W. Harkness — Magnitude of the Solar System. 231 



divided our knowledge into a number of sciences which we 

 usually regard as quite distinct from each other. Along cer- 

 tain lines, or more properly, in certain regions, these sciences 

 necessarily abut on each other, and just there lies the weakness 

 of the specialist. He is like a wayfarer who always finds 

 obstacles in crossing the boundaries between two countries, 

 while to the traveler who gazes over them from a commanding 

 eminence the case is quite different. If the boundary is an 

 ocean shore there is no mistaking it ; if a broad river or a 

 chain of mountains it is still distinct ; but if only a line of posts 

 traced over hill and dale, then it becomes lost in the natural 

 features of the landscape, and the essential unity of the whole 

 region is apparent. In that case the border land is wholly a 

 human conception of which nature takes no cognizance, and so 

 it is with the scientific border land to which I propose to invite 

 your attention this evening. 



To the popular mind there are no two sciences further apart 

 than astronomy and geology. The one treats of the structure 

 and mineral constitution of our earth, the causes of its physical 

 features and its history, while the other treats of the celestial 

 bodies, their magnitudes, motions, distances, periods of revolu- 

 tion, eclipses, order, and of the causes of their various phe- 

 nomena. And yet many, perhaps I may even say most of the 

 apparent motions of the heavenly bodies are merely reflections 

 of the motions of the earth, and in studying them we are really 

 studying it. Furthermore, precession, nutation and the., phe- 

 nomena of the tides depend largely upon the internal structure 

 of the earth, and there astronomy and geology merge into each 

 other. Nevertheless the methods of the two sciences are 

 widely different, most astronomical problems being discussed 

 quantitatively by means of rigid mathematical formulae, while 

 in the vast majority of cases the geological ones are dis- 

 cussed only qualitatively, each author contenting himself with 

 a mere statement of what he. thinks. With precise data the 

 methods of astronomy lead to very exact results, for mathe- 

 matics is a mill which grinds exceeding fine ; but after all, 

 what comes out of a mill depends wholly upon what is put 

 into it, and if the data are uncertain, as is the case in most cos- 

 mological problems, there is little to choose between the 

 mathematics of the astronomer and the guesses of the geologist. 



If we examine the addresses delivered by former presidents 

 of this Association, and of the sister — perhaps it would be 

 nearer the truth to say the parent Association on the other side 

 of the Atlantic, we shall find that they have generally dealt 

 either with the recent advances in some broad field of science, 

 or else with the development of some special subject. This 

 evening I propose to adopt the latter course, and I shall invite 



