xii CONTENTS 



SECTION PAGB 



33. Radium Produces Heat 49 



34. The Sun 50 



35. The Sun the Source of All Energy 51 



36. Theories . . .52 



37. The Laws of Motion 53 



38. Effects of Two or More Forces Acting at the Same Time . 55 



39. Universal Gravitation .56 



40. The Center of Gravity . 57 



41. The Effect of Two Forces Acting at Right Angles to Each 



Other - _ , . 58 



42. The Measurement of Rotary Motion . . , . . . 59 



43. Theories of the Evolution of the Solar System . ... . 61 



44. The Solar System 63 



45. The Planets and the Earth 63 



46. Age of the Earth 65 



47. The Shape and Size of the Earth . ._.,, 65 



48. The Heat of the Earth . . .'*..'.. . 67 



49. Direction Latitude and Longitude 68 



50. Motions of the Earth . . . . , . . .69 



51. Time ,. . . " . 71 



52. The Seasons . 72 



53. The Calendar 74 



54. The Moon ' . . . . .75 



55. Tides . ., . . . . .... . . .76 



56. Meteors 77 



57. Comets . . . / . . . 78 



58. The Stars . . ]'.. ' t . , 79 



59. Distances of the Stars . v . . v , 80 



60. The Earth as a Whole 80 



61. The Earth as a Magnet 81 



62. Other Magnets .83 



63. The Northern Lights 85 



64. Sources of Electricity 86 



65. Applications of Electricity 88 



66. Chemical Effects of Electricity . . . . . ..,90 



67. Heat and Light from Electricity 91 



68. Heat Produces Light 94 



69. Light and Vision 95 



70. Reflection 96 



