Cause of the Glacial Period 
conditions of which we are in search under the hypothesis of a North Atlantic current taking the place of 
the Gulf Stream, but with a temperature ten degrees lower than the latter. If we suppose that the 
annual mean temperature of all those regions influenced by the Gulf Stream as far south as (say) the 
parallel of 40° N. is reduced by about ten degrees below its present range, then we shall have the present 
isotherm of (let us say) 32° Fahr. taking the position of that of 42° Fahr., and that of 42° taking the 
position of that of 52°; there will, in fact, be a general advance of cold southward. Then by observing 
the climatic conditions of the regions crossed by the present isotherms of 32° and 42° we shall be able to 
form an approximate idea of the climate, under the hypothetical conditions of temperature we are here 
considering. Iam well aware that this mode of determination would not, in all cases, give strictly 
accurate results. Climates depend not only on temperature, but on relations of land and sea, on levels, 
prevalent winds, and other conditions ; but temperature is a main factor, and the mode of determination 
here suggested will probably afford fairly reliable results. 
Isotherms.—Of all the isothermal lines representing annual mean temperature that may be drawn 
across the chart of the Northern Hemisphere, none is more important than that of 32° Fahr., the freezing- 
point of water. This isotherm, according to Berghaus,! crosses America, from lat. 58° N. on the west 
coast, to Cape Charles, lat. 52° 35’ N. on the east ; skirts the southern coast of Greenland and crosses the 
Atlantic by the northern coast of Iceland ; entering Europe near the North Cape ; then trending south- 
wards along the coast of Norway to the south of the Arctic Circle, it crosses the Europe-Asian Continent 
nearly along the 6oth parallel to the coast of China. This isotherm is everywhere to the south of the 
Arctic Circle except in that part of the Atlantic bordering the coast of Norway and lying to the south-east 
of Iceland, where it passes the Circle along the arm of the Gulf Stream which, even in these high 
latitudes, gives evidence of its power to ameliorate the rigour of the climate. 
The isotherm of 32° Fahr. may be regarded as a convenient line of demarcation between the 
permanently glacial regions and those which enjoy a temperate climate. To the-north of this line are 
situated the frozen regions of Hudson Bay, Labrador, Baffin Bay, and Davis Strait (regions only 
accessible during two or three months in the year), the continental island of Greenland enshrouded in 
eternal snow and ice,” the Greenland Sea blocked by ice-floes, the glacial isles of Spitzbergen and Franz 
Josef Land, Novaia Zemlia and Liakov Isles (New Siberia), with the surrounding Arctic Sea, whose 
surface of ice is only penetrable during three months in the year ; lastly, the mountains of North Norway 
and Lapland and the frigid tracts of Siberia bordering the ocean, where the soil is permanently frozen a 
few inches below the surface. Of all these regions it may be said, that if their temperature ever rises 
above freezing-point of water, it is only to the extent ofa very few degrees during the three summer months.® 
Isotherm of 42° Fahr.—Very different is the climate enjoyed by those regions lying under the 
isothermal line of 42°. Leaving the western coast of America at Vancouver it crosses that continent by 
the Great Lakes to Nova Scotia in lat. 45° N.; then driven northwards by the Gulf Stream it crosses the 
6oth parallel half-way between Iceland and Scotland and reaches the coast of Norway a little north of 
Bergen ; then curving round the southern shores of Scandinavia, passes into Russia south of St. Petersburg, 
and crosses Central Asia near the intersection of the parallel of 45° N. by the meridian of go° E. This 
isotherm, except when it passes over the Atlantic, is characterised by extremes of heat and cold ; but the 
heat predominates, and glacial conditions are impossible except at high altitudes such as are afforded by 
the mountains of Norway. 
1 Physikalischer Atlas (Gotha, 1892). See map (Plate X.). 
2 Except along the margin, and inland valleys as shown by Nansen, Northern Mists (passim). 
3 The following are some of the temperature observations within the Polar regions :— 
Three Three Annual 
Summer Winter 
Months. Months. ‘Temperature. 
Banks Land-. 3 0 0 6 e a Latn74 N= +35° — 3°.7 +1°.8 Fahr. 
Parry Island. 6 . 0 op. ME OS +37° — 10°.6 a TeeA ess 
Cornwallis Island . ‘ 6 0 7 0 mp Pas’ + 36° — 8°.6 Co op 
Northumberland Sound 6 2 P op Re +30°.8 —11°.8 SB gy 
