190 ON THE MAGNITUDE OF 



traveller 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 fea- 

 tures and its history, while the other treats of the celestial bodies, 

 their magnitudes, motions, distances, periods of revolution, 

 eclipses, order, and of the causes of their various phenomena. 

 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 phenomena of the 

 tides depend largely upon the internal structure of the earth, and 

 there astronomy and geology merge into each other. Neverthe- 

 less 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 discussed 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 mathematics 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 

 cosmological 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 



