CHAPTER IV.

THE MOTIVE POWER.

SECTION I.--RUBBER MOTORS.

§ 1. Some experiments with rubber cord. § 2. Its extension under various weights. § 3. The laws of elongation (stretching)--Permanent set. § 4. Effects of elongation on its volume. § 5. "Stretched-twisted" rubber cord--Torque experiments with rubber strands of varying length and number. § 6. Results plotted as graphs--Deductions--Various relations--How to obtain the most efficient results--Relations between the torque and the number of strands, and between the length of the strands and their number. § 7. Analogy between rubber and "spring" motors--Where it fails to hold. § 8. Some further practical deductions. § 9. The number of revolutions that can be given to rubber motors. § 10. The maximum number of turns. § 11. "Lubricants" for rubber. § 12. Action of copper upon rubber. § 12A. Action of water, etc. § 12B. How to preserve rubber. § 13. To test rubber. § 14. The shape of the section. § 15. Size of section. § 16. Geared rubber motors. § 17. The only system worth consideration--Its practical difficulties. § 18. Its advantages 24

SECTION II.--OTHER FORMS OF MOTORS.

§ 18A. _Spring motors_; their inferiority to rubber. § 18B. The most efficient form of spring motor. § 18C. _Compressed air motors_--A fascinating form of motor, "on paper." § 18D. The pneumatic drill--Application to a model aeroplane--Length of possible flight. § 18E. The pressure in motor-car tyres. § 19. Hargraves' compressed air models--The best results compared with rubber motors. § 20. The effect of heating the air in its passage from the reservoir to the motor--The great gain in efficiency thereby attained--Liquid air--Practical drawbacks to the compressed-air motor. § 21. Reducing valves--Lowest working pressure. § 22. The inferiority of this motor compared with the steam engine. § 22A. Tatin's air-compressed motor. § 23. _Steam engine_--Steam engine model--Professor Langley's models--His experiment with various forms of motive power--Conclusions arrived at. § 24. His steam engine models--Difficulties and failures--and final success--The "boiler" the great difficulty--His model described. § 25. The use of spirit or some very volatile hydrocarbon in the place of water. § 26. Steam turbines. § 27. Relation between "difficulty in construction" and the "size of the model." § 28. Experiments in France. § 29. _Petrol motors._--But few successful models. § 30. Limit to size. § 31. Stanger's successful model described and illustrated. § 32. One-cylinder petrol motors. § 33. _Electric motors_ 39