108--MARCONI AND HIS WIRELESS-TELEGRAPH SENDING AND RECEIVING INSTRUMENTS 109--DIAGRAM OF WIRELESS-TELEGRAPH SENDING APPARATUS 110--DIAGRAM OF MARCONI WIRELESS-TELEGRAPH RECEIVING APPARATUS 111--RECEIVER OF BELL'S PHOTOPHONE 112--A GAS FLAME IS SENSITIVE TO ELECTRIC WAVES 113--CAPTAIN INGERSOLL ON BOARD THE U. S. BATTLE-SHIP "CONNECTICUT" USING THE WIRELESS TELEPHONE 114--INCANDESCENT ELECTRIC LAMP LIGHTED THOUGH NOT CONNECTED TO ANY BATTERY OR DYNAMO 115--AN ELECTRIC DISCHARGE AT A PRESSURE OF 12,000,000 VOLTS, A CURRENT OF 800 AMPERES IN THE SECONDARY COIL 116--AN ELECTRIC DISCHARGE SIXTY-FIVE FEET IN LENGTH 117--A PHYSICIAN EXAMINING THE BONES OF THE ARM BY MEANS OF X-RAYS 118--X-RAY PHOTOGRAPH OF THE EYE 119--PHOTOGRAPH MADE WITH RADIUM INTRODUCTORY NOTE Great inventions are a never-failing source of interest to all of us, and particularly to the boy in his teens. The dynamo, the electric motor, the telegraph, with and without wires, the telephone, air-ships, and many other inventions excite in him an interest which is deeper than mere curiosity. He wants to know how these things work, and how they were invented. The man is so absorbed in the present that he cares little for the past. Not so with the boy. He cares for the history of inventions, and in this he is wiser than the man, for it is only by a study of its origin and growth that we can understand the larger significance of a great invention. Great inventions have their origin in great discoveries. The story of great inventions, therefore, includes the story of the discoveries out of which they have arisen. The stories of the discoveries and the inventions are inseparable from the lives of the men who made them, and so we must deal with biography, which in itself is of interest to the boy. Such a story is the story of physical science in the service of humanity. The interest of the youth in great inventions is unquestioned. Shall we stifle this interest by overemphasis of technical detail, or shall we minister to it as a thing vital in the life of the youth of to-day?