﻿392 Scientific Intelligence. 



portion of the second link in the chain of causation must exist ; 

 and this should proceed 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, Phil. Mag., 1878). 



However this may be, so remarkable a doctrine as the instabil- 

 ity 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 its acceptability. 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 astro- 

 nomical 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 diffi- 

 culty he proposes a modification. According to his view, large 

 eccentricity in the earth's orbit will only produce glaciation when 

 accompanied by favorable 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- 

 favorable, 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, while there is instability in moderate climates. I 

 cannot perceive that we have much reason from physical consider- 

 ations for accepting these remarkable propositions, and the accept- 

 ance 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, 



