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computation of the times which will he given in the pages of your Register 
for May next, both phenomena occupy a little over two hours, and differ only 
a few minutes ; and the accuracy of your computed times may be confirmed 
by future observation about 11th May, [1864,] at which time Jupiter will be 
again in opposition. 
“(6.) It appears then, that Mr. Reddie must either deny Jupiter's orbital 
motion, for precisely the same reasons as led him to conclude the Sun to be 
motionless, or that he has misapplied the 4 well-known mechanical principles 
and necessities of the laws of space and motion.’ ” 
30. To this letter I replied in the April number of the 
Astronomical Register, as follows : — * 
44 (1.) As regards paragraph one of D.’s letter, I think he must yet confess 
that I indulge in nothing 4 fanciful’ 4 The test of observation ’ (in par. 2) I 
accept. I agree with paragraphs three and four except the last two lines, in 
which D. draws a false conclusion from his own premises, as probably he has 
already discovered. But, since his argument stands on record, I must 
answer it. 
44 (2.) Suppose, therefore, the Sim to be at rest, and that Jupiter revolves 
round it, with a mean velocity of 29,000 miles an hour ; also that his second 
satellite revolves round him with a mean velocity of 32,000 miles an hour. 
Then, doubtless (according to well-known mechanical principles, and the 
necessities of the laws of space and motion), the satellite must move, when 
it is being eclipsed at the rate of 29,000 + 32,000 = 61,000 miles an hour 
direct , and during a transit at the rate of 29,000 — 32,000 = 3,000 miles 
an hour retrograde. So far, I beg leave to assume D. goes with me ; as I 
am merely quoting what he himself truly says, though he puts it forward 
only as an argumentum ad hominem against me. 
44 (3.) But he adds (par. 4) : — 4 Therefore, the time occupied by the transit 
of the satellite should be somewhere about twenty times that occupied by 
the occultation ! ’ To this (as I have hinted) I feel it almost unnecessary to 
reply. D. has inadvertently overlooked the effect of Jupiter's own proper 
motion, and forgotten that the time of the occultation or transit only indicates 
the apparent and relative motions of the planet and satellite : i. e., the time 
in which they cross one another. 
44 (4.) I therefore pass over par. 5 of D.’s letter, and come to par. 6. There 
he says : — 4 It appears, then, that Mr. Reddie must either deny Jupiter’s 
orbital motion, for precisely the same reasons as led him to conclude the 
Sun to be motionless, or [admit] that he has misapplied the well-known 
mechanical principles, and the necessities of the laws of space and motion.’ 
Now, I do not take advantage of the error in reasoning already noticed, upon 
which the first of these alternative propositions is based ; but will frankly 
admit that there is a fair analogy between the solar system with a moving 
Sun, and the motion of Jupiter and his satellites. I assume, also, that 
D. will now give up his second proposition, taking for granted that (assuming 
