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his own data in par. 3) he now sees that the real motions of Jupiter’s second 
satellite must differ precisely by 58,000 miles an hour, during an occultation 
and transit respectively, and that these greatly varying velocities are con- 
firmed by ‘ the test of observation.’ 
“ (5.) Bat should D. not admit these assumptions, then I would beg leave 
to turn his own argument against himself thus : — If Jupiter’s real motion 
be 29,000 miles an hour direct, and we suppose the real motion of his second 
satellite during an occultation to be 32,000 miles in the same direction — 
then the apparent and relative velocity of the satellite (i. e. the rate at which 
it will pass behind the planet), will be only 3,000 miles an hour direct ; 
whereas, if during a transit, while Jupiter is moving at the rate of 29,000 
miles an hour direct , we suppose his satellite really to move at 32,000 
retrograde , — then (to apply the test of observation) the apparent speed with 
which they would cross one another would be 29,000 -f* 32,000 = 61,000 
miles an hour ; and, in that case, the eclipse would certainly occupy twenty 
times the period of the transit. The latter would be over in little more 
than an hour, the occultation would take more than twenty hours. But 
1 the test of observation ’ refutes these absurd suppositions and their results, 
and proves what D. had questioned. 
“ (6.) The analogy adduced by D. is nothing new to me. In a Paper on 
this subject, which I submitted to Section A of the British Association this 
year, I said ‘ The motion of the moon round the earth, as it moves in its 
orbit round the sun, is analogous to the motion of the earth round the sun, 
if the sun moves in space.’ .... In my former letter, I only noticed a few 
very salient points, in order to induce others to think. 
“ (7.) If D. had said that such immensely varying angular velocities as 
those of Jupiter’s satellites are thus shown to be, while revolving round their 
primary and but slightly varying their respective distances, cannot be recon- 
ciled with the current dogmas of physical astronomy, any more than the 
varying velocities of the earth, if the sun moved in space, to which I have 
objected, I could not have gainsaid the proposition. But the facts as to the 
motions of J upiter’s satellites being what they are, — assuming D.’s own data, 
and applying his own test, — I trust that he is not prepared to say, c so much 
the worse for the facts,’ and to cling to irreconcileable theories. 
“ (8.) D. should also recollect that the motions of Jupiter and his satellites, 
like those of all the other planets and satellites, and the comets, would them- 
selves be greatly complicated and confused by the motion of the sun as their 
centre. Their old aphelion and perihelion velocities would all be upset if the sun 
so moved ; and all the elliptical orbits converted into complicated, impossible 
paths, that could only be characterized as Vermicular. For simplicity and 
clearness, I have chiefly argued only as to the earth and moon. To do more 
would be like attempting to explain the obscurum per obscurius. After all, we 
do know somewhat more of this dull earth than of Jupiter and his satellites. 
These may whirl about in looped curves, -with alternate points of rest and 
great velocity, and yet continue, as we see they do, in the heavens. But 
what would happen if this massive earth were thus arrested in its orbit, or 
