724 THE POPULAR SCIENCE MONTHLY. 



their relations to the atmosphere and ocean in respect to their move- 

 ments. The trade-winds owe their existence to the difference of tcm- 

 peratui-e between the equator and the poles. Whatever increases this 

 difference increases the strength and volume of the winds, whether in 

 the Northern or Southern Hemisphere. The coming on of a period of 

 northern glacial cold would be concurrent with increasincf violence of 

 the northeast trade-wind. It would sweep at its maximum far beyond 

 the equator, for the Southern Hemisphere would be heated and the line 

 of greatest equatorial heat would be southward from its present posi- 

 tion. A deflection of the great equatorial current of the ocean would 

 occur corresponding with this, and its vast volume of heated waters 

 would pour into the southern instead of into the northern ocean. 

 Tlie Gulf Stream would cease to flow, or flow only with greatly dimin- 

 ished volume. " In the severest droughts it never fails," said Prof. 

 Maury, but it may fail from other causes, and leave half a hemisphere 

 rigid from the austerity of cold. Depleted of the Gulf Stream, the sur- 

 face-waters of the North Atlantic would be warmed only by the di- 

 rect rays of the sun, and would rapidly approximate to the low tem- 

 peratures which now prevail only a few fathoms beneath the surface. 

 The mean temperature of Scotland for January is 28 higher than its 

 normal, and 15 above its normal for the year. The loss of the Gulf 

 Stream would change all this. 



Theories which have made the Ice period depend wholly on cold 

 are shown to be untenable by Prof. Tyndall, who calls attention to 

 the fact that great heat is as necessary to the production and growth 

 of glaciers as intense cold, the one being needed to produce vapor, the 

 other to condense and freeze it. Any accumulation of snow aud gla- 

 cial ice is impossible without this combination of circumstances ; but 

 these constitute an integral part of Mr. Croll's theory, which assigns a 

 mild climate to one hemisphere while the other is wrapped with ice. 



Altei-nation of climates in geological time is as certain as diversity 

 of climate at the present day. Evidence of it is found in the geologi- 

 cal record, and eras of glaciation have succeeded each other, but each 

 one has buried or erased many traces of preceding ones, and only the 

 last one is before us, the monumental history of which reveals its 

 startling and wonderful features. 



It is held indeed by Mr. Croll that eras of cold and glaciers alter- 

 nating with those of temperate climate are fully accounted for by the 

 causes stated. His conclusions, however, are not accepted by many 

 eminent geologists. Prof. Dana says that climatic changes effected 

 by the Gulf Stream have been brought about, " not by diversions of 

 the current from the ocean, and its restoration to it again, but by 

 variations in the amount and height of arctic lands, in one case 

 closing and the other opening the arctic regions to the tropical 

 stream, and the same for the Pacific current." While this view may 

 not call in question the warming influences of the stream, it assigns 



