Sept. 2, 1886] 
NATURE 
421 
But if climate be unstable, and if, from some of those tempor- 
ary causes for which no reasons can as yet be assigned, there 
occurs a short period of cold, then surely some even infinitesimal 
portion of the second link in the chain of causation must exist ; 
and this should proceed as in the first case to augment the 
departure from the original condition, and the climate must 
change. 
In a matter so complex as the weather, it is at least possible 
that there should be instability when the cause of disturbance is 
astronomical, whilst there is stability in an ordinary sense. If 
this is so, it might be explained by the necessity for a prolonged 
alteration in the direction of prevailing winds in order to affect 
oceanic currents (Zoppritz, Phi’. Mag. 1878). 
However this may be, so remarkable a doctrine as the insta- 
bility of climate must certainly be regarded with great suspicion, 
and we should require abundant proof before accepting it. Now 
there is one result of Mr. Croll’s theory which should afford 
almost a crucial test of itsacceptability. In consequence of the 
precession of the equinoxes the conditions producing glaciation 
in one hemisphere must be transferred to the other every 10,000 
years. If there is good geological evidence that this has actually 
been the case, we should allow very great weight to the astrono- 
mical theory, notwithstanding the difficulties in its way. Mr. 
Croll has urged that there is such evidence, and this view has 
been recently strongly supported by M. Blytt (NATURE, July 8 
and 15, 1886). Other geologists do not, however, seem convinced 
of the conclusiveness of the evidence. 
Thus Mr. Wallace (‘Island Life’’), whilst admitting that 
there was some amelioration of climate from time to time during 
the last glacial period, cannot agree in the regular alternations of 
cold and warm demanded by Mr. Croll’s theory. To meet this 
difficulty he proposes a modification. According to his view, 
large eccentricity in the earth’s orbit will only produce glaciation 
when accompanied by favourable geographical conditions. And 
when extreme glaciation has once been established in the hemi- 
sphere which has its winter in aphelion, the glaciation will persist, 
with some diminution of intensity, when precession has brought 
round the perihelion to the winter. In this case, according to 
Wallace, glaciation will be simultaneous on both hemispheres. 
Again, he contends that, if the geographical conditions are 
not favourable, astronomical causes alone are not competent to 
produce glaciation. 
There is agreement between the two theories in admitting in- 
stability of climate at first, when glaciation is about to begin 
under the influence of great eccentricity of the orbit, but after- 
wards Wallace demands great stability of climate. Thus he 
maintains that there is great stability in extreme climates, either 
warm or cold, whilst there is instability in moderate climates. I 
cannot perceive that we have much reason from physical con- 
siderations for accepting these remarkable propositions, and the 
acceptance or rejection of them demands an accurate knowledge 
of the most nicely balanced actions, of which we have as yet 
barely an outline. 
Ocean currents play a most prominent part in these theories, 
but at this moment our knowledge of the principal oceanic circu- 
lation, and of its annual variability, is very meagre. In the 
course of a few years we may expect a considerable accession to 
our knowledge, when the Meteorological Office shall have com- 
pleted a work but just begun—viz. the analysis of ships’ lozs for 
some s.xty years, for the purpose of laying down in charts the 
oceanic currents. 
With regard to the great atmospheric currents even the general 
scheme is not yet known. Nearly thirty years ago Prof. James 
Thomson gave before this Association at Dublin an important 
suggestion on this point. Asit has been passed over in complete 
silence ever since, the present seems to be a good opportunity of 
redirecting attention to it. 
According to Halley’s theory of atmospheric circulation, the 
hot air rises at the equator and floats north and south in two 
grand upper currents, and it then acquires a westward motion 
relatively to the earth’s surface, in consequence of the earth’s 
rotation. Also the cold air at the pole sinks and spreads out 
over the earth’s surface in a southerly current, at first with a 
westerly tendency, because the air comes from the higher regions 
of the atmosphere, and afterwards due south, and then easterly, 
when it is left behind by the earth in its rotation. 
Now Prof. Thomson remarks that this theory disagrees with 
fact in as far as that in our latitudes, the winds, though westerly, 
have a poleward tendency, instead of the reverse. 
In the face of this discrepancy he maintains that ‘‘the great 
circulation already described does actually occur, but occurs sub- 
ject to this modification, that a thin stratum of air on the surface 
of the earth in the latitudes higher than 30°—a stratum in which 
the inhabitants of those 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 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” (Brit. Assoc. Report, Dublin, 1857, 
pp: 35; 39). 
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 distribution 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 difficulties 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 
geolozy. 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 inten-ity 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 geological time are of three kinds. I shall refer 
first to that which has been called the argument from tidal fric- 
tion ; 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 accu e€ 
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 vilue 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 complexion to Adams's opinion, and 
being quoted without a statement of the premises, 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 :—‘‘ It is proper to add that Adams lays but little stress 
on the actual numerical values which have been used in this 
computation, and is of opinion that the amount of tidal retarda- 
tion of the earth's rotation is quite uncertain.” Thus, in the 
