3i8 POPULAR SCIENCE MONTHLY. 



Consider, for an instant, what is involved in the theory of revolving 

 material crystal shells. The stars are at an immense distance, all 

 fixed to a crystal surface, which revolves once in twenty-four hours. 

 The sun is situated on the surface of another shell, but it can not 

 be in one fixed spot on the surface, for we see it rise and set at different 

 points of our horizon at different times of the year. What kind of 

 a crystal shell is it upon which the sun can glide so far and no farther ? 

 No wonder that certain medieval writers felt the necessity of imagin- 

 ing two shells for each luminary between which the motion took place 

 with freedom, beyond which there was no passage. What sort of 

 shells are those that correspond to the planets, each of which moves 

 at various rates in varied directions — sometimes eastward, sometimes 

 westward, sometimes north, sometimes south ? The details of a scheme 

 like this are literally unthinkable. It must be accepted, if at all, by 

 faith — by a faith founded in phrases. 



The ancient astronomers did not, in general, seek knowledge for 

 its own sake. They were either concerned about some practical matter, 

 as the length of the year, the prediction of the seasons and the like; 

 or else sought acquaintance with some aspect of divine or partly divine 

 matter, such as formed the planets and the stars. The science of the 

 middle ages has been summarized in a sentence: 'It was all divination, 

 clairvoyance, unsubjected to our modern exact formulas, seeking in 

 an instant of vision to concentrate a thousand experiences' (Pater). 

 A few of the ancients, Archimedes and Aristarchus, for example, had 

 what we call the modern spirit. Roger Bacon was the first to formulate 

 it. Newton may be taken as its first thorough-going representative, 

 for even Kepler and Galileo were deeply tinged at times with the 

 medieval color. 



The Meteorologica and the De Ccelo of Aristotle (384-322 B. C.) 

 were the text-books of the middle ages. The doctrine of material 

 spheres was frankly adopted in these books and in the writings derived 

 from them. The geometric scheme of Eudoxus was transformed into 

 a clumsy mechanism, and its complexity was further increased by the 

 addition of other spheres, so that fifty-six in all were necessary to 

 explain celestial motions. "The glorious philosopher, to whom 

 nature opened her secrets most freely, proved in the second chapter of 

 his De Coelo, that this world, the earth, is of itself stable and fixed to 

 all eternity. . . . Let it be enough to know, upon his great authority, 

 that this earth is fixed and does not revolve, and that it, with the sea, 

 is the center of the heavens. These heavens revolve round this center 

 continuously even as we see" (Dante, Convito, iii., chap. v.). Until we 

 remember that mechanics was an unknown science to the ancients and 

 in the middle ages, it is almost impossible to conceive how professors 

 could teach, or students accept, a system like Aristotle's that was, in 

 essence, unintelligible. While Cremonini was expounding the De Ccelo 



