THE RETURN OF HALLEY’S coMET IN 1910. 33 
The revolution just completed is much the shortest on 
record, and this is due to the fact that both in 1834 and in 
1836 Jupiter was placed exactly hehind the comet and reduced 
its speed considerably on each occasion. ‘That a record passage 
should be the result of reduced speed sounds rather paradoxical, 
but the case is like that of a thrower’s arm being held when he 
throws a stone into the air; it will not rise so high and comes 
back to earth sooner. So the comet has not gone so far into 
space as usual on the last revolution, and the sun’s attraction 
has been able to bring it back more quickly. Our own earth 
assisted in the shortening of the revolution to the extent of one 
week, the comet having been near us in October, 1835. One of 
the chief points of interest in the calculation is to see whether 
there is any indication of unknown forces acting on the comet. 
The actual return is three days later than the calculated one, 
and from the precautions taken, it is inferred that at least two 
of these days are due to some unknown cause, not to errors in 
the calculation. The unknown cause may be a planet beyond 
Neptune, or a resisting medium or the reaction produced on the 
nucleus by the emission of the tail-matter. 
Two quotations from Sir G. Airy’s address or presenting the 
Astronomical Society’s medal to Professor Rosenberger in 1835 
come in very appropriately here: “How are these wild bodies 
to be disciplined to our service? They are to be sent forth as 
spies; they are to go in directions in which no planets move ; 
they are to explore spaces in which no other bodies are known 
to’ exist; and they are to return bringing us an account, such 
as the physical astronomer can read, of the forces to which they 
have been subjected, and of the nature of the spaces through 
which they have passed. Have the anomalous motions of 
Uranus caused some astronomers to suspect the existence of a 
large planet beyond him? Then may we hope that Halley’s 
or Olbers’ comet will, in some revolution, feel its effect while 
far beyond our sight, and wil 1eturn to our eyes still bearing in 
its disturbed motions a trace of the perturbations which it has 
undergone. Has it for ages past been conjectured that some 
matter exists in the planetary space which in time may 
sensibly affect the motions of the most dense bodies? Then 
will the comparative insignificance of the comets be more likely 
to feel its effects.” 
“We have seen a comet whose last appearance it is probable 
that no man living can distinctly recollect—whose period 
exceeds the limit of ordinary life—whose path extends into 
spaces far beyond any which in other parts of physical 
