INTRODUCTION 



has the scientific problem of the genesis of the solar sys- 

 tem been in a more chaotic and discouraging state than 

 at this very time, in spite of, or perhaps because of, the 

 distracting multiplicity of rival cosmogonies. 



THE COMETS 



These objects remain as great a mystery to men of 

 science of the twentieth century as to Newton and his gen- 

 eration. It is true that some new information has been 

 gleaned from the spectra of comets, and also from a crit- 

 ical study of their orbital vagaries ; but as to their origin 

 and nature, and the causes of the peculiarities exhibited 

 by their tails, practically nothing has yet been satisfac- 

 torily determined. Young sums it up by saying (Art. 

 737) : "We have little certain knowledge on the subject." 



THE TIDES 



There are two criteria, and two only, for testing the 

 correctness of Newton's tidal theory. One of these was 

 pointed out about a century and a half ago by the German 

 philosopher, Immanuel Kant, who, relying upon the 

 theory, demonstrated that the earth's rotation should be 

 gradually slowing, and our day consequently lengthening. 

 Since Kant's day, however, mathematicians have con- 

 vinced themselves that the day has not lengthened, even 

 infinitesimally ; hence the tidal theory cannot on this 

 count be sustained. The second, and doubtless more 

 satisfactory, test is that of comparing the computations 

 made on the basis of theory with the observations. This 

 test has been repeatedly applied in anxious efforts to cor- 

 roborate Newton, but invariably with results precisely 

 the reverse. Sir George H. Darwin (son of the great 

 naturalist), recently deceased, and the late Lord Kelvin, 

 who to-day are everywhere esteemed as among the 

 highest, if not the very highest, authorities on the tides, 

 for a score of years collaborated together on this subject. 

 It goes without saying that if these gentlemen, imbued as 

 they were to the very last with unquestioning faith in 

 Newton's tidal theories, could have found anything in the 



