traced in their revolutions by no means sets the question at rest. For it might well be that an effect totally imperceptible until after the lapse of countless ages, as regards the solid orbs of our system, might be obvious in the movements of bodies like comets of small mass and great bulk; just as a feather or a gauze veil at once yields its motion to the resistance of the air, while a cannon-ball cuts its way through with comparatively slight loss of velocity. It will thus be seen that issues of the most momentous character hang on the _time-keeping_ of comets; for plainly all must in some degree suffer the same kind of hindrance as Encke's, if the cause of that hindrance be the one suggested. None of its congeners, however, show any trace of similar symptoms. True, the late Professor Oppolzer announced,[248] in 1880, that a comet, first seen by Pons in 1819, and rediscovered by Winnecke in 1858, having a period of 2,052 days (5·6 years), was accelerated at each revolution precisely in the manner required by Encke's theory. But M. von Haerdtl's subsequent investigation, the materials for which included numerous observations of the body in question at its return to the sun in 1886, decisively negatived the presence of any such effect.[249] Moreover, the researches of Von Asten and Backlund[250] into the movements of Encke's comet revealed a perplexing circumstance. They confirmed Encke's results for the period covered by them, but exhibited the acceleration as having _suddenly diminished_ by nearly one-half in 1868. The reality and permanence of this change were fully established by observations of the ensuing return in March, 1885. Some physical alteration of the retarded body seems indicated; but visual evidence countenances no such assumption. In aspect the comet is no less thin and diffuse than in 1795 or in 1848.