94 ON THE MAGNITUDE OF THE SOLAR SYSTEM. 



constitution of our earth, tlie causes of its physical features and its 

 history, while the other treats of the celestial bodies, their magnitudes, 

 motions, distauces, jieriods 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 retlectious 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 otlier. 

 Nevertheless, the meth'ods of the two sciences are widely diftei-ent, 

 most astronomical problems being discussed quantitatively by means 

 of rigid mathematical formuhe, while in the vast majority of cases the 

 geological ones are discussed only qiuilitatively, 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 tine; 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 j)residents of this 

 association, and of the sister — perhaps it would be nearer the truth to 

 say the inir^nt — association on the other side of the Atlantic, avc shall 

 Hud that they have generally dealt either with the recent advances 

 in some broad field of science, or else Avith the development of some 

 special subject. This evening 1 propose to adopt the latter course, and 

 I shall invite your attention to the present condition of our knowledge 

 respecting the nuignitude of the solar system; but in so doing it will be 

 necessary to introduce some considerations derived from laboratory 

 experiments upon the luminiferous ether, others derived from exi)eri- 

 ments upon ponderable matter, and still others relating both to the 

 surface i^henomena and to the internal structure of the earth, and thus 

 we shall deal largely with the border land where astronomy, i)hysics, 

 and geijlogy merge into each other. 



The relative distauces of the various bodies which compose the solar 

 system can be determined to a considerable degree of approximation 

 with very crude instruments as soon as the true plan of the system 

 becomes known, and that plan was taught by Pythagoras more than 

 five hundred years before Christ. It must have been known to the 

 Egyptians and Chaldeans still earlier, if Pythagoras really acquired 

 his knowledge of astronomy from them, as is- affirmed by some of the 

 ancient writers, but on that point there is no certainty. In public 

 Pythagoras seemingly accepted the current belief of his time, which 

 made the earth the center of the universe, but to his own chosen disci- 

 ples he communicated the true doctrine that the sun occupies the center 

 of the solar system and that the earth is only one of the planets revoly- 



