viii CONTENTS. 



PAGE 



52. Newton's conjectures as to the existence of comets of long 

 periods. 53. Halley's researches. 54. Halley predicts its re- 

 appearance in 1758-9 161 



CHAP. III. Halley's prediction (continued). 55. Great advance of 

 mathematical and physical sciences between 1682 and 1759. 56. 

 Exact path of the coiiet on its return, and time of its perihelion, 

 calculated and predicted by Clairaut and Lalande. 57. Remark- 

 able anticipation of the discovery of Uranus. 58. Prediction of 

 Halley and Clairaut fulfilled by reappearance of the comet in 

 1758-9. 59. Disturbing action of a planet on a comet explained. 

 60. Effect of the perturbing action of Jupiter and Saturn on Hal- 

 ley's comet between 1682 and 1758. 61. Calculations of its return 

 in 1835-36. 62. Predictions fulfilled. 63. Elements of the orbit 

 of Halley's comet. 64. Pons's comet of 1812. 65. Olbers's 

 comet of 1815. 66. De Vice's comet of 1846. 67. Brorsen's 

 comet of 1847. 68. Westphal's comet of 1852. 69. Data 

 necessary to determine the motions of these six comets. 70. 

 Diagram of their orbits. 71. Planetary characters are nearly 

 effaced in these orbits. IV. ELLIPTIC COMETS, WHOSE MEAN 



DISTANCES EXCEED THE LIMITS OP THE SOLAR SYSTEM : 72. Syn- 



opsis of twenty-one elliptic comets of great eccentricity and long 

 period. 73. Plan of the form and relative magnitude of the orbits. 

 V. 74. HYPERBOLIC COMETS : VI. 75. PARABOLIC COMETS. 

 VII. PHYSICAL CONSTITUTION OP COMETS : 76. Apparent form 

 head and tail. 77. Nucleus. 78. Coma. 79. Origin of the 

 name. 80. Magnitude of the head. 81. Magnitude of the 

 nucleus. 82. The tail. 83. Mass, density, and volume of 

 comets 177 



CHAP. IV. 84. Light of comets. 85. Enlargement of magnitude on de- 

 parting from the sun. 86. Professor Struve's drawings of Encke's 

 Comet. 87. Remarkable physical phenomena manifested by 

 Halley's comet. 88. Struve's drawings of the comet approaching 

 the sun in 1835. 89. Its appearance on the 29th of Sept. 90. 

 Appearance on Oct. 3. 91. Appearance on Oct. 6. 92. Ap- 

 pearance on Oct. 9. 93. Appearance on Oct. 10. 94. Appear- 

 ance on Oct. 12. 95. Appearance on Oct. 14. 96. Appearance 

 on Oct. 29. 97. Appearance on Nov. 5. 98. Sir J. Herschel's 

 deductions from these phenomena. 99. Appearance of the comet 

 after perihelion. 100. Observations and drawings of MM. Maclear 

 and Smith. 101. Appearance on Jan. 24. 102. Appearance on 

 Jan. 25. 103. Appearance on Jan. 26. 104. Appearance on 

 Jan. 27. 105. Appearance on Jan. 28. 106. Appearance on 

 Jan. 30. 107. Appearance on Feb. 1. 108. Appearance on 

 Feb. 7. 109. Appearance on Feb. 10. 110. Appearance on 

 Feb. 16 and 23. 111. Number of comets. 112. Duration of 

 the appearance of comets. 113. ^Near approach of comets to the 

 earth , 193 



ERRATA. 



lu Crust of the Earth, page 59, lines 1 to 19 inclusive, for "species" read 

 "genera" or "geuus" everywhere without exception. 



