The Sun.
London, England, 1869-08-19 00:00:00

THE BRITISH _ASSOCIATION.

EXETER, WEDNESDAY.
Professor Stokes is now the full-fledged President of the
British .Association. At eight o'clock this evening., in the

Victoria Hall, Dr. Hooicii resigned the chair which he
had held during the past year to the Cambridge mathe-
matician, who at once proceeded to deliver the
address which is given below. In personal ap-
.

pearance there is so little that is striking about the
Lucasian Professor, that in an ordinary assembly he
would attract much less attention than other men with

only a tenth part of his brains. Somewhat sturdy in build,
and of medium height, he carries his fifty years remarkably
well, though there are not wanting traces, when his face
is closely scanned, of the undoubtedly hard work to which
he has subjected himself. The platform was, as usual,
crowded with notabilities, and the attendance of mem-

- -
bers in the body of the hall was limited only by its

cspabilities.
Professor STORES commenced his address with a few

words in explanation of the objects for which the British
Association was instituted. Having intimated that he
should not attempt to give an account of the recent pro-
gress of science in general, but should select from those
branches with which he was more familiar some examples
of recent progress which would, he hoped, prove of pretty
general interest, he proceeded to say :—Among the
various branches of physical science, astronomy occupies
in many respects a foremost rank. Newton's great
discovery of universal gravitation raised it from the rank
of a science of observation to that of one admitting of the
most exact mathematical deduction ; and Ms investigation
of the consequences of this law, and the explanation
thereby of the lunar and planetary disturbances, have
afforded a field for the exercise of the highest mathemati-

cal powers on the part of Newton and his successors.
Gradually the apparent anomalies, as they might
have been deemed, in the motions of the
heavenly bodies were shewn to be necessary con-
sequences of the one fundamental law ; and at last, as.the
result of calculations of enormous labour, tables were con-
stractied enabling the plates of those bodies at any given
time to be determined years beforehand with astonishing
precision. A still more striking step was taken. When
it had been shewn by careful calculation that the appa-
rent motion of the remotest of the planets then known to
belong to our system could not be wholly explained on
the theory of gravitation, by taking account of the
disturbing powers of the other known planets,
Adams in our own country, and Le Verrier in France,
boldly reversed the problem, and instead of deter-
mining the disturbing effect of a known planet, set them-
selves to inquire what must be the mass and orbit of an
unknown planet which shall be capable of producing by
its disturbing knee the unexplained deviations in the
position of Uranus from its calculated place. The result
of these inquiries is too well known to require notice.
The science of astronomy is indebted to that of optics for
the principles which regulate the construction of those
optical instruments which are so essential to the astro-
nomer. It repaid its debt by furnishing to optics a result
which it is important we Ebou'd keep in view in consider-
ing the nature of light. It is to astronomy that we are
indebted for the first proof we obtained of the finite
velocity of light, and fur the first numerical determination
of that enormous velocity. Astronomy, again, led, forty-
four years later, to a second determination of that
velocity in the remarkable phenomenon of aberration dig-
covered by Bradley, a phenomenon presenting special
points of interest in relation to the nature of light, and
which a s given rise to some discussion, extending even to the
present clay, so that the Astronomer Royal has not deemed
it unworthy of investigation, laborious as he foresees the trial
is likely to prove, to determine the constant of aberration
by means of a telescope having its tube filled with water.
If in respect of these phenomena optics received much
aid from astronomy, the latter science has been indebted
to the former for information which could not otherwise
have been obtained. The motions and the masses of the
heavenly bodies are revealed to us more or less fully by
astronomical observations ; but we could not thus become
acquainted with the chemical nature of these distant
objects. Yet, by the application of the spectroscope to
the scrutiny of the heavenly bodies, evidence has been
obtained of the existence therein f various elements
known to us by the chemical examination of the materials
of which our own earth is composed ; and not only so, but
light is thrown on the sf.ate in which matter is there
existing which, in the case of nebulae especially, led to the
formation of new ideas respecting their constitution, and
the rectification of astronomical speculations pre-
viously entertained. I shall not, however, dwell
further on this part of the subject, which is now
of some years' standing, and has been mentioned by
more than one of your former presidents, but will pass on
to newer researches in the same direction. We are accus-
tomed to apply to the stars the epithet fixed. How shall
we determine whether any particular star is approaching
to or receding from our sun? It is clear that astronomy
alone is powerless to aid us here, since such a motion
would be unaccompanied by change of angular position.
Here the science of optics comes to our aid in a remark-
able manner. The pitch of a musical note depends, as
we know, on the number of vibrations which reach the car
in a given time, such as a second. Suppose, now, that a
body, such as a bell, which is vibrating a given number
of times per second, is at the same time moving from the
observer, the air being calm. Since the successive pulses
of sound travel all with the velocity of sound, but
diverge from different centres, namely, the successive
points in the bell's path at which the bell was when those
pulses were first excited, it is evident that the scnnd-waves
will be somewhat more spread out on the side from which
the bell is moving, and inure crowded together on the side
towards which it is inovivg, than if the Len had been at
rest. Consequently the number of vibrations per second
which re..eli the ear of an observer situated in the former of
these directions will be somewhat smaller, and the number
which reach an observer situated in the opposite
direction somewhat greater, than if the bell had
Leen at rest. Hence to the former the pitch will be
somewhat lower, and to the latter somewhat higher, than
the natnral pitch of the bell. And the snme thing will
happen if the observer be in motion instead of the bell, or
if both be in motion ; in fact, the effect depends only on
the relatice motion of the observer and the bell in the
direction of a line joining the two—in other words, on the
velocity of recession or approach of the observer and the
bell. The effect may be perceived in standing by a rail-
way when a train in which the steam-whistle is sounding
passes by at full speed, or bet ter still, if the observer be
seated in a train which is simultaneously moving in the
opposite direction. Having noticed the fact that the
present state of optical science was such as to furnish
evidence of the strongest kind that light consists of a
tremor or vibratory movement propagated in an elastic
medium, filling the planetary and stellar spaces, a medium
which fulfils for light an office similar to that of air for
sound, the President drew attention to the splendid
researches of Professor Kirchhoff, to whom i 3 owing
the inference from an extension of Prevost's theory
of exchanges that a glowing medium which
emits bright light of any particular refrangi-
bility necessarily (at that temperature at least)
acts as an absorbing medium, extinguishing light of the
same refrangibility-. In relation to radiant heat (from
whence the transition to light is easy) Kirchhoff was, said
Professor Stokes, preceded, though unconsciously, by our
own countryman, Mr. Balfour Stewart, and the inference
which Kirchhoff drew from Prevost's theory thus
extended led him to make a careful comparison of the
places of the dark lines of the solar spectrum with those
of bright lines produced by the incandescent gas or vapour
of known elements ; and the coincidences were in many
cases so remarkable as to establish almost to a certainty
the existence of several of the known elements in the solar
atmosphere, p' cn by their absorbing action the dark
lines coinciding with the bright lines observed. Among o.Uer
elements may be mentioned in particular hydrogen,
the spectrum of which, when traversed by an electric
discharge, shows a bright line or band exactly coinciding
with the dark line C, and another with the line I'. By
careful and repeated observations with a telescope fur-
nished with a spectroscope of high dispersive power Mr.
Huggins found that the F line, the one selected for obser-
vation, in the spectrum of Sirius did not exactly coincide
with the Corresponding bright line of a hydrogen spark,
which latter agrees in position with the solar F, but was a
little less refrangible, while preserving the same general
appearance. What conclusion, thenm, are we to draw from
the result? Surely it would' be ost unreasonable te
attribute the dark lines in the spectra of the sun and of
Sirius to distinct cantles, and to regard their almost exact
coincidence as purely fortuitous, when we have in proper
motion a vcra eausa to account for a minute difference. And
if, as Kirchhoff's labours render almost certain, the dark
solar line depends cn the existence of hydrogen in the
atmosphere of our sun, we are led to infer that that ele-
ment, with which the chemist working in his laboratory
is so familiar, exists and is subject to the same physical laws
in that distant star, so distant, that, judging by the most
probable value of its annual parallax, light which would
go seven times round our earth in one second would take
fourteen years to travel from the star. What a grand
conception of the unity of plan pervading the universe
do such conclusions present to our minds! Assuming.
then, that the small difference of refrangibility observed
between the solar F and that of Sirius is due to proper
motion, Mr. Huggins concludes from his measures of the
minute difference of position that at the time of the
observation Sirius was receding from the earth at the rate
of 41.4 miles pereecond. A part of this was due to the
motion of the earth in its orbit ; and on deducting the
orbital velocity of the earth, resolved in the direction of a
line drawn from the star, there remained 29'4 miles per
second as the velocity with which Sirius and our sun are
mutually receding from each other. Considering the
minuteness of the quantity on which the result depends,
it is satisfactory to find that Mr. Huggins's results as tc
the motion of Sirius have been confirmed by the observa-
tions of Father Secchi, made at Rome with a different
instrument. Turning to another application of
spectral analysis, the President referred to the
subject of total solar eclipses, especially dwelling upon the
results obtained by means of the equatorially-mounted
telescope provided with a spectroscope procured by the
Royal Society for the purpose of observing the eclipse of
August 17th last year. Shortly before the conclusion of
the meeting of the Association at Norwich last year, the
first results of the observations were made known to the
meeting through the agency of the electric telegraph.
In'h telegram sent by M. Janssen to the President of
the Royal Society, it was announced that the spectrum
of the prominences was very remarkable, chewing
fright lines, while that of the corona chewed none.
Brief as the message necessarily was, one point was
settled. The promimmi could not be clouds in the strict
ef";otlitnet
.b
ro below. I.'
ill hey i3tr coonn-
sist of incandeeceie. matter in the gatente form. It op-
hseenaste, oro lt,iyielfiges.lY'resflittlegil
post from the various observers, and put togethei at let-
peared from the more detailed accounts received by
sure. that except in the immediate neighbourhood of the
mu the light of the prominences consisted mainly of
three bright lines, of which two coinckied with C and I',
and the intermediate one nearly, but, as subsequent re-
searches chewed, riot exactly with 1). The bright lines
coinciding with C and F indicate the presence of glowing
hydrogen. Several of the other lines were identified
with those which would he produced by the incandescent
vapour of certain other elements. But valuable as these
observations were, it is obvions that we should have had
long to wait before we could have become acquainted with
the usual behaviour of these objects, bond their possible
relation to changes which may be .going on at the surface
of the sun, if we had been dependent on the rare and brief
phenomenon of a total ao!ar eclipse for gathering infor-
mation respecting them. But how, the question might
beasked, shall weever be able so to subdue the overpowering
glare of our great luminary and the dazzling jllumination
which it produces in our atmosphere when we look nearly
in its direction. as to perceive objects which are coropara-