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Apparatus for testing the direction of air currents, 35 14. The ends of screw blades, 36 15. The manner of building up the large screws, 39 16. A fabric-covered screw, 40 17. The hub and one of the blades of the screw on the Farman machine, 42 18. Section of screw blades having radial edges, 43 19. Form of the blade of a screw made of sheet metal, 44 20. New form of hub, 45 21. Small apparatus for testing fabrics for aeroplanes, 50 22. Apparatus for testing the lifting effect of aeroplanes and condensers, 51 23. Apparatus for testing aeroplanes, condensers, &c., 52 24. Cross-sections of bars of wood, 53 25. Sections of bars of wood, 54 26. A flat aeroplane placed at different angles, 55 27. Group of aeroplanes used in experimental research, 56 28. An 8-inch aeroplane which did very well, 57 29. Resistance due to placing objects in close proximity to each other, 58 30. Cross-section of condenser tube made in the form of Philipps’ sustainers, 60 31. The grouping of condenser tubes made in the form of Philipps’ sustainers, 61 32. Machine with a rotating arm, 63 33. A screw and fabric-covered aeroplane in position for testing, 64 34. The rotating arm of the machine with a screw and aeroplane attached, 65 35. The little steam engine used by me in my rotating arm experiments, 66 36. The machine attached to the end of the rotating shaft, 68 37. Marking off the dynamometer, 69 37a. Right- and left-hand four-blade screws, 70 38. Apparatus for indicating the force and velocity of