4 Dr. L. N. G. Filon on Comets : 
which had appeared in 1531, 1607, and 1682, followed very 
nearly the same path; and he also found that there were records 
of great comets in 1066, 1145, 1801, 1456, 1531. The inference 
was that all these were really different appearances of one and 
the same comet, returning at intervals of about seventy-six years, 
intervals such as 1145-1301 (156 years), and 1301-1456 (155 
years), were accounted for by the comet not having been seen 
under sufficiently favourable conditions at the intermediate return. 
Halley predicted that this comet moved, not in an accurate 
parabola—no comet moving in a parabola can ever return—but 
in a very long ellipse, and he announced its return for 1758. His 
prediction was fulfilled, but Halley did not live to see it; he 
died sixteen years before the reappearance of his comet. 
Since then the latter has been observed at every return. It 
was last seen in 1835, and is due again in the summer of 1910. 
The existence of Halley’s comet shows that all comets do not 
move in parabolas and go out of our ken for ever, but that some 
move round the sun like the planets and are permanent mem- 
bers of the solar system. 
Since Halley’s day, several other comets, returning at com- 
paratively short intervals, have been discovered. Out of two 
hundred and seventy comets whose paths have been calculated, 
fifty have been found to move in ellipses, and more are doubtful, 
Only thirteen, however, have actually been observed more than 
once. 
The accompanying woodcut (Fig. 2), which is reproduced from 
Professor Young's ‘ General Astronomy ’ (slide exhibited), shows 
the paths of some of the best known of these comets. The cross- 
cut on each of the orbits marks the node or point where the 
paths of the comets cross the plane of the solar system. 
These nodes are all close to the orbit of Jupiter. Jupiter is the 
largest and heaviest of the planets and his attraction disturbs 
the comets and pulls them down nearer his own orbit. 
The comet of shortest known period is Encke’s comet, dis- 
covered in 1818. It returns at intervals of three years and four 
months. It presents a curious and hitherto unexplained anomaly: 
its period is being gradually accelerated, so that it threatens, 
eventually, to fall into the sun. 
Encke’s comet is a faint object. All the bright periodic 
comets return at such long intervals that the ancients never 
thought of identifying them. 
Structure of Comets. —For purposes of description we dis- 
tinguish three parts in a comet :— 
(A) The Head.—This is a hazy cloud of faintly shining matter, 
On one occasion (1882) it was preceded by a fainter forerunner 
or ‘‘sheath,” a luminous shadow, so to speak. The diameter 
varies usually from 40,000 to 100,000 miles. 
