CONTENTS. vii 



PAGE 



CHAP. IV. 68. Effect of the screw-propeller reaction on the vessel. 

 69. Their best practical proportion. 70. Their varying pitch. 

 71. Relative advantages of screw and paddle-wheels. 72. 

 Their effects in long sea-voyages. 73. Experiments with the 

 "Rattler" and "Alecto." 74. These experiments continued. 

 75. Admiralty experiments. -76. Government report. 77. 

 Application of the screw in the commercial marine. 78. Appli- 

 cation of screw to mail-vessels.79. Geared and direct action. 

 80. Geared-engines. 81. Fairbairn's internal gearing. 82. 

 Subdivision of the power among several cylinders. 83. Protec- 

 tion from shot. 84. Regulation of slides. 85. Relative speed 

 of screw and vessel. 86. Engines of the "Great Britain." 87. 

 Engines of the "Arrogant" and "Encounter." 88. Various 

 forms of screw-engines. 89. Cross action of H. M.'s screw steam- 

 packet "Plumper." 90. Auxiliary steam-power. 91. Effect of 

 screw- vessels head to wind. 92. Nominal and real horse-power. 

 93. Official tables of the strength of the steam-navy . . 161 



THUNDER AND LIGHTNING, AND THE AURORA 

 BOREALIS. 



1. Atmospheric electricity. 2. The air generally charged with 

 positive electricity. 3. Subject to variations and exceptions. 

 4. Diurnal variations of electrical intensity. Observations 

 of Quetelet. 5. Irregular and local variations and excep- 

 tions. 6. Variations dependent on the season and weather. 7. 

 Methods of observing atmospheric electricity. 8. Methods of 

 ascertaining the electrical condition of the higher strata. 9. 

 Remarkable experiments of Romas, 1757. 10. Electrical change 

 of clouds varies. 11. Thunder and lightning. 12. Form and 

 extent of the flash of lightning. 13. Cause of the rolling of 

 thunder. 14. Affected by the zigzag form of lightning. 15. 

 Affected by the varying distance of different parts of the flash . 

 16. Affected by echo and by interference. 17. Inductive action of 

 clouds on the earth. 18. Formation of Fulgurites explained. 

 19. Accidents of the surface which attract lightning. 20. Light- 

 ning follows conductors by preference its effects on buildings. 

 21. Conductors or paratonnerres for the protection of buildings. 

 22. Effects of lightning on bodies which it strikes. 23. The 

 Aurora Borealis the phenomena unexplained. 24. General 

 character of the meteor. 25. Description of auroras seen in the 

 polar regions by M. Lottin . . . . . . .177 



ELECTRO-MOTIVE POWER. 



CHAP. I. 1. Prospects of improvement in motive power by the appli- 

 cation of electricity. 2. Example of its practical application in 

 the workshop of Mons. Froment, mathematical instrument maker 

 in Paris. 3. Mention of it in Catalogue of the Great Exhibition 



