416 Chonicles of Science. [July, 
Professor Haughton remarks that the law received by astronomers 
is—“ That the quantity of heat received per annum from the sun 
varies inversely as the minor axis of the orbit,” that is to say, the 
heat ¢nereases with the eccentricity. Mr. Croll’s view* was based 
on a contrary opinion—namely, that the greater the eccentricity 
the colder the chmate. In point of fact, however, he has not only 
to prove that astronomers are wrong, but also that the difference 
in temperature owing to the eccentricity of the orbit is sufficiently 
great to be the determining cause of a Glacial Period. 
It is satisfactory to learn that the conclusions of mathematicians 
respecting the ice-cap theory correspond with those arrived at by a 
study of facts. Indeed, Mr. Croll has never yet, we believe, 
attempted to show that the known phenomena of the Glacial Period 
square with his theories ; and unless he can do that, they would not 
advance us a step, no matter how beautifully symmetrical or 
mathematically exact they might be. At present we are merely 
discussing whether they are extraneously possible; their intrinsic 
probability is quite another question. 
Mr. John Evans, of flint implement celebrity, has lately pro- 
pounded before the Royal Society another theory of the cause of 
supposed changes in the earth’s axis of rotation. He takes the 
case of a sphere consisting of a mass of viscid or fluid matter, sur- 
rounded by a crust of inconsiderable thickness, and ina condition of 
revolution about a given axis. He then supposes the equilibrium 
to be disturbed by the protrusion of a portion of the crust some- 
where between one of the poles of the sphere and its equator. The 
result would be, as Mr. Evans states, that the greater centrifugal 
force possessed by this protruded portion would tend to bring it 
towards the equator, and thus alter the axis of rotation of the 
external crust, which would adjust itself about the internal plastic 
and still spherical mass within. The rotation of the sphere and this 
excess of centrifugal force possessed by the protruded portion would 
cause the latter to describe a spiral, as it were, of gradually 
increasing dimensions, until at last it became in the line of the 
equator of the sphere, and described a circle, when the axis of 
rotation would again become fixed until some fresh cause of dis- 
turbance produced a repetition of the process. In the same manner 
Mr. Evans supposes the upheaval of large mountain-masses to have 
operated on the earth’s axis of rotation, the only distinction he 
draws being that, from the spheroidicity of its figure, there would 
be more difficulty in the crust adjusting itself over the fluid nucleus. 
This theory looks very plausible, but if we inquire a little further 
into the case of the supposed sphere, we shall get an indication of 
the kind of difficulty it fails to meet when applied to the earth. 
‘Imagine a second mass to be protruded after the first shall have 
* «Phil. Mag.,’ Jan., 1866. 
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