The sun. [volume] (New York [N.Y.]) 1833-1916, 1891-12-06
SOME NEW BOOKS.

SOME NEW BO0KA
TIE Causes OF . aissisi Epoch.
There 10 scarcely any question Of wider eel
entinc interest than this: How long ago 90
CURED the glacial era, Of which 90 many YES
tires remain in the northern hemisphere, ant
what was its relation to geography. geology
and anthropology7 Hitherto. the English
writer's most conspicuously connected with tho
discussion OF the subject have been Prof
James Geikie. author OF Tho Great Ice Age"
and DR.. James Croll. author Of Climate and
Time" II Sir John Lubbock. the editor Of the
New Modern Science Series" tAppletons).
has thought that there was room for new
book on the Same topic. II is because serious
error had crept into the astronomical theory Of
tho Ice age. and It lie the discoverer OF that err
TOR. Sir ROBERT BALL. who has been invited to
expound the theme anew in q small octave
volume Of less than 200 pages. We should say
at the outset that the error does not invalidate
the deductions drawn by Messrs. Geikie and
Croll from tho data before them. The mistake
made was ono OF understatement and tho
correction Of By Sir Robert Ball not only con-
firms. but emphasizes the conclusions which
they roach.
7.
The calculations By which Sir Robert Ball
arrived at the fundamenta discovery to which
We shall presently refer arc set forth in an ap-
psndix. being intended for tho eyes Of students
conversant with the higher mathematics.
Thcre 10. however. simple method Of investl.
eating the conditions Of the question which
will be perfectly Ineligible to the general
reader. The essential point necessary for the
production Of an Ice age in one hemisphere
and the simultaneous production OF . genial
age in the opposite hemisphere can b entirely
expressed By the difference in the length Of
the seasonA Tho essence Of tho astronomical
theory resides in the faet that the total heat
from the sun received during the summer In
ono hemisphere 10 Q constant quantity and
that tho total heat received during tho winter
Is also . constant quantity. Tho highest
degree OF contrast between tho climatic
condition OF the two seasons is, there
fore obtained by making the difference
in the d.rattons OF the seasons as great
As possible. Tho difference between tho
lengths Of the seasons ia. as q mathema-
tician would say. o function Of two other inde-
pendent quantities: In other words. It depends
partly on tho eccentrioity Of the earth's orbit
and partly on the position Of the line Of eQui-
noxes with respect to the longer axis Of the
esrth's orbit When the position Of tho line Of
equinoxes is perpendicular to the major axis
Of the earth's orbit the inequality between tho
duration Of tho two seasons 10 as great as is
compatible with the eccentricity at the time
1fthere were no eccentricity there could be no
difference In tho lengths Of the seasons. no
matter where the line Of equinoxes may lie.
Taking the eccentricity and the longitude Of
the perihelion-that Is to say. the position OF
the line Of eauinoxes with respect to the |
longer axis Of the earth's orbit-as these two |
elements OF the calculation are found at any !
moment. the number Of days constituting |
the difference in the duration OF the sea- |
sons Is Q determined point In the |
course Of this book 1t Is demonstrated that |
the number 80 denotes the maximum number |
OF days which. by . combination Of favorable |
circumstances. express tho difference In dura- |
ton Of summer and winter respectively. From |
this maximum tho difference may descend |
step By step to its present value OF seven days.
or even to zero. In which case the two seasons /
would be Of equal duration. Tho greater tho |
difference. the moro intense becomes tho con. |
trast between tho climatic conditions Of the /
two hemispheres. That hemisphere which |
has the long summer and the short winter will |
be In Q genial condition. while that with the /
short summer and the long winter will b |
glaciatedL Other things being equal. It may bs |
affirmed with certainty that the greater tho |
number OF days In this difference the more in |
tense will be the severity Of the corresponding |
Zoe age. 30 long. however. IS the eccentricity |
Of the earth's orbit remains In its present |
value. tho difference between the lengths of |
the seasons will only nuotuate between the ex-
treme values of q winter seven days |
longer than summer and Q summer seven
days longer than winter. Between these
limits there can be no Ice age: II there was
possibility Of suoh our southern hemisphere
should be at this moment In q glacial state,
and. OF course. the northern hemisphere In
corresponding genial state When. on the
other hand. the difference in the duration OF
the seasons. which Is at present only seven
days. shall make oome considerable approach
toward its maximum value. which is nearly
five weeks. then our globe would again be
placed In the condition In which ice ages and
genial ages will oscillate between ono hemis-
phere and the other. the interval between the
time OF the maximum Of glacistion and the
maximum Of the opposite condition in either
hemisphere being 10000 years.
II.
Lot Us see now under what conditions tho
maximum difference in tho duration Of the
seasons. or close approach to It. may be ex-
pected. Lot US. In other words. describe the
astronomical theory Of the Ice age as We find
It expounded In tho book under review. It is, |
IN the first place. pointed out that so long as
the earth's orbit retains form which Is nearly
circular. there con be no extensive glaciation;
the seasons must succeed each other accord
ing to tho same laws that they observe at PRES
emit When. however. BY the disturbing eject Of
As Other planets. the earth's orbit passes from
tho nearly circular to the elliptic shape-such
state OF things 10 only attained after i'm.
mense intervals Of ume-then II remains
decidedly elliptical during a period which
Is long enough for several ice ages. But
this. as we have seen above. Is only
the first condition: another astronomical
adjustment must oonspire with high degree
Of ocoentrioity. When the line OF equinoxes
happens to DO perpendicular to the longer
axis OF tho ellipse. we have the Sec
oud necessary condition for the estab.
lishmont OF an leo aga Nor Is there
anything arbitrary In the assumption that tho
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