DR.. Lardner's Fourth Lecture.
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evening bv saving thai although the subject an
nou3,ed in the bills was Comtts ne should resume
the susiecr with which he closed on the pre-
viou. evening. He announced, also, that at 'he
next lecture he should introduce innovation. ana
vi AT might seem bold one. but one which ne was
convinced would be productive Of good; and as IN
the diffusion of knowledge every experiment which
promised useful results was worth trying. ne had
no hesitation in resolving hereafter 70 divide his
lecture into two parts. or, said he - - aa upon
the stage, suppose may say into TWO acts, de-
voting them 10 me consideration Of diferen; sec
lions OF knowledge. In the interval between them
the didactic models will be exhibited.


The planet said Dr.. L. upon which We were
engaged at the conclusion OF the last lecture vas
Jupi;er explained the physical cause Of MS
BeAs--showed the appearance which the Sun a,-
sunes TO his inhabitants, and proved ihatt tn. guam
.ity of light and heat enjoyed by any planet IS il
proportion to the magnitude OF the Sun $. seen
from that planet. Now the economy OF the Solar
system is in us respec, more beautifully observed
than IN this particular Ne physical principle Is
more essential 10 the healthy continuance Of or
ganic life than light and heat If there be re-
dundancy or deficiency OF either, healthy and hap
py existence i. ne longer possible. Now Jupiter
being placea at the immense distance OF 100000
900 of miles from the SUE, Is supplied with p
unes les. ligh, and heat than We receive. How
this deficiency supplied Evidently BY his moons,
three of which appear ii the same magnitude to
Jupiter. inhabitants as our Moon does 10 Us. the
fourth being one and half times larger. Here is
msst beautifully exemplitisd the principle OF com
Notwithstanding TIE existence Of saperin,end-
eace sad benevolent consideration. AS supply
is cut off another is given There is another
most remarkable arrangement, among the moons
OF Jupiter As we have but one moon, we are
deprived of it, light Tor one half Il every
month. besides during gTent part OF the other
half having osy partial light. Jupiter has four
moons and not only has she therefore more
light, bu. in observing closely detect most
remarkable circumstance attending their motions
developing one OF the mos; admirable economies
OF the planetary system. Upon observing their
motions around Jupiter fRd that they can
never all be upon the same side OF the planet at
the same time. There is certain rule, fixed
harmony establi- he'd. which renders it impossible.
Il all were on the same side Of the planet once
with tins sun, the result would ba hat the other
side would be deprived entirely OF light; but this
Is impossible, ana in consequence no part of Jupi-
ter Is ever without moonlight. There may always
! l.
two, sometimes three; but four. These four
attendants upon TIN, splendid planet obey law
which no mechanical philosophy could ever estab-
l.sh, and they travel refund, afford the greatest
possible degree OF light to the inhabitants OF the
| planet.


The diameter of Jupiter is $4,000 miles
/ than eleven times that of our Earth his magni-
tude or cubchl coateuIs equal 10 that OF i47O
globe, like the Earth. As have already explain
ed the method OF calculating the weight OF the
planets, | shall not repeat II: but the weight OF
Jupiter i found 10 be 300 times that of the Earth.
Hence We may draw singular inference: while
tue weigh, OF this planet : only 3OO times that OF
the Earth, its volume IS above 400 times greater:
consequently lis density Is only oneoErih that OF
tic Earth. By this simple reasoning, we pene-
tra,e TO the centre OF distant planets, feel what
thew a,e made OF travel to tIeir very heart and
]
our Earth. By an experiment already explained
ths weight OF the Earth wa, found TO be four
times what it would be were it composed OF
water consequently the weight Of Jupiter is that
of an equal BODY 81 water. / ne weight oi bodies
on the surface of Jupiter, Is three times that
which obtains on the Earth. This disproporlion
although considerable, Is not so great as 10 put a
yeto upon the supposilion that II IS inhabited Ii
the weight OF The planet have any Just proportion
to It's bulk the inference would be strongly against
its habtnbiiiy. The length Of Jupiter year Is
twelve of our ears that oi day ten OF our hours
Il has no seasons least none practically, ior
rs axis is but slightly inclined the plane ot is
rbit. These the Chief numerical circum-
stances connected with this planet


There is oue more arrangement toe beautiful,
too important and astonishing not 10 merit our a.
tention, and although 1t might seem hazardous 10
attempt the explanation TO popular audience,
s, have no doub, that shall make 1t clear TO
YOU Yos have all heard that hight moves with
certain speed; indeed the common expression OF
velocity which is regarded as Infinite, Is de swift
q. hsht.' Now II hr-t a.certalned By means
OF JuPi,ers Satellites not only that lish, nus
tion but that ilies with amazing velocity oi
100000 (or nlore accurately, ly2,0gO) miles IN
single second Of limo. But YOU may ask how
a velocity so enormous be ascertained This shall
be explained, ana let me say, that the velocity OF
light. great Is it i, be determined with more
relative accuracy than that OF railway tram upon
your roads | do not say that the number o.
feet passed over in givan time can De de-
termined us correctly but tae practical error
in the calculation considering tha relative dis
tances, Is less than that you would make IN
estimating the speed of one Of your trottng- horse
The manner OF lis discovery this-and | ask
for It's explanation yoar close attention. HE
moons of Jupiter. unlike eurs, move N ;he same
plane as Jupiter himself around TIE sun: 50 that
those nearest can never pass behind HIM without