xii CONTENTS 



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 ...... . . . .79 



59. Distances of the Stars ...... . .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 P^lectricity ........ 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 



