he came to devote his attention to astronomy. It appears that when
he found himself in the possession of leisure and of means, he
deliberately cast around to think how that means and that leisure
could be most usefully employed. Nor was it surprising that he
should search for a direction which would offer special scope for his
mechanical tastes. He came to the conclusion that the building of
great telescopes was an art which had received no substantial advance
since the great days of William Herschel. He saw that to construct
mighty instruments for studying the heavens required at once the
command of time and the command of wealth, while he also felt that
this was a subject the inherent difficulties of which would tax to
the uttermost whatever mechanical skill he might possess. Thus it
was he decided that the construction of great telescopes should
become the business of his life.
[PLATE: BIRR CASTLE.
PLATE: THE MALL, PARSONSTOWN.]
In the centre of Ireland, seventy miles from Dublin, on the border
between King's County and Tipperary, is a little town whereof we must
be cautious before writing the name. The inhabitants of that town
frequently insist that its name is Birr, * while the official
designation is Parsonstown, and to this day for every six people who
apply one name to the town, there will be half a dozen who use the
other. But whichever it may be, Birr or Parsonstown--and I shall
generally call it by the latter name--it is a favourable specimen of
an Irish county town. The widest street is called the Oxmantown
Mall. It is bordered by the dwelling-houses of the chief residents,
and adorned with rows of stately trees. At one end of this
distinctly good feature in the town is the Parish Church, while at
the opposite end are the gates leading into Birr Castle, the
ancestral home of the house of Parsons. Passing through the gates
the visitor enters a spacious demesne, possessing much beauty of wood
and water, one of the most pleasing features being the junction of
the two rivers, which unite at a spot ornamented by beautiful
timber. At various points illustrations of the engineering skill of
the great Earl will be observed. The beauty of the park has been
greatly enhanced by the construction of an ample lake, designed with
the consummate art by which art is concealed. Even in mid-summer it
is enlivened by troops of wild ducks preening themselves in that
confidence which they enjoy in those happy localities where the sound
of a gun is seldom heard. The water is led into the lake by a tube
which passes under one of the two rivers just mentioned, while the
overflow from the lake turns a water-wheel, which works a pair of
elevators ingeniously constructed for draining the low-lying parts of
the estate.
* Considering the fame acquired by Parsonstown from Lord Rosse's
mirrors, it may be interesting to note the following extract from
"The Natural History of Ireland," by Dr. Gerard Boate, Thomas
Molyneux M.D., F.R.S., and others, which shows that 150 years ago
Parsonstown was famous for its glass:--
"We shall conclude this chapter with the glass, there having been
several glasshouses set up by the English in Ireland, none in Dublin
or other cities, but all of them in the country; amongst which the
principal was that of Birre, a market town, otherwise called
Parsonstown, after one Sir Lawrence Parsons, who, having purchased
that lordship, built a goodly house upon it; his son William Parsons
having succeeded him in the possession of it; which town is situate
in Queen's County, about fifty miles (Irish) to the southwest of
Dublin, upon the borders of the two provinces of Leinster and Munster;
from this place Dublin was furnished with all sorts of window and
drinking glasses, and such other as commonly are in use. One part of
the materials, viz., the sand, they had out of England; the other,
to wit the ashes, they made in the place of ash-tree, and used no
other. The chiefest difficulty was to get the clay for the pots to
melt the materials in; this they had out of the north."--Chap. XXI.,
Sect. VIII. "Of the Glass made in Ireland."
Birr Castle itself is a noble mansion with reminiscences from the
time of Cromwell. It is surrounded by a moat and a drawbridge of
modern construction, and from its windows beautiful views can be had
over the varied features of the park. But while the visitors to
Parsonstown will look with great interest on this residence of an
Irish landlord, whose delight it was to dwell in his own country, and
among his own people, yet the feature which they have specially come
to observe is not to be found in the castle itself. On an extensive
lawn, sweeping down from the moat towards the lake, stand two noble
masonry walls. They are turreted and clad with ivy, and considerably
loftier than any ordinary house. As the visitor approaches, he will
see between those walls what may at first sight appear to him to be
the funnel of a steamer lying down horizontally. On closer approach
he will find that it is an immense wooden tube, sixty feet long, and
upwards of six feet in diameter. It is in fact large enough to admit
of a tall man entering into it and walking erect right through from
one end to the other. This is indeed the most gigantic instrument
which has ever been constructed for the purpose of exploring the
heavens. Closely adjoining the walls between which the great tube
swings, is a little building called "The Observatory." In this the
smaller instruments are contained, and there are kept the books which
are necessary for reference. The observatory also offers shelter to
the observers, and provides the bright fire and the cup of warm tea,
which are so acceptable in the occasional intervals of a night's
observation passed on the top of the walls with no canopy but the
winter sky.
Almost the first point which would strike the visitor to Lord Rosse's
telescope is that the instrument at which he is looking is not only
enormously greater than anything of the kind that he has ever seen
before, but also that it is something of a totally different nature.
In an ordinary telescope he is accustomed to find a tube with lenses
of glass at either end, while the large telescopes that we see in our
observatories are also in general constructed on the same principle.
At one end there is the object-glass, and at the other end the
eye-piece, and of course it is obvious that with an instrument of
this construction it is to the lower end of the tube that the eye of
the observer must be placed when the telescope is pointed to the
skies. But in Lord Rosse's telescope you would look in vain for
these glasses, and it is not at the lower end of the instrument that
you are to take your station when you are going to make your
observations. The astronomer at Parsonstown has rather to avail
himself of the ingenious system of staircases and galleries, by which
he is enabled to obtain access to the mouth of the great tube. The
colossal telescope which swings between the great walls, like
Herschel's great telescope already mentioned, is a reflector, the
original invention of which is due of course to Newton. The optical
work which is accomplished by the lenses in the ordinary telescope is
effected in the type of instrument constructed by Lord Rosse by a
reflecting mirror which is placed at the lower end of the vast tube.
The mirror in this instrument is made of a metal consisting of two
parts of copper to one of tin. As we have already seen, this mixture
forms an alloy of a very peculiar nature. The copper and the tin
both surrender their distinctive qualities, and unite to form a
material of a very different physical character. The copper is tough
and brown, the tin is no doubt silvery in hue, but soft and almost
fibrous in texture. When the two metals are mixed together in the
proportions I have stated, the alloy obtained is intensely hard and
quite brittle being in both these respects utterly unlike either of
the two ingredients of which it is composed. It does, however,
resemble the tin in its whiteness, but it acquires a lustre far
brighter than tin; in fact, this alloy hardly falls short of silver
itself in its brilliance when polished.
[PLATE: LORD ROSSE'S TELESCOPE. From a photograph by W. Lawrence,
Upper Sackville Street, Dublin.]
The first duty that Lord Rosse had to undertake was the construction
of this tremendous mirror, six feet across, and about four or five
inches thick. The dimensions were far in excess of those which had
been contemplated in any previous attempt of the same kind. Herschel
had no doubt fashioned one mirror of four feet in diameter, and many
others of smaller dimensions, but the processes which he employed had
never been fully published, and it was obvious that, with a large
increase in dimensions, great additional difficulties had to be
encountered. Difficulties began at the very commencement of the
process, and were experienced in one form or another at every
subsequent stage. In the first place, the mere casting of a great
disc of this mixture of tin and copper, weighing something like three
or four tons, involved very troublesome problems. No doubt a casting
of this size, if the material had been, for example, iron, would have
offered no difficulties beyond those with which every practical
founder is well acquainted, and which he has to encounter daily in
the course of his ordinary work. But speculum metal is a material of
a very intractable description. There is, of course, no practical
difficulty in melting the copper, nor in adding the proper proportion
of tin when the copper has been melted. There may be no great
difficulty in arranging an organization by which several crucibles,
filled with the molten material, shall be poured simultaneously so as
to obtain the requisite mass of metal, but from this point the
difficulties begin. For speculum metal when cold is excessively
brittle, and were the casting permitted to cool like an ordinary
copper or iron casting, the mirror would inevitably fly into pieces.
Lord Rosse, therefore, found it necessary to anneal the casting with
extreme care by allowing it to cool very slowly. This was
accomplished by drawing the disc of metal as soon as it had entered
into the solid state, though still glowing red, into an annealing
oven. There the temperature was allowed to subside so gradually,
that six weeks elapsed before the mirror had reached the temperature
of the external air. The necessity for extreme precaution in the
operation of annealing will be manifest if we reflect on one of the
accidents which happened. On a certain occasion, after the cooling
of a great casting had been completed, it was found, on withdrawing
the speculum, that it was cracked into two pieces. This mishap was
eventually traced to the fact that one of the walls of the oven had
only a single brick in its thickness, and that therefore the heat had
escaped more easily through that side than through the other sides
which were built of double thickness. The speculum had,
consequently, not cooled uniformly, and hence the fracture had
resulted. Undeterred, however, by this failure, as well as by not a
few other difficulties, into a description of which we cannot now
enter, Lord Rosse steadily adhered to his self-imposed task, and at
last succeeded in casting two perfect discs on which to commence the
tedious processes of grinding and polishing. The magnitude of the
operations involved may perhaps be appreciated if I mention that the
value of the mere copper and tin entering into the composition of
each of the mirrors was about 500 pounds.
In no part of his undertaking was Lord Rosse's mechanical ingenuity
more taxed than in the devising of the mechanism for carrying out the
delicate operations of grinding and polishing the mirrors, whose
casting we have just mentioned. In the ordinary operations of the
telescope-maker, such processes had hitherto been generally effected
by hand, but, of course, such methods became impossible when dealing
with mirrors which were as large as a good-sized dinner table, and
whose weight was measured by tons. The rough grinding was effected
by means of a tool of cast iron about the same size as the mirror,
which was moved by suitable machinery both backwards and forwards,
and round and round, plenty of sand and water being supplied between
the mirror and the tool to produce the necessary attrition. As the
process proceeded and as the surface became smooth, emery was used
instead of sand; and when this stage was complete, the grinding tool
was removed and the polishing tool was substituted. The essential
part of this was a surface of pitch, which, having been temporarily
softened by heat, was then placed on the mirror, and accepted from
the mirror the proper form. Rouge was then introduced as the
polishing powder, and the operation was continued about nine hours,
by which time the great mirror had acquired the appearance of highly
polished silver. When completed, the disc of speculum metal was
about six feet across and four inches thick. The depression in the
centre was about half an inch. Mounted on a little truck, the great
speculum was then conveyed to the instrument, to be placed in its
receptacle at the bottom of the tube, the length of which was sixty
feet, this being the focal distance of the mirror. Another small
reflector was inserted in the great tube sideways, so as to direct
the gaze of the observer down upon the great reflector. Thus was
completed the most colossal instrument for the exploration of the
heavens which the art of man has ever constructed.
[PLATE: ROMAN CATHOLIC CHURCH AT PARSONSTOWN.]
It was once my privilege to be one of those to whom the illustrious
builder of the great telescope entrusted its use. For two seasons in
1865 and 1866 I had the honour of being Lord Rosse's astronomer.
During that time I passed many a fine night in the observer's
gallery, examining different objects in the heavens with the aid of
this remarkable instrument. At the time I was there, the objects
principally studied were the nebulae, those faint stains of light
which lie on the background of the sky. Lord Rosse's telescope was
specially suited for the scrutiny of these objects, inasmuch as their
delicacy required all the light-grasping power which could be
provided.
One of the greatest discoveries made by Lord Rosse, when his huge
instrument was first turned towards the heavens, consisted in the
detection of the spiral character of some of the nebulous forms.
When the extraordinary structure of these objects was first
announced, the discovery was received with some degree of
incredulity. Other astronomers looked at the same objects, and when
they failed to discern--and they frequently did fail to discern--the
spiral structure which Lord Rosse had indicated, they drew the
conclusion that this spiral structure did not exist. They thought it
must be due possibly to some instrumental defect or to the
imagination of the observer. It was, however, hardly possible for
any one who was both willing and competent to examine into the
evidence, to doubt the reality of Lord Rosse's discoveries. It
happens, however, that they have been recently placed beyond all
doubt by testimony which it is impossible to gainsay. A witness
never influenced by imagination has now come forward, and the
infallible photographic plate has justified Lord Rosse. Among the
remarkable discoveries which Dr. Isaac Roberts has recently made in
the application of his photographic apparatus to the heavens, there
is none more striking than that which declares, not only that the
nebulae which Lord Rosse described as spirals, actually do possess
the character so indicated, but that there are many others of the
same description. He has even brought to light the astonishingly
interesting fact that there are invisible objects of this class which
have never been seen by human eye, but whose spiral character is
visible to the peculiar delicacy of the photographic telescope.
In his earlier years, Lord Rosse himself used to be a diligent
observer of the heavenly bodies with the great telescope which was
completed in the year 1845. But I think that those who knew Lord
Rosse well, will agree that it was more the mechanical processes
incidental to the making of the telescope which engaged his interest
than the actual observations with the telescope when it was
completed. Indeed one who was well acquainted with him believed Lord
Rosse's special interest in the great telescope ceased when the last
nail had been driven into it. But the telescope was never allowed to
lie idle, for Lord Rosse always had associated with him some ardent
young astronomer, whose delight it was to employ to the uttermost the
advantages of his position in exploring the wonders of the sky. Among
those who were in this capacity in the early days of the great
telescope, I may mention my esteemed friend Dr. Johnston Stoney.
Such was the renown of Lord Rosse himself, brought about by his
consummate mechanical genius and his astronomical discoveries, and
such the interest which gathered around the marvellous workshops at
Birr castle, wherein his monumental exhibitions of optical skill were
constructed, that visitors thronged to see him from all parts of the
world. His home at Parsonstown became one of the most remarkable
scientific centres in Great Britain; thither assembled from time to
time all the leading men of science in the country, as well as many
illustrious foreigners. For many years Lord Rosse filled with marked
distinction the exalted position of President of the Royal Society,
and his advice and experience in practical mechanical matters were
always at the disposal of those who sought his assistance. Personally
and socially Lord Rosse endeared himself to all with whom he came in
contact. I remember one of the attendants telling me that on one
occasion he had the misfortune to let fall and break one of the small
mirrors on which Lord Rosse had himself expended many hours of hard
personal labour. The only remark of his lordship was that "accidents
will happen."
The latter years of his life Lord Rosse passed in comparative
seclusion; he occasionally went to London for a brief sojourn during
the season, and he occasionally went for a cruise in his yacht; but
the greater part of the year he spent at Birr Castle, devoting
himself largely to the study of political and social questions, and
rarely going outside the walls of his demesne, except to church on
Sunday mornings. He died on October 31, 1867.
He was succeeded by his eldest son, the present Earl of Rosse, who
has inherited his father's scientific abilities, and done much
notable work with the great telescope.
AIRY.
In our sketch of the life of Flamsteed, we have referred to the
circumstances under which the famous Observatory that crowns
Greenwich Hill was founded. We have also had occasion to mention
that among the illustrious successors of Flamsteed both Halley and
Bradley are to be included. But a remarkable development of
Greenwich Observatory from the modest establishment of early days
took place under the direction of the distinguished astronomer whose
name is at the head of this chapter. By his labours this temple of
science was organised to such a degree of perfection that it has
served in many respects as a model for other astronomical
establishments in various parts of the world. An excellent account
of Airy's career has been given by Professor H. H. Turner, in the
obituary notice published by the Royal Astronomical Society. To this
I am indebted for many of the particulars here to be set down
concerning the life of the illustrious Astronomer Royal.
The family from which Airy took his origin came from Kentmere, in
Westmoreland. His father, William Airy, belonged to a Lincolnshire
branch of the same stock. His mother's maiden name was Ann Biddell,
and her family resided at Playford, near Ipswich. William Airy held
some small government post which necessitated an occasional change of
residence to different parts of the country, and thus it was that his
son, George Biddell, came to be born at Alnwick, on 27th July, 1801.
The boy's education, so far as his school life was concerned was
partly conducted at Hereford and partly at Colchester. He does not,
however, seem to have derived much benefit from the hours which he
passed in the schoolroom. But it was delightful to him to spend his
holidays on the farm at Playford, where his uncle, Arthur Biddell,
showed him much kindness. The scenes of his early youth remained
dear to Airy throughout his life, and in subsequent years he himself
owned a house at Playford, to which it was his special delight to
resort for relaxation during the course of his arduous career. In
spite of the defects of his school training he seems to have
manifested such remarkable abilities that his uncle decided to enter
him in Cambridge University. He accordingly joined Trinity College
as a sizar in 1819, and after a brilliant career in mathematical and
physical science he graduated as Senior Wrangler in 1823. It may be
noted as an exceptional circumstance that, notwithstanding the
demands on his time in studying for his tripos, he was able, after
his second term of residence, to support himself entirely by taking
private pupils. In the year after he had taken his degree he was
elected to a Fellowship at Trinity College.
Having thus gained an independent position, Airy immediately entered
upon that career of scientific work which he prosecuted without
intermission almost to the very close of his life. One of his most
interesting researches in these early days is on the subject of
Astigmatism, which defect he had discovered in his own eyes. His
investigations led him to suggest a means of correcting this defect
by using a pair of spectacles with lenses so shaped as to counteract
the derangement which the astigmatic eye impressed upon the rays of
light. His researches on this subject were of a very complete
character, and the principles he laid down are to the present day
practically employed by oculists in the treatment of this
malformation.
On the 7th of December, 1826, Airy was elected to the Lucasian
Professorship of Mathematics in the University of Cambridge, the
chair which Newton's occupancy had rendered so illustrious. His
tenure of this office only lasted for two years, when he exchanged it
for the Plumian Professorship. The attraction which led him to
desire this change is doubtless to be found in the circumstance that
the Plumian Professorship of Astronomy carried with it at that time
the appointment of director of the new astronomical observatory, the
origin of which must now be described.
Those most interested in the scientific side of University life
decided in 1820 that it would be proper to found an astronomical
observatory at Cambridge. Donations were accordingly sought for this
purpose, and upwards of 6,000 pounds were contributed by members of
the University and the public. To this sum 5,000 pounds were added
by a grant from the University chest, and in 1824 further sums
amounting altogether to 7,115 pounds were given by the University for
the same object. The regulations as to the administration of the new
observatory placed it under the management of the Plumian Professor,
who was to be provided with two assistants. Their duties were to
consist in making meridian observations of the sun, moon, and the
stars, and the observations made each year were to be printed and
published. The observatory was also to be used in the educational
work of the University, for it was arranged that smaller instruments
were to be provided by which students could be instructed in the
practical art of making astronomical observations.