APPENDIX. 393 



risk of error may be much diminished by taking the mean of 

 the observed intervals of time. A comparison between observa- 

 tions between stations whose height is known, made under 

 different conditions, by day and night, and in different states of 

 weather, might, I think, contribute to diminish our ignorance as 

 to the variable conditions of the atmosphere at different heights 

 above the surface. 



APPENDIX B. 



REMARKS ON MR. CROLL'S THEORY OF SECULAR CHANGES OF 

 THE earth's climate. 



Most scientific readers are familiar with the theory respecting 

 the influence of changes in the eccentricity of the earth's orbit 

 on the climate of the globe, which has been sustained with 

 remarkable ability by Mr. James CroU. The views originally 

 advanced in various scientific periodicals were presented to the 

 public in a connected form in the volume entitled " Climate and 

 Time," wherein the author has brought a wide knowledge of the 

 principles of physics, and of the whole field of geological science, 

 to the support of his theory. Even those who have not given 

 especial attention to the subject are also acquainted with the 

 conclusions which Sir Charles Lyell drew from the discussion 

 of Mr. Croll's arguments, and which are contained in the 

 thirteenth chapter of the tenth edition of his " Principles of 

 Geology," and also with the more recent examination of the 

 subject which is to be found in Mr. Alfred Wallace's important 

 work, " Island Life." 



I need not say that a theory so important in. its bearing on 

 some of the most obscure problems of geology has been dis- 

 cussed, in more or less detail, by many other writers. To most 

 of the objections presented to his theory, Mr. CroU has replied 

 with his usual ability ; and I believe that at present the prevail- 

 ing tendency among geologists is towards a partial acceptance 

 of his views, subject to the limitations assigned by Mr. Wallace. 



