2. Repeated load displacements of land and sea, and oonsoquent litoral action.
3. The melting of great circumpolar glaciers, and the drifting of floating ice.
During the period of nearly twenty years which has elapsed since the first publication of the preceding portion of this article, much has been done on the subject, both by mathematicians and geologists. The consequences of admitting the hypothesis of the refrigeration of the globe, and the history of the gravel containing sea-shells alluded to above, and now called the "northern drift," hare been rigorously investigated. Until within this period, the only change of climate which bad been recognised by geologists as baying taken place during the earth's geological history was one from a higher to a lower tempe rature ; and for those who believed in the prinlitive heat of the globe, however originated, that heat afforded one obvious cause for this higher temperature at remote geological epochs. When, however, an examination of the phenomena of the " glacial epoch " (first conceived by Agassiz) during and on the gradual termination of which that drift was accumulated. rendered it necessary to recognise a change of climate in our own region of the globe (as indicated by the facts noticed in the proposition 1 above) from a lower temperature during that period to a higher subsequent temperature, new conditions were added to the problem. As we have seen, two other causes had been already sug gested, which might poseibly aceouot not only for a change from a higher to a lower superficial temperature, but also for oscillatory changes. One of these assigned causes rested on the fact of the trans latory motion of the whole solar eyetem in space, and the hypothesis proposed by Poisson of the variable temperature of the different regions through which it might pass; thus the other cause assigned, pointed to in propositions 1 and 2 above, was the influence of different configurations of land and sea on the climatal state of particular portions of the earth s surface. No attempt, however, was made to examine the efficiency of these different causes to account for all the phenomena which might be referable to them, until Mr. William Hopkins, F.R.S., of St. Peter's College, Cambridge, when president of the Geological Society of London, undertook a remarkable investigation contained in a paper communicated to that society in 1351, and pub lished in its Quarterly Journal,' vol. viii., pp. 56-92, On the Causes which may have produced Changes in the Earth's Superficial Tempe-. rature.' He also returned to the subject in his anniversary address to the society in the following year, printed in the same volume. From these papers is derived the following view of what may be regarded as the actual condition of the theory of the refrigeration of the globe; hut we have stated the subject in a somewhat different order from that adopted by the author.
If there be any propositions in experimental science which may be deemed incontrovertible, the following, Mr. Hopkins conceives, is one of them :—" If a mass of matter—such, for instance, as the earth with its water and its atmosphere—be placed in space of which the tempe rature is lower than its own, it will necessarily lose a portion of its heat by radiation, until its temperature ultimately approximates to that of the circumambient space, unless this reduction of temperature be prevented by the continued generation of heat." This proposition, it will be seen, is equivalent to an explicit statement of the theory of the refrigeration of the globe in all its generality. Now, assuming the primitive temperature of the globe to have been very much greater than at present, there is manifestly no difficulty in accounting for any higher superficial temperature than the present, at past epochs, pro vided those epochs be sufficiently remote. They must, however, be exceedingly remote to enable us thus to account for a variation of temperature which should sensibly affect the climatal conditions in any part of the earth. If the cooling of the earth were to oontioue for an indefinite period of time,—assuming the temperature of external space, the sun, and the earth'e atmosphere to remain as at present,— the superficial temperature would approximate indefinitely near to a certain limit. The difference between that limit and the earth's present euperficial temperature is the effect due to the remains of the primitive heat. Theory gives us a simple relation between the amount
of this effect and the rate of increase of temperature in descending beneath the earth's surface. Knowing the one, we can immediately determine the other, and thus, having ascertained the rate of increase, we know the amount of superficial temperature which is now due to the earth's primeval heat, assuming always that heat to be the cause of the existing internal temperature of the globe. This amount is thus proved not to exceed about the one-thirtieth part of a degree of the Centigrade thermometer, so nearly has the earth's superficial tem perature approximated to that ultimate limit beyond which it could never descend, supposing external conditions to remain the same. It has been shown, en high mathematical authority, that to reduce the euperficial temperature of the earth by one-halt of the above amount, or one-sixtieth of a centesimal degree, it would require the enormous period of one hundred thousand millions of years. " It would doubt less," says Mr. Hopkins, " require ue to go back into the plat some such immense period as this to arrive at the epoch when the super ficial temperature should have exceeded its present amount by even one or two degrees. At the same time, the rate of increase of tempe rature in descending beneath toe surface would be much more rapid than at present. [The present rate is about 1 Fahr., or :* C. for every 60 feet of depth.) If the superficial temperature amounted to 2 C. above its ultimate limit, instead of having of a degree, the rate at which the temperature would increase in descending would be about sixty times as great as at present ; that is, there would be an increase of I• C. for little more than 1 foot of depth." " It must be recollected that this state of terrestrial temperature, if duo to the cause we are considering," that is, to the refrigeration of the globe, " could only have existed at times which, even in a geological sense, must have been extremely remote. The important peculiarity of this state of the earth would seem to consist in the simultaneous existence of a superficial temperature, and therefore of climatal con ditions, very nearly the same as at present, with an internal tempera ture at the depth of a few hundred feet and upwards, immensely greater than at present." With regard to the reason assigned by Poisson for the higher tempe rature below the surface of the earth and its changes, as cited above in Dr. Wliewell's observations, Mr. Hopkins remarks " What may have been the possible effect of this cause in the lapse of indefinite time, it is impossible to say; but I cannot understand how it could be very considerable without a totally different distribution of the group of stars to which the sun should belong, or the near approach of the solar system to some individual star. The latter hypothesis, however, would be inconsistent with the integrity of the solar system as it now exists, if we suppose the proximity to any single etar to become such as to produce any material modification of terrestrial climate ; and perhaps it may be difficult to conceive how the first hypothesis should escape a similar objection. At all events it may be regarded as certain that according to neither of these hypotheses can any considerable effects have been produced by this cause on terrestrial temperature within the later Tertiary period, and that we cannot thus account for the cold of the glacial epoch." With respect to the third supposed cause of changes in the earth's superficial temperature, we need not follow Mr. Hopkins, as it does not belong to the subject immediately before us • his masterly ex position of it will be found in the papers already cited. For the special history of the glacial epoch, we may refer to Sir C. Lyell's Manual of Elementary Geology,' nfth edition, ch. xi. and sii. Mr. Hopkins, in continuing his researches in physical geology, has inves tigated the effects due to the remains of the earth's "primitive heat ;" an account of his results, together with the principal known facts of the earth's internal temperature, will be given in the article TEMPERA TURE. TERRESTRIAL, DISTRIBUTION OF.