74 W. Hopkins on Changes of Climate. 
3. ) The basin of the Atlantic from the Tropic to the North 
Sea converted into land, uniting the old and new continents. ~~ 
rge portions of the continents of Euro Nor 
Ametica submerged beneath the surface oe the ocean, and 7 
Gulf-stream diverted into some other cour 
e consideration of st eases will siniaey the first Sect 
of this part of the mem 
10. The snow-line, or pone line on the side of a mountain abe 
which the snow never disappears at any season of the year, bears 
aft important relation to glaciers, since it divides that higher re- 
gion, in which productive agencies prevail in augmenting super- 
ficially the mass of a glacier, from the lower region, in whi 
the destructive agencies predominate. It is essential to our in- 
vestigations to know the vertical distance which existing glaciers 
usually descend beneath the snow-line, that we may be able to 
judge by analogy of the selacars distances to which ancient gle 
ciers may have descend i 
As Ke heorw from the surface of the earth, the mea 
temperature decreases according to laws which have ber approx: — 
imately dodaci’ by observation. If this temperature, there 
fore, at any proposed place be greater than 32° F., we shall arrit® 
at this latter temperature only at a certain elevation above the 
earth’s surface. A line on the side of a erie, or an imagiu- 
ary line in space, along which the mean annual temperature 8 — 
32° F., I define as the dine of 32° F. Its height can be approx 
imately calculated for any place at which the mean annual tear 
perature is known. In sufficiently low latitudes it will be ata 
considerable height above the earth’s surface, but will descend 
the surface along the mean annual isothermal of 32° F. as | 
proceed into higher latitudes. The relative heights of this line 
and of the snow-line at the present time depend on ciret 
ces. It is essential to ascertain these circumstances and their in- 
fluence, that we may be the better able to estimate the height of f 
the snow-line in the hypothetical cases which I purpose to com — 
sider. We shall then be able, as intimated in the preceding par 
agraphh, to estimate the height above the level of the a to which 
the ancient glaciers may have descended. ©The seco 
of this Part will be vs Rae with the consideration of | a 
seo 22 dn Soe dbg third he caine Section I shall offer some * 
ive claims of the different | 
peng 
fire sae Seathien ms to form tt ation of geological ra thearies 
§ I. On the Positions of tl the 5 Weineeeba Lines in the above: 
mentioned hypothetical cases. _ 
UL. Taking the first ease; that of the absence of the Gul 
sired, lot os tate 4 bable cot January isother 
mal of 32° F oceedit 
it aftains its these ay see 
