PREFACE ix 



Mr. J. P. Hill and Dr. Arthur Willcy. Professor W. N. Parker 

 has very kindly read the whole of the proof-sheets and favoured us 

 with many valuable suggestions, besides acting as referee in 

 numerous minor difficulties which would otherwise have cost a 

 delay of many weeks. 



It is a mere truism to say that a text-book can never really 

 reflect the existing state of the science of which it treats, 

 but must necessarily be to some extent out of date at the time 

 of publication. In the present instance, the revises of the earlier 

 pages, giving the last opportunity for any but minor alterations, 

 were corrected in the latter part of 1895, and the sheets passed 

 for press in the middle of 1896. We are, therefore, fully alive to 

 the fact that much of our work already needs a thorough revision, 

 and can console ourselves only by reflecting that " to travel hope- 

 fully is a better thing than to arrive, and the true success is to 

 labour." 



We may mention, in conclusion, that, whatever may be the merits 

 or demerits of the book, it enjoys the distinction of being unique 

 in one respect. The two authors have been separated from one 

 another, during the greater part of their collaboration, by a 

 distance of 1200 miles, and the manuscript, proofs, and drawings 

 have had to traverse half the circumference of the globe in their 

 journeys between the authors on the one hand, and the publishers, 

 printers, artist, and engravers on the other. It will, therefore, be 

 readily believed that all persons concerned have had every oppor- 

 tunity, during the progress of the work, of exercising the supreme 

 virtue of patience. 



