1904-5.] Dr J. Halm on the Nebular Hypothesis. 559 
moment of momentum, the centrifugal force could not have over- 
balanced the centripetal force until the solar body had shrunk 
far wi thin the orbit of Mercury. This argument against the 
Laplaceian view seems to me unanswerable, and I agree with 
Mr Moulton when he contends that it “points to a mode of 
development quite different from, and much more complicated 
than, that postulated in the nebular theory.” The present view is 
not exposed to this difficulty ; on the contrary, the slow rotation 
of the sun follows of necessity from the mode in which rotation 
is supposed to have been brought into the system. 
A further point in favour of the hypothesis seems to be the reason- 
ing by which the existence of comets and the peculiarities of their 
orbits may be explained. Laplace, as is well known, considered the 
comets as bodies not belonging to our system. He arrived at this 
conclusion by investigating the question what form of cometary 
orbits should be the most probable if they are bodies launched 
upon us from outside space. He found that the most probable 
orbit must be the parabola ; and since this is indeed the typical 
form of cometary orbits, he concluded that his supposition on 
their origin was correct. Subsequently, however, Schiaparelli has 
proved that Laplace committed an error in his analysis, and that 
the result to be expected from Laplace’s supposition should be 
exactly opposite to his conjecture. Schiaparelli showed that the 
parabola is in fact the least probable curve in which a foreign 
body may intrude upon our system, and that under Laplace’s 
supposition the great majority of orbits should be hyperbolic. 
His researches leave scarcely any doubt that the comets are 
members of our own system ; that at practically infinite distance 
there exists a cosmic cloud travelling with our sun through space 
with practically the same speed and in the same direction ; and 
that all the comets originate from this mysterious appendage. To 
explain these facts by the Laplaceian hypothesis seems to me 
extremely difficult ; but they are rendered almost obvious by the 
present theory. I have shown in my paper on temporary stars 
that through the catastrophe the star becomes surrounded with 
an expanding atmosphere of gases and vapours. We have the 
strongest possible evidence of the presence of this atmosphere in 
the absorption-bands of the spectra of new stars, which by their 
