1851.] HOPKINS ON CHANGES OF CLIMATE. 61 



the temperature of the region in which the earth is placed, imme- 

 diately surrounding the sun. The question, however, more imme- 

 diately before us, is restricted to the consideration of the probability 

 of this region having been so much colder during the glacial period 

 than at present, as the phaenomena of that period would, if thus 

 accounted for, require. This question admits of a very definite 

 answer. 



Whatever may be the indefiniteness of geological periods of time, 

 we are not altogether without means of obtaining a rude estimate of 

 the time which must have been required for the production of some 

 of the later geological phaenomena. Sir Charles Lyell has estimated 

 the time necessary for the formation of the delta of the Mississippi, 

 and the accumulation of the alluvial matter of the plain above, at about 

 100,000 years. The Rhone must have been performing incessantly 

 a similar work at the head of the Lake of Geneva, and many other 

 rivers at the heads of many other lakes, probably ever since the 

 glacial period. Can they have been longer in eifecting their compa- 

 ratively minute operations, than the great American river has been 

 performing its great work of deposition? It would seem improbable. 

 But to render our reasonings the more definite, let us suppose 100,000 

 years to have elapsed since the period of low temperature in western 

 Europe. 



The distance which the solar system would traverse in space during 

 that time, with the rate of motion above given, as calculated by M. 

 O. Struve, would be equal to about ^th of the sun's mean distance 

 from the stars of the first magnitude. Conceive an imaginary sphere, 

 having the earth in its centre, and of which this distance should be 

 the radius. Then the question is, — What variation of the temperature 

 due to stellar radiation may there be within this imaginary sphere ? 



We have no means of estimating the whole effect of the aggregate 

 radiation of all the stars in space ; but we know that any sensible 

 variation of temperature due to that cause within a limited space like 

 that of our imaginary sphere, must be entirely due to the influence 

 of stars more immediately in its vicinity. Now we know that the 

 effect of radiation from these nearer stars, at their present distances, 

 on the temperature of any point within the solar system, is exceed- 

 ingly small, and therefore, that the minimum temperature at any point 

 within the above-mentioned sphere can differ only by an exceedingly 

 small, and probably inappreciable, quantity from the temperature of 

 the portion of space now occupied by the earth's orbit, — the tempe- 

 rature here spoken of being always understood to be that which is 

 due to stellar, and independent of solar radiation. A similar con- 

 clusion would not be necessarily applicable to the maximum tempe- 

 rature ; because some part of the boundary of any proposed limited 

 space, like that of our imaginary sphere, might so far approximate 

 to one particular star as to come under the influence of a radiation 

 from it, indefinitely greater than that from any other star. With 

 centres of heat there must be local spaces of very high temperature, 

 but there are no corresponding limited spaces of very low tempe- 

 rature, because there are no corresponding centres of cold. These 



