10 Dr. L. N. G. Filon on Comets: 
phenomenon was repeated, under the same circumstances, in 
November, 1886. The motion of the meteor stream is the same 
as that of the original comet, and there seems little doubt that 
they are the scattered remnants of Biela’s comet. 
Other comets have since then been found to break up. (Slides 
were shown of Brook’s comet, 1893 (1V.), breaking up; and of 
Swift’s comet, 1892 (1.), developing a second nucleus.) 
Danger of the Earth from Comets, — The possible danger to the 
Karth from comets has been the subject of much discussion, and 
has given rise in recent years to various ‘‘ comet scares.”’ 
There are two possible causes of such danger :— 
(a) The Earth may strike a comet. This is quite possible: 
according to calculation, it is likely to occur, on an average, 
once in 15,000,000 years. If it did occur, it is highly probable 
that the result would be simply a grand display of shooting stars, 
the matter of the comet not being solid enough to hurt the Harth 
much. The Earth has actually passed through the tail of a 
comet without any result. 
(6) A comet may fall into the sun, and the heat developed 
may be such as to be a grave danger to organic life on the Karth. 
Apart from the improbability of such a collision (it can be shown 
that such an event is far more unlikely than a collision with the 
Earth, because the comet must initially be heading straight 
towards the sun, if it is to occur at all), the mass of a comet is 
so small that it would only generate as much heat as the sun 
will radiate in eight or nine hours. But this heat will be 
developed inside the sun, below its photosphere, and will only 
be gradually liberated. The effect on the EHarth will be practi- 
cally nil. 
So that, after all, comets are very harmless objects, although 
at first sight they make such a brave show. We, however, are 
wiser than our ancestors, and look upon them as mines of in- 
formation rather than as harbingers of death. It is one great 
regret of astronomers that, since astronomical photography has 
been extensively introduced, no great bright comet has been kind 
enough to sit before our cameras. ; 
