x^cxi.] LIMITS OF SCIENTIFIC METHOD. 747 



decrease of the rotational velocity, so that ultimately the 

 terrestrial day will become identical with the year, just as 

 the i)eriods of revolution of the moon upon its axis and 

 around the earth have already become equal. Secondly, 

 there can be little doubt that certain manifestations of 

 electricity upon the earth's surface depend upon the 

 relative motions of the planets and the sun, which give rise 

 to periods of increased intensity. Such electrical pheno- 

 mena must result in the production and dissipation of heat, 

 the energy of which must be drawn, partially at least, from 

 the moving bodies. This effect is probably identical (p. 570) 

 with the loss of energy of comets attributed to the so-called 

 resisting medium. But whatever be the tlieoretical expla- 

 nation of these phenomena, it is almost cei'tain that there 

 exists a tendency to the dissipation of tlie energy of the 

 planetary system, which will, in the indefinite course of 

 time, result in the fall of the planets into the sun. 



It is hardly probable, however, that the planetary system 

 will be left undisturbed tliroughout the enormous interval 

 of time required for the dissipation of its energy in this way. 

 Conflict with other bodies is so far from being improbable, 

 that it becomes approximately certain when v\e take very 

 long intervals of time into account. As regards conietary 

 conflicts, I am by no means satisfied with the negative 

 conclusions drawn from the remarkable display on the 

 evening of the 27th of November, 1872. We may often 

 have passed through the tail of a comet, the light of which 

 is probably an electrical manifestation no inore substantial 

 than the aurora borealis. Every remarkable shower of 

 shooting stars may also be considered as proceeding from a 

 cometary body, so that we may be said to have passed 

 through the thinner parts of innumerable comets, liut the 

 earth has probably never passed, in tinjes of which we liave 

 any record, through the nucleus of a comet, which consists 

 perhaps of a dense swarm of small meteorites. AVe can 

 only speculate upon the effects which might be ])roduced 

 by such a conflict, but it would jjroliably be a much more 

 serious event than any yet registered in history. The 

 probability of its occurrence, too, cannot be assigned ; for 

 though the probaliility of conflict with any one cometary 

 nucleus is almost iiifinite>imal, yet the number of comets 

 is immensely great (p. 408J. 



