which were not
simultaneous, and had therefore to be corrected by an ingenious process
which Bradley devised expressly for this purpose. And finally, his skill
in the management of instruments is shown by his measuring the diameter of
the planet Venus with a telescope actually 212-1/4 feet in length. It is
difficult for us to realise in these days what this means; even the
longest telescope of modern times does not exceed 100 feet in length, and
it is mounted so conveniently with all the resources of modern
engineering, in the shape of rising floors, &c., that the management of it
is no more difficult than that of a 10-foot telescope. But Bradley had no
engineering appliances beyond a pole to hold up one end of the telescope
and his own clever fingers to work the other; and he managed to point the
unwieldy weapon accurately to the planet, and measure the diameter with an
exactness which would do credit to modern times. A few words of
explanation may be given why such long telescopes were used at all. The
reason lay in the difficulty of getting rid of coloured images, due to the
composite character of white light. Whenever we use a _single_ lens to
form an image, coloured fringes appear. Nowadays we know that by making
two lenses of different kinds of glass and putting them together, we can
practically get rid of these coloured fringes; but this discovery had not
been made in Bradley's time. The only known ways of dealing with the evil
then were to use a reflecting telescope like Newton and Gregory, or if a
lens was used, to make one of very great focal length; and hence the
primary necessity for these very long telescopes. They had another
advantage in producing a large image, or they would probably have given
way to the reflector. This advantage is gradually bringing them back into
use, and perhaps in the eclipse of 1905 we may use a telescope as long as
Bradley's; but we shall not use it as he did in any case. It will be laid
comfortably flat on the ground, and the rays of light reflected into it by
a coelostat.
[Sidenote: Bradley appointed at Oxford, but continues to work at Wansted.]
In 1721 Bradley was appointed to the Savilian Professorship of Astronomy
at Oxford, vacant by the death of Dr. John Keill. Once it became clear
that there was no chance of securing his uncle for this position, Bradley
himself was supported enthusiastically by all those whose support was
worth having, especially by the Earl of Macclesfield, who was then Lord
Chancellor; by Martin Foulkes, who was afterwards the President of the
Royal Society; and by Sir Isaac Newton himself. He was accordingly elected
on October 31, 1721, and forthwith resigned his livings. His resignation
of the livings was necessitated by a definite statute of the University
relating to the Professorship, and not by the existence of any very
onerous duties attaching to it; indeed such duties seem to have been
conspicuously absent, and after Bradley's election he passed more time
than ever with his uncle in Wansted, making the astronomical observations
which both loved; for there was not the vestige of an observatory in
Oxford. His uncle's death in 1724 interrupted the continuity of these
joint observations, and by an odd accident prepared the way for Bradley's
great discovery. He was fain to seek elsewhere that companionship in his
work which had become so essential to him, and his new friend gave a new
bent to his observations.
[Sidenote: Samuel Molyneux.]
[Sidenote: Attempts to find stellar parallax.]
Samuel Molyneux was a gentleman of fortune much attached to science, and
particularly to astronomy, who was living about this time at Kew. He was
one of the few, moreover, who are not content merely to amuse themselves
with a telescope, but had the ambition to do some real earnest work, and
the courage to choose a problem which had baffled the human race for more
than a century. The theory of Copernicus, that the earth moved round the
sun, necessitated a corresponding apparent change in the places of the
stars, one relatively to another; and it was a standing difficulty in the
way of accepting this theory that no such change could be detected. In the
old days before the telescope it was perhaps easy to understand that the
change might be too small to be noticed, but the telescope had made it
possible to measure changes of position at least a hundred times as small
as before, and still no "parallax," as the astronomical term goes, could
be found for the stars. The observations of Galileo, and the measures of
Tycho Brahe, as reduced to systematic laws by Kepler, and finally by the
great Newton, made it clear that the Copernican theory was _true_: but no
one had succeeded in proving its truth in this particular way. Samuel
Molyneux must have been a man of great courage to set himself to try to
crack this hard nut; and we can understand the attraction which his
enterprise must have had for Bradley, who had just lost the beloved
colleague of many courageous astronomical undertakings. His co-operation
seems to have been welcomed from the first; his help was invited and
freely given in setting up the instrument, and he fortunately had the
leisure to spend considerable time at Kew making the observations with
Molyneux, just as he had been wont to observe with his uncle.
I must now briefly explain what these observations were. There is a bright
star [gamma] Draconis, which passes almost directly overhead in the
latitude of London. Its position is slowly changing owing to the
precession of the equinoxes, but for two centuries it has been, and is
still, under constant observation by London astronomers owing to this
circumstance, that it passes directly overhead, and so its position is
practically undisturbed by the refraction of our atmosphere.
[Sidenote: The instrument.]
[Sidenote: Expected results.]
It was therefore thought at the time that, there being no disturbance from
refraction, the disturbance from precession being accurately known, and
there being nothing else to disturb the position but "parallax" (the
apparent shift due to the earth's motion which it was desirable to find),
this star ought to be a specially favourable object for the determination
of parallax. Indeed it had been announced many years before by Hooke that
its parallax had been found; but his observations were not altogether
satisfactory, and it was with a view of either confirming them or seeing
what was wrong with them that Molyneux and Bradley started their search.
They set up a much more delicate piece of apparatus than Hooke had
employed. It was a telescope 24 feet long pointed upwards to the star, and
firmly attached to a large stack of brick chimneys within the house. The
telescope was not absolutely fixed, for the lower end could be moved by a
screw so as to make it point accurately to the star, and a plumb-line
showed how far it was from the vertical when so pointing. Hence if the
star changed its position, however slightly, the reading of this screw
would show the change. Now, before setting out on the observations, the
observers knew what to expect if the star had a real parallax; that is to
say, they knew that the star would seem to be farthest south in December,
farthest north in June, and at intermediate positions in March and
September; though they did not know _how much_ farther south it would
appear in December than in June--this was exactly the point to be decided.
[Illustration: FIG. 2.]
[Sidenote: Unexpected results.]
The reason of this will be clear from Fig. 2. [Remark, however, that this
figure and the corresponding figure 4 do not represent the case of
Bradley's star, [gamma] Draconis: another star has been chosen which
simplifies the diagram, though the principle is essentially the same.] Let
A B C D represent the earth's orbit, the earth being at A in June, at B in
September, and so on, and let K represent the position of the star on the
line D B. Then in March and September it will be seen from the earth in
the same direction, namely, D B K; but the directions in which it is seen
in June and December, viz. A K and C K, are inclined in opposite ways to
this line. The farther away the star is, the less will this inclination or
"parallax" be; and the star is actually so far away that the inclination
can only be detected with the utmost difficulty: the lines C K and A K are
sensibly parallel to D B K. But Bradley did not know this; it was just
this point which he was to examine, and he expected the greatest
inclination in one direction to be in December. Accordingly when a few
observations had been made on December 3, 5, 11, and 12 it was thought
that the star had been caught at its most southerly apparent position, and
might be expected thereafter to move northwards, if at all. But when
Bradley repeated the observation on December 17, he found to his great
surprise that the star was still moving southwards. Here was something
quite new and unexpected, and such a keen observer as Bradley was at once
on the alert. He soon found that the changes in the position of the star
were of a totally unexpected character. Instead of the extreme positions
being occupied in June and December, they were occupied in March and
September, just midway between these. And the range in position was quite
large, about 40"--not a quantity which could have been detected in the
days before telescopes, but one which was unmistakable with an instrument
of the most moderate measuring capacity.
[Sidenote: Tentative explanations.]
What, then, was the cause of this quite unforeseen behaviour on the part
of the star? The first thought of the observers was that something might
be wrong with their instrument, and it was carefully examined, but without
result. The next was that the apparent movement was in the plumb-line, the
line of reference. If the whole earth, instead of carrying its axis round
the sun in a constant direction, were to be executing an oscillation, then
all our plumb-lines would oscillate, and when the direction of a star like
[gamma] Draconis was compared with that of the plumb-line it would seem to
vary, owing actually to the variation in the plumb-line. The earth might
have a motion of this kind in two ways, which it will be necessary for us
to distinguish, and the adopted names for them are "nutation of the axis"
and "variation of latitude" respectively. In the case of nutation the
North Pole remains in the same geographical position, but points to a
different part of the heavens. The "variation of latitude," on the other
hand, means that the North Pole wanders about on the earth itself. We
shall refer to the second phenomenon more particularly in the sixth
chapter.
[Sidenote: Nutation?]
[Sidenote: Anomalous refraction.]
But it was the first kind of change, the nutation, which Bradley
suspected; and very early in the series of observations he had already
begun to test this hypothesis. If it was not the star, but the earth and
the plumb-line, which were in motion, then other stars ought to be
affected. The telescope had been deliberately restricted in its position
to suit [gamma] Draconis; but since the stars circle round the Pole, if we
draw a narrow belt in the heavens with the Pole as centre, and including
[gamma] Draconis, the other stars included would make the same circuit,
preceding or following [gamma] Draconis by a constant interval. Most of
them would be too faint for observation with Bradley's telescope; but
there was one bright enough to be observed, which also came within its
limited range, and it was promptly put under _surveillance_ when a
nutation of the earth's axis was suspected. Careful watching showed that
it was not affected in the same way as [gamma] Draconis, and hence the
movement could not be in the plumb-line. Was there, then, after all, some
effect of the earth's atmosphere which had been overlooked? We have
already remarked that since the star passes directly overhead there
should be practically no refraction; and this assumption was made by
Molyneux and Bradley in choosing this particular star for observation. It
follows at once, if we assume that the atmosphere surrounds the earth in
spherical layers. But perhaps this was not so? Perhaps, on the contrary,
the atmosphere was deformed by the motion of the earth, streaming out
behind her like the smoke of a moving engine? No possibility must be
overlooked if the explanation of this puzzling fact was to be got at.
[Illustration: FIG. 3.]
The way in which a deformation of the atmosphere might explain the
phenomenon is best seen by a diagram. First, it must be remarked that rays
of light are only bent by the earth's atmosphere, or "refracted," if they
enter it obliquely.
If the atmosphere were of the same density throughout, like a piece of
glass, then a vertical ray of light, A B (see Fig. 3), entering the
atmosphere at B would suffer no bending or refraction, and a star shining
from the direction A B would be seen truly in that direction from C. But
an oblique ray, D E, would be bent on entering the atmosphere at E along
the path EF, and a star shining along D E would appear from F to be
shining along the dotted line G E F. The atmosphere is not of the same
density throughout, but thins out as we go upwards from the earth; and in
consequence there is no clear-cut surface, B E, and no sudden bending of
the rays as at E: they are gradually bent at an infinite succession of
imaginary surfaces. But it still remains true that there is no bending at
all for vertical rays; and of oblique rays those most oblique are most
bent.
[Illustration: FIG. 4.]
Now, suppose the atmosphere of the earth took up, owing to its revolution
round the sun, an elongated shape like that indicated in diagram 4, and
suppose the star to be at a great distance away to the right of the
diagram. When the earth is in the position labelled "June," the light
would fall vertically on the nose of the atmosphere at A, and there would
be no refraction. Similarly in "December" the light would fall at C on the
stern, also vertically, and there would be no refraction. [The rays from
the distant star in December are to be taken as sensibly parallel to those
received in June, notwithstanding that the earth is on the opposite side
of the sun, as was remarked on p. 98.] But in March and September the rays
would strike obliquely on the sides of the supposed figure, and thus be
bent in opposite directions, as indicated by the dotted lines; and the
extreme positions would thus occur in March and September, as had been
observed. The explanation thus far seems satisfactory enough.
But we have assumed the star to lie in the plane of the earth's orbit; and
the stars under observation by Bradley did not lie in this plane, nor did
they lie in directions equally inclined to it. Making the proper allowance
for their directions, it was found impossible to fit in the facts with
this hypothesis, which had ultimately to be abandoned.
[Sidenote: Delay in finding real explanation.]
[Sidenote: Bradley sets up another instrument at Wansted.]
[Sidenote: Finds the right clue.]
[Sidenote: A wind-vane on a boat.]
It is remarkable to find that two or three years went by before the real
explanation of this new phenomenon occurred to Bradley, and during this
time he must have done some hard thinking. We have all had experience of
the _kind_ of thinking if only in the guessing of conundrums. We know the
apparent hopelessness of the quest at the outset: the racking of our
brains for a clue, the too frequent despair and "giving it up," and the
simplicity of the answer when once it is declared. But with scientific
conundrums the expedient of "giving it up" is not available. We must find
the answer for ourselves or remain in ignorance; and though we may feel
sure that the answer when found will be as simple as that to the best
conundrum, this expected simplicity does not seem to aid us in the search.
Bradley was not content with sitting down to think: he set to work to
accumulate more facts. Molyneux's instrument only allowed of the
observation of two stars, [gamma] Draconis and the small star above
mentioned. Bradley determined to have an instrument of his own which
should command a wider range of stars; and by this time he was able to
return to his uncle's house at Wansted for this purpose. His uncle had
been dead for two or three years, and the memory of the loss was becoming
mellowed with time. His uncle's widow was only too glad to welcome back
her nephew, though no longer to the old rectory, and she allowed him to
set up a long telescope, even though he cut holes in her floor to pass it
through. The object-glass end was out on the roof and the eye end down in
the coal cellar; and accordingly in this coal cellar Bradley made the
observations which led to his immortal discovery. He had a list of
seventy stars to observe, fifty of which he observed pretty regularly. It
may seem odd that he did not set up this new instrument at Oxford, but we
find from an old memorandum that his professorship was not bringing him in
quite L140 a year, and probably he was glad to accept his aunt's
hospitality for reasons of economy. By watching these different stars he
gradually got a clear conception of the laws of aberration. The real
solution of the problem, according to a well-authenticated account,
occurred to him almost accidentally. We all know the story of the apple
falling and setting Newton to think about the causes of gravitation. It
was a similarly trivial circumstance which suggested to Bradley the
explanation which he had been seeking for two or three years in vain. In
his own words, "at last, when he despaired of being able to account for
the phenomena which he had observed, a satisfactory explanation of them
occurred to him all at once when he was not in search of it." He
accompanied a pleasure party in a sail upon the river Thames. The boat in
which they were was provided with a mast which had a vane at the top of
it. It blew a moderate wind, and the party sailed up and down the river
for a considerable time. Dr. Bradley remarked that every time the boat put
about 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. He observed this
three or four times without speaking; at last he mentioned it to the
sailors, and expressed his surprise that the wind should shift so
regularly every time they put about. The sailors told him that the wind
had not shifted, but that the apparent change was owing to the change in
the direction of the boat, and assured him that the same thing invariably
happened in all cases. This accidental observation led him to conclude
that the phenomenon which had puzzled him so much was owing to the
combined motion of light and of the earth. To explain exactly what is
meant we must again have recourse to a diagram; and we may also make use
of an illustration which has become classical.
[Illustration: FIG. 5.]
[Sidenote: Analogy of rain.]
If rain is falling vertically, as represented by the direction A B; and if
a pedestrian is walking horizontally in the direction C D, the rain will
appear to him to be coming in an inclined direction, E F, and he will find
it better to tilt his umbrella forwards. The quicker his pace the more he
will find it advisable to tilt the umbrella. This analogy was stated by
Lalande before the days of umbrellas in the following words: "Je suppose
que, dans un temps calme, la pluie tombe perpendiculairement, et qu'on
soit dans une voiture ouverte sur le devant; si la voiture est en repos,
on ne recoit pas la moindre goutte de pluie; si la voiture avance avec
rapidite, la pluie entre sensiblement, comme si elle avoit pris une
direction oblique." Lalande's example, modified to suit modern conditions,
has been generally adopted by teachers, and in examinations candidates
produce graphic pictures of the stationary, the moderate-paced, and the
flying, possessors of umbrellas.
[Sidenote: Aberration.]
Applying it to the phenomenon which it is intended to illustrate, if light
is being received from a star by an earth, travelling across the direction
of the ray, the telescope (which in this case represents the umbrella)
must be tilted forward to catch the light. Now on reference to Fig. 4 it
will be seen that the earth is travelling across the direction of rays
from the star in March and September; and in opposite directions in the
two cases. Hence the telescope must be tilted a little, in opposite
directions, to catch the light; or, in other words, the star will appear
to be farthest south in March, farthest north in September. And so at last
the puzzle was solved, and the solution was found, as so often happens, to
be of the simplest kind; so simple when once we know, and so terribly hard
to imagine when we don't! It may comfort us in our struggles with minor
problems to reflect that Bradley manfully stuck to his problem for two or
three years. It was probably never out of his thoughts, waking or
sleeping; when at work it was the chief object of his labours, and when on
a pleasure party he was ready to catch at the slightest clue, in the
motion of a wind-vane on a boat, which might help him to the solution.
[Sidenote: Results of discovery.]
The discovery of aberration made Bradley famous at a bound. Oxford might
well be proud of her two Savilian Professors at this time, for they had
both made world-famous discoveries--Halley that of the periodicity of
comets, and Bradley of the aberration of light. How different their tastes
were and how difficult it would have been for either to do the work of the
other! Bradley was no great mathematician, and though he was quite able to
calculate the orbit of a comet, and carried on such work when Halley left
it, it was probably not congenial to him. Halley, on the other hand,
almost despised accurate observations as finicking. "Be sure you are
correct to a minute," he was wont to say, "and the fractions do not so
much matter." With such a precept Bradley would never have made his
discoveries. No quantity was too small in his eyes, and no sooner was the
explanation of aberration satisfactorily established than he perceived
that though it would account for the main facts, it would not explain all.
There was something left. This is often the case in the history of
science. A few years ago it was thought that we knew the constitution of
our air completely--oxygen, nitrogen, water vapour, and carbonic acid gas;
but a great physicist, Lord Rayleigh, found that after extracting all the
water and carbonic acid gas, all the oxygen and all the nitrogen, there
was something left--a very minute residuum, which a careless experimenter
would have overlooked or neglected, but which a true investigator like
Lord Rayleigh saw the immense importance of. He kept his eye on that
something left, and presently discovered a new gas which we now know as
argon. Had he repeated the process, extracting all the argon after the
nitrogen, he might have found by a scrutiny much more accurate still yet
another gas, helium, which we now know to exist in extremely minute
quantities in the air. But meantime this discovery was made in another
way.
[Sidenote: Still something to be explained.]
[Sidenote: Probably nutation.]
[Sidenote: His nineteen years' campaign.]
When Bradley had extracted all the aberration from his observations he
found that there was something left, another problem to be solved and some
more thinking to be done to solve it. But he was now able to profit by his
previous labours, and the second step was made more easily than the first.
The residuum was not the parallax of which he had originally been in
search, for it did not complete a cycle within the year; it was rather a
progressive change from year to year. But there was an important clue of
another kind. He saw that the apparent movements of all stars were in
this case the same; and he knew that a movement of this kind can be
referred, not to the stars themselves, but to the plumb-line from which
their directions are measured. He had thought out the possible causes of
such a movement of the plumb-line or of the earth itself, and had realised
that there might be a _nutation_ which would go through a cycle in about
nineteen years, the period in which the moon's nodes revolve. He was not
mathematician enough to work out the cause completely, but he saw clearly
that to trace the whole effect he must continue the observations for
nineteen years; and accordingly he entered on this long campaign without
any hesitation. His instrument was still that in his aunt's house at
Wansted, where he continued to live and make the observations for a few
years, but in 1732 he removed to Oxford, as we shall see, and he must have
made many journeys between Wansted and Oxford in the course of the
remaining fifteen years during which he continued to trace out the effects
of nutation. His aunt too left Wansted to accompany Bradley to Oxford, and
the house passed into other hands. It is to the lasting credit of the new
occupant, Mrs. Elizabeth Williams, that the great astronomer was allowed
to go on and complete the valuable series of observations which he had
commenced. Bradley was not lodged in her house; he stayed with a friend
close by on his visits to Wansted, but came freely in and out of his
aunt's old home to make his observations. How many of us are there who
would cheerfully allow an astronomer to enter our house at any hour of the
night to make observations in the coal-cellar! It says much, not only for
Bradley's fame, but for his personal attractiveness, that he should have
secured this permission, and that there should be no record of any
friction during these fifteen years. At the end of the whole series of
nineteen years his conclusions were abundantly verified, and his second
great discovery of nutation was established. Honours were showered upon
him, and no doubt the gentle heart of Mrs. Elizabeth Williams was uplifted
at the glorious outcome of her long forbearance.
[Sidenote: Residence at Oxford.]
But we may now turn for a few moments from Bradley's scientific work to
his daily life. We have said that in 1732, after holding his professorship
for eleven years, he first went definitely to reside in Oxford. He
actually had not been able to afford it previously. His income was only
L140 a year, and the statutes prevented him from holding a living: so
that he was fain to accept Mrs. Pound's hospitable shelter. But in 1729 an
opportunity of adding to his income presented itself, by giving lectures
in "experimental philosophy." The observations on nutation were not like
those on aberration: he was not occupied day and night trying to find the
solution: he had practically made up his mind about the solution, and the
actual observations were to go on in a quiet methodical manner for
nineteen years, so that he now had leisure to look about him for other
employment. Dr. Keill, who had been Professor of Astronomy before Bradley,
had attracted large classes to lectures, not on astronomy, but on
experimental philosophy: but had sold his apparatus and goodwill to Mr.
Whiteside, of Christ Church, one of the candidates who were disappointed
by Bradley's election. In 1729 Bradley purchased the apparatus from
Whiteside, and began to give lectures in experimental philosophy. His
discovery of aberration had made him famous, so that his classes were
large from the first, and paid him considerable fees. Suddenly therefore
he changed his poverty for a comfortable income, and he was able to live
in Oxford in one of two red brick houses in New College Lane, which were
in those days assigned to the Savilian Professors (now inhabited by New
College undergraduates). His aunt, Mrs. Pound, to whom he was devotedly
attached, came with him, and two of her nephews. In his time of prosperity
Bradley was thus able to return the hospitality which had been so
generously afforded him in times of stress.
[Sidenote: Astronomer Royal at Greenwich.]
[Sidenote: Letter from Earl of Macclesfield.]
Before he completed his observations for nutation another great change in
his fortunes took place. In 1742 he was elected to succeed Halley as
Astronomer Royal. It was Halley's dying wish that Bradley should succeed
him, and it is said that he was even willing to resign in his favour, for
his right hand had been attacked by paralysis, and the disease was
gradually spreading. But he died without any positive assurance that his
wish would be fulfilled. The chief difficulty in securing the appointment
of Bradley seems to have been that he was the obvious man for the post in
universal opinion. "It is not only my friendship for Mr. Bradley that
makes me so ardently wish to see him possessed of the position," wrote the
Earl of Macclesfield to the Lord Chancellor; "it is my real concern for
the honour of the nation with regard to science. For as our credit and
reputation have hitherto not been inconsiderable amongst the astronomical
part of the world, I should be extremely sorry we should forfeit it all at
once by bestowing upon a man of inferior skill and abilities the most
honourable, though not the most lucrative, post in the profession (a post
so well filled by Dr. Halley and his predecessor), when at the same time
we have amongst us a man known by all the foreign, as well as our own
astronomers, not to be inferior to either of them, and one whom Sir Isaac
Newton was pleased to call the best astronomer in Europe." And again, "As
Mr. Bradley's abilities in astronomical learning are allowed and confessed
by all, so his character in every respect is so well established, and so
unblemished, that I may defy the worst of his enemies (if so good and
worthy a man have any) to make even the lowest or most trifling objection
to it."
"After all," the letter goes on, "it may be said if Mr. Bradley's skill is
so universally acknowledged, and his character so established, there is
little danger of opposition, since no competitor can entertain the least
hope of success against him. But, my lord, we live in an age when most men
how little soever their merit may be, seem to think themselves fit for
whatever they can get, and often meet with some people, who by their
recommendations of them appear to entertain the same opinion of them, and
it is for this reason that I am so pressing with your lordship not to lose
any time."
Such recommendations had, however, their effect: the dreaded possibility
of a miscarriage of justice was averted, and Bradley became the third
Astronomer Royal, though he did not resign his professorship at Oxford.
Halley, Bradley, and Bliss, who were Astronomers Royal in succession, all
held the appointment along with one of the Savilian professorships at
Oxford; but since the death of Bliss in 1761, the appointment has always
gone to a Cambridge man.
[Sidenote: Instruments very defective.]
When Bradley went to Greenwich, in June 1742, he was at first unable to do
much from the wretched state in which he found the instruments. Halley was
not a good observer: his heart was not in the work, and he had not taken
the trouble to set the instruments right when they went wrong. The
counterpoises of that instrument which ought to have been the best in the
world at the time rubbed against the roof so that the telescope could
scarcely be moved in some positions: and some of the screws were broken.
There was no proper means of illuminating the cross-wires, and so on. With
care and patience Bradley set all this right, and began observations. He
had the good fortune to secure the help of his nephew, John Bradley, as
assistant, and the companionship seems to have been as happy as that
previous one of James Bradley and his uncle Pound. John Bradley was able
to carry on the observations when his uncle was absent in Oxford, and the
work the two got through together in the first year is (in the words of
Bradley's biographer Rigaud) "scarcely to be credited." The transit
observations occupy 177 folio pages, and no less than 255 observations
were taken on one night. And at the same time, it must be remembered,
Bradley was still carrying on his nutation observations at Wansted, still
lecturing at Oxford, and not content with all this, began a course of
experiments on the length of the seconds' pendulum. Truly a giant for hard
work!
[Sidenote: New instruments.]
But, in spite of his care in setting them right, the instruments in the
Observatory were found to be hopelessly defective. The history of the
instruments at the Royal Observatory is a curious one. When Flamsteed was
appointed the first Astronomer Royal he was given the magnificent salary
of L100 a year, and no instruments to observe with. He purchased some
instruments with his own money, and at his death they were claimed by his
executors. Hence Halley, the second Astronomer Royal, found the
Observatory totally unprovided in this respect. He managed to persuade the
nation to furnish the funds for an equipment; but Halley, though a man of
great ability in other ways, did not know a good instrument from a bad
one; so that Bradley's first few years at the Observatory were wasted
owing to the imperfection of the equipment. When this was fully realised
he asked for funds to buy new instruments, and such was the confidence
felt in him that he got what he asked for without much difficulty. More
than L1000, a large sum for those days, was spent under his direction,
the principal purchases being two quadrants for observation of the
position of the stars, one to the north and the other to the south. With
these quadrants, which represented the perfection of such apparatus at
that time, Bradley made that long and wonderful series of observations
which is the starting-point of our knowledge of the movements of the
stars. The instruments are still in the Royal Observatory, the more
important of the two in its original position as Bradley mounted it and
left it.
[Sidenote: Work at Greenwich.]
It seems needless to mention his work as Astronomer Royal, but I will give
quite briefly a summary of what he accomplished, and then recall a
particular incident, which shows how far ahead of his generation his
genius for observation placed him. The summary may be given as follows.
We owe to Bradley--
1. A better knowledge of the movements of Jupiter's satellites.
2. The orbits of several comets calculated directly from his own
observations, when such work was new and difficult.
3. Experiments on the length of the pendulum.
4. The foundation of our knowledge of the refraction of our atmosphere.
5. Considerable improvements in the tables of the moon, and the promotion
of the method for finding the longitude by lunar distances.
6. The proper equipment of our national Observatory with instruments, and
the use of these to form the basis of our present knowledge of the
positions and motions of the stars.
Many men would consider any one of these six achievements by itself a
sufficient title to fame. Bradley accomplished them all in addition to his
great discoveries of aberration and nutation.
[Sidenote: Might have found variation of latitude.]
And with a little more opportunity he might have added another great
discovery which has shed lustre on the work of the last decade. We said
earlier in this chapter that the axis of the earth may move in one or two
ways. Either it may point to a different star, remaining fixed relatively
to the earth, as in the nutation which Bradley discovered; or it may
actually change its position in the earth. This second kind of movement
was believed until twenty years ago not to exist appreciably; but the
work of Kuestner and Chandler led to the discovery that it did exist, and
its complexities have been unravelled, and will be considered in the sixth
chapter. Now a century and a half ago Bradley was on the track of this
"variation of latitude." His careful observations actually showed the
motion of the pole, as Mr. Chandler has recently demonstrated; and,
moreover, Bradley himself noticed that there was something unexplained.
Once again there was a _residuum_ after (first) aberration and (next)
nutation had been extracted from the observations; and with longer life he
might have explained this residuum, and added a third great discovery to
the previous two. Or another coming after him might have found it; but
after the giant came men who could not tread in his footsteps, and the
world waited 150 years before the discrepancy was explained.
[Sidenote: Oxford's tardy recognition of Bradley.]
The attitude of our leading universities towards science and scientific
men is of sufficient importance to justify another glance at the relations
between Bradley and Oxford. We have seen that Oxford's treatment of
Bradley was not altogether satisfactory. She left him to learn astronomy
as he best could, and he owes no teaching to her. She made him Professor
of Astronomy, but gave him no observatory and a beggarly income which he
had to supplement by giving lectures on a different subject. But when he
had disregarded these discouragements and made a name for himself, Oxford
took her share in recognition. He was created D.D. by diploma in 1742; and
when his discovery of nutation was announced in 1748, and produced
distinctions and honours of all kinds from over the world, we are told
that "amidst all these distinctions, wide as the range of modern science,
and permanent as its history, there was one which probably came nearer his
heart, and was still more gratifying to his feeling than all. Lowth
(afterwards Bishop of London), a popular man, an elegant scholar, and
possessed of considerable eloquence, had in 1751 to make his last speech
in the Sheldonian Theatre at Oxford as Professor of Poetry. In recording
the benefits for which the University was indebted to its benefactors, he
mentioned the names of those whom Sir Henry Savile's foundation had
established there: 'What men of learning! what mathematicians! we owe to
Savile, Briggs, Wallis, Halley; to Savile we owe Greaves, Ward, Wren,
Gregory, Keill, and one whom I will not name, for posterity will ever have
his name on its lips.' Bradley was himself present; there was no one in
the crowded assembly on whom the allusion was lost, or who did not feel
the truth and justice of it; all eyes were turned to him, while the walls
rung with shouts of heartfelt affection and admiration; it was like the
triumph of Themistocles at the Olympic games."
[Sidenote: The study of "residual phenomena."]
These words of Rigaud indicate the fame deservedly acquired by an earnest
and simple-minded devotion to science: but can we learn anything from the
study of Bradley's work to guide us in further research? The chief lessons
would seem to be that if we make a series of careful observations, we
shall probably find some deviation from expectation: that we must follow
up this clue until we have found some explanation which fits the facts,
not being discouraged if we cannot hit upon the explanation at once, since
Bradley himself was puzzled for several years: that after finding one
_vera causa_, and allowing for the effect of it, the observations may show
traces of another which must again be patiently hunted, even though we
spend nineteen years in the chase: and that again we may have to leave the
complete rectification of the observations to posterity. But though we may
admit the general helpfulness of these directions, and that this patient
dealing with residual phenomena seems to be a method capable of frequent
application, we cannot deduce any universal principle of procedure from
them: witness the difficulty of dealing with meteorological observations,
for instance. It is not always possible to find any orderly arrangement of
the residuals which will give us a clue to start with. When such an
arrangement is manifested, we must certainly follow up the clue; it would
almost seem that no expense should be prohibitive, since it is impossible
to foresee the importance of the result.