ADVANCE OF ASTRONOMY. 203 



rid of, that the moon might be habitable like the 

 earth, and yet there seems no unanimous answer to 

 the question: — Has the moon no atmosphere, or one 

 of extreme tenuity ? We have got far from the belief 

 of Shroter, who imagined he had discovered a lunar 

 city; what were called seas are now said to be cov- 

 ered with dry rock ; what are called rills are now said 

 to be great clefts or gorges certainly waterless, but we 

 remain in doubt as to the meaning of the broad white 

 rays which diverge for hundreds of miles from some 

 of the principal " ring-plains," and there are many 

 who attribute to glaciation what others confidently 

 interpret as due to volcanic action. Perhaps the 

 most interesting observations are the few which point 

 — though with insufficient security — to some slight 

 changes on the moon^s apparently changeless face. 



Similarly, there are maps of Mars now in circu- 

 lation, which surpass in detail those available in re- 

 gard to Africa a century ago. And though the pre- 

 cision of these Martian maps may be fallacious the 

 same is true of many of the early maps of Africa, 

 and we cannot gainsay the impression of a greatly 

 increased intensity of observation. To what is this 

 due ? To more powerful telescopes, to the use of the 

 spectroscope, and polariscope, to the development of 

 photography, and to an exact knowledge of the times 

 (in " opposition " to the sun, i. e., nearest the earth) 

 when Mars can be studied to best advantage. 



The study of Mars illustrates the growing intensity 

 of observational study, while the imaginary super- 

 structure reared by some on the supposed existence of 

 an intricate system of canals illustrates the danger 

 of outstripping the evidence. 



PHYSICAL ATTD CHEMICAL PROBLEMS. 



Beginnings of Physical Astronomy. — In 1610, 



