EXPECTED METEORIC DISPLAY. 251 



the solid form is probably not many inches in diameter (I am speaking, 

 be it remembered, of the meteors producing displays of ordinary 

 shooting stars or falling stars, not of those masses which thrust their 

 way through the upper regions of the air, and, exploding, cast their 

 fragments often over many square miles of the earth's surface). It 

 will be understood how small is the chance that a flight of bodies so 

 minute compared with the average space occupied by each could cause 

 the dispersion of a mass so rare, and therefore so free to pass through 

 a meteor-flight without disintegration or disturbance, as a comet. 



How Biela's comet came actually to be divided into two distinct 

 bodies, and later to be so far dissipated as to be no longer visible even 

 in the most powerful telescopes and under the most favorable circum- 

 stances, will probably be understood when we know the nature of 

 those processes of repulsion which lead to the formation of comets' 

 tails. For our present purpose it is only necessary to observe that 

 these processes of repulsion do most obviously carry away parts of the 

 substance of a comet's head to enormous distances, and that, in some 

 way, Biela's comet was divided, even as it were under the eyes of 

 astronomers, into two distinct comets ; for we thus learn to recognize 

 the further disintegration of the comet as part of a process undoubt- 

 edly commenced in 1846 and undoubtedly competent to effect the 

 dissipation of the comet's substance. As the comet was searched for 

 in vain in 1866 in the region which unquestionably it would have 

 traversed had it remained unchanged, there can be no reason for 

 doubting that it had thus been thoroughly dissipated and disinte- 

 grated. If anything could have made this more certain, it would have 

 been the circumstance that in 1872, also, the comet was searched for 

 in vain. Remembering that the observations made during the first 

 few weeks after the comet's discovery in 1826 gave astronomers such 

 a mastery over its motions that they could successfully predict its 

 return in 1832, and show precisely where it would appear, nay, even 

 calculate back its path and recognize its identity with a comet discov- 

 ered by Montaigne in 1772, and rediscovered (though not recognized 

 as the same) by Pons in 1805, it is obvious that in 1866, after several 

 carefully observed returns and nearly a century after its first discoveiy, 

 the comet's motions must have been much more thoroughly under- 

 stood. It would have been much more easily detected that year than 

 in 1846 and 1852, even as Halley's comet was much more easily de- 

 tected at its return in 1835 than at its return in 1759. 



If the comet had been like most of its fellows, astronomers must 

 have given up all idea of obtaining further information respecting it. 

 But in one important respect it differed from them. It is one of the 

 few known comets whose paths cross, or at least pass very close to, 

 the track of the earth. Already in 1832 attention had been called to 

 this circumstance. Indeed, fears had been excited among those un- 

 familiar with astronomical relations by the announcement that the 



