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may have been a hundred years more or less than this. It must therefore have been visible at its preceding return sometime in the first century before Christ, but there is no record by which it could be identified. It may be expected again in the thirty-eighth or thirty-ninth century.

A very remarkable case of several comets moving in very nearly the same orbit is afforded by the comets of 1843, 1880, and 1882. The first of these was one of the most memorable comets on record, as it passed so near the sun as almost to graze the surface. In fact, it must have passed quite through the outer portions of the solar corona. It came into view with remarkable suddenness in the neighbourhood of the sun, about the end of February. It was visible in full daylight. By a singular coincidence it appeared shortly after the well-known prediction of Miller that the end of the world was to come in the year 1843. Those who had been alarmed by this prediction saw in the comet an omen of the approaching catastrophe.

The comet disappeared from view in April, so that the time of observation was rather short. The period of revolution now became a subject of interest. It was found, however, that its orbit did not differ sensibly from the parabola. But the time of observation was so brief that any estimate of the period would be somewhat uncertain. All that could be said was that the comet would not return for several centuries.

Great, therefore, was the surprise when, thirty-seven years later, a comet was seen by observers in the southern