RAGNAROK. 
167 
true causes might be added another in the known phenomena 
of shore and river ice. These four causes are undoubtedly at 
work now in different districts of the earth’s surface. How 
they were combined, and in what manner they were enlarged 
and intensified so as to produce the glacial deposits, is a 
matter for fair discussion and fuller research. 
But can the same in any sense whatever be said of a 
comet ? In other words, can a comet be said to be a vera 
causa at all ? The physics of the cometary system have 
become much better known in late years than formerly ; but 
still much remains to be known. What is known, however, 
utterly negatives the idea that one of these erratic visitors 
caused the Drift. This book speaks of a comet “ many times 
larger than the mass of the earth” (p. 264); and again 
(p. 399) of “ the attractive power of the comet great enough 
to hold its gigantic mass* in place through the long reaches 
of the fields of space.” Whereas it is perfectly well known 
that the mass of existing and recorded comets must be very 
small compared with that of other bodies in the Solar system. 
Otherwise the perturbations caused in planetary motions 
must necessarily have been observed by astronomers. Indeed, 
our author quotes from the “ Edinburgh Beview ” that “ In 
the years 1767 and 1779 Lexell’s comet passed through the 
midst of Jupiter’s satellites and became entangled temporarily 
among them. But not one of the satellites altered its move¬ 
ments to the extent of a hair’s breadth, or the tenth of an 
instant.” This shows that the mass of this particular comet 
must have been very small. Moreover, the extreme tenuity 
of cometary matter is shown by other things ; the writer of 
these lines himself saw the star Arcturus shining brilliantly 
through Donati’s comet in 1858 ; and it is a well-known fact 
that a star of the fifth magnitude was clearly seen, with no 
abatement of its light, through the centre of the nucleus of 
the comet discovered by Miss Mitchell in 1847. 
Nor, indeed, does the now commonly received hypothesis 
that some comets are attended in their orbits by meteoric 
masses, which occasion displays of shooting stars, in any 
degree support the idea that a comet caused the glacial 
deposits. Meteors and aerolites are very different things 
from boulders. 
It may of course be urged that known comets of modern 
days can be no measure of the gigantic results which accrued 
from them ages ago. But, surely, if they were of such 
gigantic mass as Bagnarok represents, they would have been 
* Is it possible that the author here has confounded the word ‘mass’ 
with * magnitude ’ ? Magnitude may be immense when mass is small. 
