INTRODUCTION. 5 



Section VII. Secular Changes of Intensity. — Spots on the Sun's Disk. Lever- 

 rier's secular values of the Eccentricity connected with slight changes of Intensity. 

 Tabular differences of annual Intensity 10,000 years ago from the present amount. 

 Intensity during Summer and Winter influenced by the place of the Earth's 

 Perihelion. Change of Intensity since the time of Hipparchus, 1 28 B. C. Con- 

 clusion that great Geological Changes must be referred to other causes than the 

 Secular Inequalities of the Earth's Orbit. A probable result of the motion of the 

 whole Solar System in space. 



Section VIII. Local and Climatic Changes. — More equable Intensity in the 

 Northern Hemisphere. A slight local inequality produced by daily change of the 

 Sun's Declination. Of the Maximum and Minimum, or mid-summer and mid- 

 winter Intensity. Climate of the Pole. The question of an open Arctic Sea. 



Section IX. Duration of Sunlight and Twilight. — Perturbation of the annual 

 Duration of Sunlight — its Epochs — its Analytic Expression. Of Civil and Astro- 

 nomic Twilight. The Twilight Bow. Height of the Atmosphere calculated from 

 Twilight. Limits of Twilight. Formulas for its Annual Duration. Tables of 

 the Diurnal and Annual Values for the Northern Hemisphere. Delineations by 

 Geometrical Curves. 



