after superstition had departed. At various periods, great alarm has been created in the popular mind. To soothe this alarm, mathematicians have calculated the chances of a collision, and have shewn the improbability of such a catastrophe. Still, they have not been able to shew the impossibility; and the popular mind is sometimes as much alarmed at the possible as at the probable. Astronomers may give an assurance that a comet will come in collision with the earth only once in 280,000,000 of years, but then they can give no positive assurance that that one time may not be in our day, and this possibility is, to many minds, a sufficient ground of alarm. Our great security is, that the comets are not confined to one plane, as the planets are. If they were, they would, in crossing our path, subject us constantly to the danger of collision. There is, however, one known comet, Biela's, thus awkwardly circumstanced. Its orbit may be compared to a level-crossing over a railway. The danger of such crossings is well known, and hence the stringency of the law in requiring, generally, that the public road should pass above or under the line, and not on the same level. All other comets, as far as is known, either pass above or under the path of the earth, but this one passes so nearly on a level, that it is always a question of time every seven years whether we are to come into collision. Much apprehension was created in 1832, when it was thought the case of our earth was very critical. We, how-
