PROGEESS IN ASTEONOMY. 139 



Enoug-h hiis ])oon stilted to .show that there is not likely to l)e any 

 breach of continuity in the treatment of this most important problem. 



Another attack on the moon, and incidentally its motion, has 

 recently been made b\^ another anah'st — Prof. George Darwin. Grap- 

 pling with all the consequences of tidal friction, he has been able to 

 present to us the past and future history of our satellite. Beginning 

 as a part of the material congeries from which subsequently, some 

 50,000,000 years ago, both earth and moon, as separate bodies, were 

 formed, it has ever since been extending its orbit, and so retreating 

 farther away from its center of motion, while the period of the earth's- 

 rotation has been increasing at the same time from a possible period 

 of some three hours when the moon was born to one of 1,-iOO hours 

 when the day and month will be equal, something- like 150,000,000 

 3'ears being required for the process. 



STELLAR EVOLUTION. 



It was only in the eighties, after thousands of observations of the 

 spectra of stars, nebula, and comets had been secured, that the full 

 meaning of the revelations of the spectroscope began to dawn upon 

 the world. 



Before the introduction of spectrum anah'sis all stars were supposed 

 to be suns, and the onh^ difference recognized among them was one of 

 brilliancy, and the variation of brillianc}^ in the case of some of them. 



It ultimately came out that great classes might be recognized b}' the 

 differences of their spectra, which were ultimately traced to differences 

 in their chemistr}" and in their temperature, as determined by the 

 extension of the spectra in the ultraviolet, the whiter stars being- 

 hotter than the red ones, as a white-hot poker is hotter than a red-hot 

 poker. 



Next there was evidence to show that a large proportion of the stars 

 were not stars at all like the sun, but swarms of meteorites; and in 

 this wa}^ the m3\sterious new stars which appear from time to time in 

 the heavens, and a large number of variable stars, were explained as 

 arising from collisions among such swarms. 



The inquiry which dealt with the spectroscopic results, having thus 

 introduced the ideas of meteor swarms and collisions to explain many 

 stellar phenomena, went further and showed that the various chemical 

 changes observed in passing from star to star might also be explained 

 by supposing the whole stellar constitution to arise from cool meteoritic 

 swarms represented by nebulae, the changes up to a certain point being 

 explained by a rise of temperature due to condensation toward a cen- 

 ter. Here the new view was opposed to that of Laplace, advanced 

 during the last century, that the stars were produced by condensation 

 and cooling; but Kelvin had shown, before the new view was enun- 

 ciated, that Laplace's view was contrary to thermodynamics, a branch 



