Pingré, whose great knowledge of comets adds weight to his testimony. The vibrations commenced at the head and appeared to traverse the whole length of the comet in a few seconds. It was long supposed that the cause was connected with the physical nature of the comet itself; but Ollers pointed out that such appearances could only be fairly attributed to the effects of the Earth's atmosphere, and for this reason:—"the various portions of the tail of a large comet must often be situated at widely different distances from the Earth; so that it will frequently happen that the light would require several minutes longer to reach us from the extremity of the tail than from the end near the nucleus. Hence, if the coruscations were caused by some electrical emanation from the head of the comet, even if it occupied but one second in passing over the whole surface, several minutes must necessarily elapse before we could see it reach the tail. This is contrary to observations,[1] the pulsations being almost instantaneous." Instances of this phenomenon are not very common; Coggia's Comet of 1874 is the most important modern example. An English observer at Hereford named With, well known for his astronomical mirrors, noticed an "oscillatory motion of the fan-shaped jet upon the nucleus as a centre, which occurred at intervals of from 3 to 8 seconds. The fan seemed to 'tilt over' from the preceding to the following side, and then appeared sharply defined and fibrous in structure; then it became nebulous, and all appearance of structure vanished."[2] A flickering of the tail of this comet was observed by Newall.
The mention of the word "jet" in the preceding quotation suggests the necessity of something more being said, based on this word. Without being able exactly to dogmatise on the subject, it seems certain that not a few of the larger comets which have been subjected to telescopic scrutiny during the last half-century have exhibited changes which can only be compared to the appearance of a jet of water rising from the nozzle of a fountain and rising higher and higher, until at last
