CONTENTS. 



barometer to measure heights. 15. Density of strata affected by 

 their temperature. 16. Fall of barometer in the balloon ascent 

 of Gay Lussac. 17. Extreme variations of the barometric 

 column in a given place. 18. Its diurnal variation. 19. How- 

 it may prognosticate the weather. 20. Fallacy of popular rules. 

 21. Barometric prognostics 177 



THE SAFETY LAMP. 



1. Introductory observations. 2. Fire-damp Sir Humphry Davy 

 invents the Safety Lamp. 3. Nature and laws of flame 

 investigated and discovered by him, and rendered subservient to 

 his invention. ......... 188 



WHITWORTH'S MICROMETRIC APPARATUS . .190 



STEAM. 



1. Power of steam manifested by natural evaporation. 2. Early 

 attempts at its mechanical application Inventions of Watt. 3. 

 Influence of steam power on mankind. 4. Its agency in 

 commerce and the arts. 5. Analysis of its operation. 6. Com- 

 bustion of coals. 7. Force developed by the evaporation of 

 water. 8. Steam an invisible aeriform fluid. 9. Measure of 

 the mechanical force developed. 10. Heat absorbed in 

 evaporation. 11. Latent heat of steam. 12. Effect of varying 

 pressure. 13. Mechanical force independent of the pressure. 

 14. Force developed by expansion. 15. Force developed by 

 condensation. 16. Application of these three forces in steam 

 engines. 17. Steam may act variously on piston. 18. Furnaces 

 and boilers. 19. Evaporating power of fuel. 20. Means of 

 economising heat. 21. Prevention of loss by radiation. 22. 

 Inspection and reports of Cornish engines. 23. Greatly improved 

 efficiency. 24. Actual mechanical virtue of coals. 25. Illustra- 

 tions Pyramids of Egypt. 26. Menai Bridge. 27. Railway 

 engines. 28. Exhaustion of coal mines improbable. 29. 

 Prospects of scientific discovery . - -f ' . . . . 193 



