TRANSACTIONS OF SUCTION A. 513 



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.* 



However this may be, so remarkable a doctrine as the instability 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 afibrd almost a crucial test of its acceptability. In consequence of the pre- 

 cession of the equinoxes the conditions producing glaciation in one hemisphere must 

 be transferred to the other every 1 0,000 years. If there is good geological evidence 

 that this has actually been the case, we should allow very great weight to the 

 astronomical 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.^ Other geologists do not, however, seem convinced of the conclusive- 

 ness of the evidence. 



Thus Mr. Wallace,' 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 hemisphere 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 instability of climate 

 at first, when glaciation is about to begin under the influence of great eccentricity 

 of the orbit, but afterwards Wallace demands gi-eat 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 considerations for accepting these remarkable proposi- 

 tions, 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 important part in these theories, but at this moment 

 our knowledge of the principal oceanic circulation, 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 completed a work but 

 just begim — viz., the analysis of ships' logs for some sixty 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 Professor James Thomson gave before this 

 Association at Dublin an important suggestion on this point. As it has been 

 passed over in complete silence ever since, the present seems to be a good oppor- 

 tunity 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_ Professor 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 alreadv 

 described does actually occur, but occurs subject to this modification, that a thin 



' Zoppritz, Phil. Mag. 1878, "- Nature, July 8 and 15, 1886. 



' liland Li/e. 



1886. L L 



