IS On the Causes of Change in the 



centl v before you, " On the Causes of Change in the Earth's Super- 

 ficial Temperature ." You will recollect that, until very recently, 

 the only change of climate which had been recognised by geologists 

 as having taken place during the earth's geological history was one 

 from a higher to a lower temperature, and, for those who believed 

 in the primitive heat of the globe, 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 rendered it necessary to recognise a change of climate in our 

 own region of the globe from a lower temperature during that 

 period to a higher subsequent temperature, new conditions were 

 added to the problem, which rendered the cause formerly assigned 

 manifestly inadequate for its solution. Two other causes, how- 

 ever, had been previously suggested, which might possibly ac- 

 count, not only for a change from a higher to a lower superficial 

 terrestrial temperature, but also for oscillatory changes. One of 

 these assigned causes rested on the hypotheses of motion of the 

 whole solar system in space, and the variable temperature of the 

 different regions through which it might thus pass ; the other cause 

 assigned was the influence of different configurations of land and 

 sea on the climatal state of particular portions of the earth's sur- 

 face. Thus of the three causes above alluded to, speaking of them 

 with reference to the earth's surface, one was internal, another ex- 

 ternal, and the third superficial. No attempt, however, had been 

 made to examine the efficiency of these different causes to account 

 for all the phenomena which may be referable to them. It was 

 to remedy this defect that I undertook the investigations contained 

 in the paper of which I am speaking. 



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 pre- 

 sent, at past epochs, provided 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. The terrestrial 

 temperature, to the depth of about 70 feet, varies with the pro- 

 gress of the seasons, the variation becoming less as the depth is 

 greater, until, at about the depth just mentioned, it is no longer 

 sensible, so that a thermometer placed there would indicate a con- 

 stant temperature during the whole year. A second thermometer 

 at a greater depth would also indicate a constant temperature 

 throughout the year, but higher than that indicated by the pre- 

 ceding one. If this second thermometer were placed at a still 

 greater depth, it would indicate a still higher constant tempera- 

 ture ; and the increase of temperature between the two thermo- 

 meters would be proportional to the distance between them, i.e., 

 the temperature in descending below th? first thermometer would 

 increase at a constant uniform rale 



