Dr.. Lardner's Fourth Lecture.
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evening by saying that 2!though the suolecl az
nounced in the bills was Comets, he should resume
the subject with which he closed on the pre-
vious evening. Re announced, also, that at the
next lecture ne should introduce an innovation. and
what might seem a bold one but one which he was
convinced would be productive Of good; and az in
the diffusion of knowledge every experiment which
promised useful results was worth trying, he had
no hesitation in resolving hereafter TO divide Ms
lecture into two parts or, said he =" as AN ups"
the stage suppose may say into two ac''' ne'
voting them TO the consideration OF dihereHl sc
,ions of knowledge. In the interval between thei'
the didactic models will be exhibited.


The planet. said DR.. L. upon which we were
engaped \, the conclusion of the last lectaEe was
.Jupiter. 1 explained the physical cause OF ms
bshs-showed the appearance which the Sun a.
s,mes TO his inha5ltants, and proved ih4tthe quar-
,ity of light and hea; enjoyed by any planet is il
proportion to the magnitude Of the Sun .een
from that planet. Now the economy of the Solar
System /, in no respect ,eautifully observed
than in This particular. Ne physical principle is
more essential to the healthy coniinuancc Of or
ganic life than light and heat. If there be a
dundancy or deficiency Of either, hcnilhy and hap-
py existence is ue longer possible. Now Jupiter
being placed at the immense distance Of 800 COS
0OO Of miles from the Sun, is supplied with %
times less light and heat ihan receive. Howl
this deficiency supplied Evidently by his moons,
three of which appear ef the same magnitude t.
Jupiter. inhabitants rs our Moon does to Us, tn"
fourth being one and d half times larger. Here Is
most beautifully cxemplifsd the principle Of com
pensgtion-,howing the existence OF suPeriniend-
encc aud benevolent consideration. As one supply
is cut off unother is given. There is another
most remarkable arrangement among the moons
OF Jupiter. As have but one moon, we are
deprived of its light for one half ef ever!
month, besides during great pant Of the other
half having only partial light. Jupiter has four
moons; ami only has she therefore more
light, but in observing closely we detec; most
remarkable circumstance attending their motons-
developing one Of the most admirable economies
of the planetary system. Upon observing their
motions around Jupiter, find that they
never all be upon the side Of the planet al
the sume time. There a certain rule, fixed
harmony establish he'd, which renders it Impossible
Ii all were or the SAM side of the planet AT once
with the sun, the result would be liat The Other
side would he de pr.ed entirely Of light but tins
is impossible, and in consequence part OF Jup
ter is ever without moonlight. There may always
be seen in her armament one moon, sometimes
two, sometimes three but never four. lhese four
attendants upon this splendid planet obey law
which no mechanical philosophy could ever estab-
lsh, and as they travel round afford the greatest
possible degree of light to the inhabitants oi the
Illnet.


The diameter of jupiter is 90 600 miles; more
than eleven times thut of our Earth: his magn-
tude or cubical contents equal to that OF li70
globes like the Earth. As ! have already eXPlaiu-
ed the method of calculating the weight OF the
planets, shall net repeat ii: but the weight oi
Jupiter is found TO be 800 times that OF the Earth.
Hence VIC may draw singular inference: while
the weight OF this planet is only 800 times that OF
the Earth, its volume Is above llOO times greater
consequently its density only encosrth that OF
the Earth. By this simple reasoning, we pene-
trate to the centre of distant planets, feel what
they are made of travel TO tseir very heart ana
thus point out the differences between them aud
our Earth. By an experiment already explained,
the weight of the Earth was found to be four



water: consequently the weight of Jupiter i, thar
of an equal body OF water. The weight OF bodies
on the surface of Jupiter, Is three times That
which obtains on the Earth This dsproporion |
although considerable is not so great TO put
veto upon the suppo. ton that 1t is inhabited ii
the weight of the planet have any Just proportion
10 its bulk the inference would be strongly against
its habitabty. The length of Jueiter's year i
twelve Of our years that oi day ten OF our hours
1t has no seasons or at least practically, for
its axis is but slightly inclined to the plane OF Its
orbit These are the chiet numerical circum
stances connected with this planet.


There is one more arrangement toe beautiful
too important and astonishing not TO merit our at
tcntion and although 1t might seem hazardous to
attempt the explanation TO popular audience,
still | have no doubt that | shall make it clear TO
you Ye4 have all heard that hight moves with u
certain speed; indeed the common expression OF Il
velocity which is regarded infinite Is as swift
a. light. Now ii WAS hr-t ascertaIaed by means
of Jupiers Satellites not only that hight has u mt
non, but that IL ies with the amazing velocity oi
100000 Tor more accurately, 1872, ,090) miles IN
single second of time. Bat you may ask how can
velocity enormous be asceriained his ShaIi
be explained, ana let me say, that the velocity oi
light great US il is, can be determined with more
relative accuracy than that of railway tram upon
YOUR roads | do not say that the number OF
feet passed over in given time can be de-
termined as correctly; but the practical error
in the calculations, considering tha relative dis
tances, is less than that YOU would make III
estimating the speed OF one OF your trotting horses
The manner of Its discovery this-aud / ASK
for its explanation your close attention. The
moons OF Jupiter, unlike ours, move IN tue same
plane as Jupiter himself around the sun: thar
those nearest can never pass behind him withou
ban, arI,n.,I rnrr.in al