The Liverpool Standard and General Commercial Advertiser.
Liverpool, Merseyside, England, 1838-10-30 00:00:00

LIVERPOOL MECHANICS' INSTITUTION.

LECTURES ON ASTRONOMY.

On 'Wednesday evening, John Wallis, Esq. commenced a
popular course upon this grand science.
The science (said Mr. \Valiis) to which I have the honour
of calling your attention in this course, correctly speaking,
ought to be called one of hypothesis. It differs, therefore,
very widely in its nature, and the evidence of which it is
susceptible, from all those branches of science to whir b your
attention is generally directed in this theatre; for in the lat•
ter you have in most cases had the various iheories demon-

strated by analytical and synthetical proofs. Now, strictly
speaking, we can bting neither the one nor the other of these
species of evidence to bear upon astronomical science. My
business, therefore, is to explain the appearances which the
material substances with which we have to deal present to us,
and therefore our science differs from the other branches of

natural philosophy. In chemistry or any other science the
demonstrator produces to you the simple elements from which
he may compound a body, or he takes the compound body,
decomposes it, and proves the atoms of which it consists.
Now in one respect we may see that it is impossible in astro-
nomy to give anything like experiments,—that is, the expe-
riments which I shall introduce must be disencumbered from

so .many adventitious circumstances by which they are at-
tended, that, strictly speaking, they can only serve as general
illustrations of principle. Here, then, is one of the difficul-
ties attending our branch of enquiry, and another is that the
very magnificence and grandeur of our subject places it

beyond our reach, and is takes a very lively imagination to
grasp its various details. If I could take you under the azure
canopy of heaven, and there explain to you the sublimities
of astronomical science, I should excite your feelings so as

to command your attention; but we moat strip our subject

of its natural tnajesty and sublimity, and on many occasions
add so many appendages, and seek to explain appearances,
some of which are not in accordance with the evidence of

our senses, so that we labour under many disadvantages in
proving those circumstances which we wish to impress upon
your understanding. If you will, however, give me your
candid attention, 1 think that although we cannot demon-
strate astronomy by analytical and synthetical evidence, yet
you will understand from the nature of our subject, and the
different kinds of evidence of which it is susceptible, that
our conclusions will be as sound (this being premised) as
those to which we arrive by auy experiments of the most
demonstrative kind. _

Tins e.vening I would call your attention to a class of facts
which will throw much light upon our future progress. We
must first understand our local situation in the universe.

We conceive ourselves to be established upon a planet of the
solar system—the earth. Now we assume that this plant is
itself moving, and that we are viewing many similar bodies
which are also moving, around the same grand centre the
sun. We therefore assume now a very considerable step=
we will take a stand at once, in order to lay before you a
subsequent process, which will go much more into minute
details than that to which we now advert. We must assume
something in a science which only ascends to hypothesis.
Your view is from our own planet, and this planet is itself in
motion. Now in our subject, which treats upon the mere
elementary -parts of nature's works, we shalt proceed by
analogy. -Now analogy, when properly applied, constitutes
the very essence of philosophy : it is by it that we descend
from generals to particulars, or ascend from particulars
to generals. Now by carrying out analogy, we find
that facts which do not admit of any other mode of
reasoning have yielded to it. We find that the sun,
moon, and stars appear to be in constant motion around us.
By motion we infer a change of place of one body with re-

spect to another; but this motion may be either apparent or
real with respect to the bodies concerned. Now, it does ap-
pear to us that all the universe around us is incessantly
moving. If this motion is real, we must seek for as many
causes of motion as there are facts presented to us. We are
acquainted wills hundreds of millions offixed stars, and each
of these immense bodies, (for immense they must be to be
visible at such enormous distances as they are placed from
us,) and we must ask for some cause of the apparent motion
of these bodies. It would be very remarkable when we came
to observe millions of these bodies, if all of them should re-
volve around us in the same period of time. Here would be
one difficulty; another difficulty which presents itself to us is,
that while they all appear to revolve around us in the same
period of time, that they all appear to perform other changes
with regard to one another in their diurnal motion. Now,
we may ask whether this apparent motion of the universe
around us may not arise from our eyes being placed iu the
centre of this apparent system of motion, and that, in fact,
we ourselves may be in motion, thereby causing this appear-
ance of the universal motion of bodies. Therefore, without
encumbering you with evidence, let us at once imagine our-
selves to be spectators on the earth, andtfie earth to have a
rotation around its centre, which communicates the apparent
motion to all the universe around us in a contrary direction.
AU the difficulties of our argument are at once explained by
this; and it is my business to explain appearances by hypo-
thetical evidence, offering no physical cent radictions. Now
let us attend to what the telescope makes us acquainted with,
in regard to other bodies. Let us suppose ourselves trans-
ferred to the moon, and we have every reason to believe that
it is an inhabited body. There is no doubt whatever that
were we inhabitants on the surface of the moon, that instead

of seeing the universe going round us once in 24 hours, me
should there see the universe of son, stars, and planetary bo-
dies revolving once in 27 days, seven hours and 43 minutes,
because this is the very period in which the moon revolves
on her axis. Again, let ourselves be inhabitants of the planet
Jupiter. This planet turns upon its axis in nine hours and
56 minutes. From its surface, therefore, the whole universe
would appear to revolve around us in that time. Saturn
turns upon his axis in 10 hours and 16 minutes, consequently,
as in the other instances, that would be the time in which,
apparently to a spectator, the universe would, turn round
when viewed from that planet. Now, why should we not from
analogy consider that whilst these various planets are in mo-
tion iu their centres, our earth, which is no doubt analagous
in every point of view, appears to their inhabitants as a mere
glittering star. Under these circumstances we cannot but
conclude that it is our own earth which revolves on its axis
in 24 hours from west to east, which causes the apparent ro-
tation of the heavenly bodies in that time from east to west.
Here, then, is one simple and singular cause which goes to
explain, without any degree of perplexity or incursibrance,
the cause of the apparent revolution of six hundred mil-
lions of heavenly bodies. Again, not only have we analogy
to bear us out, but we have another very striking fact;
it is the only conceivable hypothesis which wilt explain
satisfactorily, that the universal heaven does not revolve
around the centre or the earth as it appears to do, but that
the revolution of the earth upon its own axis, produces these
appearances. This explains the reason why those stars
which are opposite the north and south axis of the earth
have no motion at all. We now take another step, and it is
this. We find by the aid of our telescopes that the planets
Jupiter and Saturn, (of whose rapid revolution upon their
centre we have the most palpable evidence) are largest in the
diameter, drawn at right angles to the axis of rotation, so
that they deviate somewhat from a globular appearauce. We
can tell in what direction these planets rotate, by various
lines and spots which are to be discerned upon their surface.
This is also the case with Saturn and Mars, as we find upon
measuring their diameters with the micrometer. It is evi-
dent that during the rotation of the earth upon the axis pass-
ing through its poles, that every part of it will he moving,
but that the portions near the equator will move faster than
any other portion. .The consequence of this is, that theeaith
cannot remain globular when put into rapid motion, on ac-
count of the centrifugal force which gives every thing situ-
ated near the equator, a certain disposition to fiy away from
the earth. This centrifugal force at the equator is very con-
siderable, because the earth moves at the rate of 25,000 miles
in 24 hours. The consequence of this is, that the earth is
less in diameter at the axis of rotation than at right angles to
it, in which there is a striking analogy to the appearances
presented to us by the other planets. We may now take
another step in our argument to prove that our earth does ro-
tate upon its axis, which causes it to be flatter at the poles
than it is at the equator. By accurate measurement we find
such to be. the case, although it was discovered by Newton
before any measurement had been made. It is evident that
gravity is a principle tending downwards. Everything upon
the surface of the earth possesses weight, because it. is near
the earth, as there is a tendency in the particles of matter to
be attracted, and to attach each other in relation to their
bulk and freedom of action. Now suppose a body at Liver-
pool has by virtue of this attraction of gravitation a ten-
dency to fall downwards, this tendency would be diminished
upon removing it to the equator, because we move here at the
rate of 500 miles an hour only, wheieas, at the equator we
should be carried with a velocity of 1,000 miles an hour.
The centrifugal force at the equator, therefore, acting in op-
position to the force of gravity, lessens the tendency of
bodies to fall to the earth at that portiun oft he globe. This
cannot be shown experimentally by the falling of bodies at
the equator; and here, because the difference is too slight for
the eye to discern, but we can calculate it accurately by
means of the pendulum. A pendulum is a mass of matter
suspended by a cord or rod. By means of this beautiful con-
trivance we observe that bodies do not fall so fast at Li-
verpool as they would fall if we carried them further from the
equator towards the pules, and that a pendulum which makes
86,400 oscillations at the equator in 24 hours, makes 86,535
oscillations at London in the same period of time. Here,
therefore, we have a direct experimental proof of the dimi-
nution of gravitation at the equator, in consequence of the
opposition of centrifugal force. This striking fact was, like
many others, accidental iu its discovery- It was the opinion
of Newton that the earth, like other planetary bodies, was
really flatter at the poles than the equator, but eassini main-
tained that the earth was a true sphere and not an oblate
spheroid. Now to settle thisdispute it was necessary that ob-
aged
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first solution was that the peneulunt beitig lengthened by the
high temperature of the equatorial regions caused the differ-
euce. Those who found that their clocks gained in time
supposod that it occurred by the shortening of the pendulum,
by the reduction of the temperature in higher or colder late-
lettere But the increase o diminutinn in the length of the
pendulum did not account tor the difference, and thus this
accidental ciictrinstance proved to them the correttness of
Newton's theory before they had time to solve it. The equa-
torial diameter is thus shown, by weeny of the pendulum, to
be 34 or 36 miles lugger than that of the polar, being ab .ut
the 300th part of the allele axis of rotation.
Now a very curious Net yet remains to be brought before
your notice—that while at the equator the diurnal motion
would lessen the force of the gravity about-the 289. h part,
the real diminution is in tact the 194:11 part. The reason is
this. If we come to suppose the attraction of a sphere, every
part at an equal distance from the centre, we find that it will
attract abu.teliyr
we coneplaseti
to he a solidatsidliviarrlistr,i).le4sa,ferostpri surface,hieisroidi iiis
equally, b
is no longer the case. Then we shall find that a body
at the poles will be more powerfully attracted than a body
placed at the equatorial regions, stile-Hee „s -
that centrifugal
force were entirely kept.out of the questieu. Consequently
eolththeenreuaoritchmait
there are two causes operating to produce the difference be-
tween the amount of gravity at the poles and at the equator,
namely the greater
ithieheeseunarttth)rutphoann
found to be currents of air which al
at the poles, and the increased centrifugal force.
rotundityit
its
wn axie
s. windsl,ll 'There
eo to
are
different seasons-of the year take different directions from
each side of the equator. Now as we find that the sun is
nearly vertical over a zone of 47 degrees in breadth, of course
this being the hottest region of the earth, the air must neces-
sarily be more rarified there. Now the atmospheric air is
attracted by the earth and carried around with it in its orbit,
the _direct. enotionsof the earth iseimpart edits
this air, and it does not indteate in any manner this relation.
But there comes in a fresh cause. The caloric of the sun's
rays we all know will expand all aeriferm substances, and
the air suffering expaosiun in the equatorial regions be-
comes specifically lighter, and ascends to higher regions of
our atmosphere. By this means the stratum of air which on
a hot summers morning will be on the surface of the earth
is by noon elevated, and a colder volume of air rushes in from
below to occupy the spiice previously occupied by the air
which has. ascended. The air which travels from higher
latitudes to supply the deficiency not having !he same velocity
of motion as the air its the equaleriol regions, will therefore
tag behind, and it will not consequently flow to the equator
from north to south, and from south to north, but from north-
east to the south-east, and consequently we shall have at dif-
ferent seasons of the year currents train the directions about
the equator. Again the air from the higher equatorial re-
gions will, as it cools,eentract in volume and flow downwards
to the north and south, but not directly so, because it has
previously acquired a velocity greater than that of the places
to which itia carried. The consequence of this is that we