514 KEPORT — 1886. 



stratum of air on the surface of the earth in the latitudes higher than 30° — a 

 stratum in which the inhabitants of tliose latitudes have their existence, and of 

 which the movements constitute the observed winds of those latitudes — being, by- 

 friction and impulses on the surface of the earth, retarded with reference to the 

 rapid whirl or vortex motion from west to east of the great mass of air above it, 

 tends to flow towards the pole, and actually does so flow to supply the partial void 

 in the central parts of that vortex, due to the centrifugal force of its revolution. 

 Thus it appears that in the temperate latitudes there are three currents at different 

 heights — that the uppermost moves towards the pole, and is part of a grand 

 primary circulation between the equatorial and polar regions ; that the lower- 

 most moves also towards the pole, but is only a thin stratum forming part of a 

 secondary circulation ; that the middle current moves from the pole, and constitutes 

 the return current for both the preceding ; and that all these three currents have a 

 prevailing motion from west to east.' ' 



Such, then, appears to be our present state of ignorance of these great terrestrial 

 actions, and any speculations as to the precise effect of changes in the annual dis- 

 tribution of the sun's heat must be very hazardous until we know more precisely 

 the nature of the thing changed. 



When looking at the astronomical theory of geological climate as a whole, one 

 cannot but admire the symmetry and beauty of the scheme, and nourish a hope 

 that it may be true ; but the mental satisfaction derived from our survey must not 

 blind us to the doubts and difiiculties with which it is surrounded. 



And now let us turn to some other theories bearing on this important point of 

 geological time. 



Amongst the many transcendent services rendered to science by Sir William 

 Thomson, it is not the least that he has turned the searching light of the theory of 

 energy on to the science of geology. Geologists have thus been taught that the 

 truth must lie between the cataclysms of the old geologists and the uniformi- 

 tarianism of forty years ago. It is now generally believed that we must look for 

 a greater intensity of geologic action in the remote past, and that the duration of 

 the geologic ages, however little we may be able mentally to grasp their greatness, 

 must bear about the same relation to the numbers which were written down in 

 the older treatises on geology, as the life of an ordinary man does to the age of 

 Methuselah, 



The arguments which Sir William Thomson has adduced in limitation of geo- 

 logical time are of three kinds. I shall refer first to that which has been called the 

 argument from tidal friction ; but before stating the argument itself it will be 

 convenient to speak of the data on which the numerical results are based. 



Since water is not frictionless, tidal oscillations must be subject to friction, and 

 this is evidenced by the delay of twenty-four to thirty-six hours, which is found 

 to occur between full and change of moon and spring-tide. An inevitable result 

 of this friction is that the diurnal rotation of the earth must be slowly retarded, 

 and that we who accept the earth as our timekeeper must accuse the moon of a 

 secular acceleration of her motion round the earth, which cannot be otherwise 

 explained. It is generally admitted by astronomers that there actually is such an 

 unexplained secular acceleration of the moon's mean motion. 



No passage in Thomson and Tait's ' Natural Philosophy ' has excited more 

 general interest than that in which Adams is quoted as showing that, with a 

 certain value for the secular acceleration, the earth must in a century fall behind a 

 perfect chronometer, set and rated at the beginning of the century, by twenty-two 

 seconds. Unfortunately this passage in the first edition gave an erroneous com- 

 plexion to Adams's opinion, and being quoted, without a statement of the premisses, 

 has been used in popular astronomy as an authority for establishing the statement 

 that the earth is actually a false timekeeper to the precise amount specified. 



In the second edition ("in the editing of which I took part) this passage has 

 been rewritten, and it is shown that Newcomb's estimate of the secular acceleration 

 only gives about one-third of the retardation of the earth's rotation, which resulted 

 from Adams's value. The last sentence of the paragraph here runs as follows : — 



» Brit. Assoc. Report, Dublin, 1857, p. 38-9. 



