Xll PREFACE TO THE FOURTH EDITION. 



b. For comparing the most important Geographical Distances ; con- 

 taining ten tables. 



c. For comparing the most important measures of Geographical Sur- 

 faces ; containing ten tables. 



Series V., containing the Meteorological Tables, now becomes Series 

 VI. , with the same name, and remains unchanged. 



Series VI., containing the Miscellaneous Tables, now becomes Series 

 VIL, and has been considerably modified. 



Table i., which formerly contained but about 60 names of observa- 

 tories, now gives the names and locations of over 150, and, in addition 

 to the data formerly given, the time west of Greenwich has been deemed 

 of sufficient importance to be placed in the table. 



Tables n., in., iv., and v. remain the same as they were. 



Table vi. is now a new tahle, giving the length of a degree of both 

 the meridian and of the parallel in the various geographical measures. 

 These have been calculated from Clarke's formula for the spheroid of 

 revolution, of 1866. 



Table vn. now contains tables for computing terrestrial surfaces, 

 which are new and are also based upon Clarke's formula. 



Table vni. is a hew table, giving a comparison of the Standards 

 of Length, of England, France, Belgium, Prussia, Russia, India, and 

 Australia, made by Capt. A. R. Clarke, at the English Ordnance Survey 

 Office, under the direction of Col. Sir Henry James, Director of the 

 Ordnance Survey. 



Table ix. is a new table, giving the length of Insolation for any 

 latitude, and for any day of the year. 



All the corrections which have been found or which have been reported, 

 have been made, and the book is now comparatively free from typo- 

 graphical errors ; but it is hoped that the same kind courtesy which has 

 prompted the friends, who have aided to make the book more perfect, 

 will be continued in the future ; and that all errors observed will be 

 reported as soon as found, so that they may be corrected at some future 

 time. 



A general Index has been prepared for the whole Volume, which will 

 greatly facilitate the use of the book, while the old plan of dividing it 

 according to Series has also been retained. 



I wish to acknowledge in this place my great indebtedness to you ; 

 and also to Mr. M. McNeill of the Princeton Observatory, for valuable 

 assistance in computation and in proof-reading. 



I am, 



Yours most respectfully, 



WILLIAM LIBBEY, JR. 



PRINCETON, K J., July, 1884. 



