[[Categoria:Pagine che usano RigaIntestazione|Scientia - Vol. VII.djvu{{padleft:251|3|0]]suppose, without straining probabilities, that these changes were entirely produced by perturbations. There is a further difficulty in the way of the solar origin; the ejected matter would leave the sun in the form of vapour, and would only liquify and solidify when it reached outer space. Probably it would solidify into particles of extreme minuteness, very much smaller than the meteoric masses that enter our atmosphere, many of which are known to have a connection with comets.

We turn next to the giant planets as possible comet-producers. The late Mr R. A. Proctor warmly advocated the view that the giant planets were the actual parents of the comet-families which are attached to them. Jupiter has a large and ever-growing family of comets, which obviously owe him allegiance, since in the case of Lexell’s and Brooks’ comets he has been caught in the act of profoundly modifying their orbits; Saturn has two, of which Tuttle’s has been observed at several returns; Uranus two, one being the comet of the November meteors; and there are six whose orbits are associated with Neptune’s in such a way as to suggest a connection. Three of these have been observed in two or more revolutions, one being the famous comet of Halley, which has been traced with tolerable certainty back to B. C. 240, and with some probability to B. C. 467 and 625. We have to go back to very remote antiquity to find a time when Neptune could have exerted any considerable influence on Halley’s comet; at present there is no near approach of their orbits, and Neptune’s influence is trifling.

Proctor argued (1) that very close approaches to the giant planets would be required, for their orbits te be transformed from an approximately parabolic form to an ellipse with a period half that of the planet; hence (2) that the number of captured comets would be a very small fraction of the total number that approach the sun. It would require an immense period to produce such a number of comets attached to the various groups. The suggestion has been made that the number might be greatly increased by the consideration that comets that have their orbits changed into ellipses, even of long period, would sooner or later make other approaches to the planet, and might undergo further shortening. But I doubt whether there is much weight in this consideration,