A Cold Store 80 Dassen Island Lighthouse 88 Measuring Heat 128 The Telewriter 184 A Miners' Rescue Team 208 Pneumatic Hammer Drill 216 An Artificial Coal Mine 224 Sectional view of a 60-pounder Gun 232 Rifles of different Nations 240 DIAGRAMS FIG. PAGE 1. Principle of Galvanometer 30 2. String Galvanometer 31 3. Duddell Thermo-Galvanometer 39 4. Construction of a Voltmeter 64 5. The Working of a Refrigerating Machine 70 6. Hertz's Machine 155 7. Hertz "Detector" 156 8. 9. 10. Wireless Waves 158 11. A Wireless Antenna 164 12. Poulsen's Machine 166 13. 14. How Pictures are sent by Wire 177 15. Message received by Telewriter 189 MARVELS OF SCIENTIFIC INVENTION CHAPTER I DIGGING WITH DYNAMITE Most people are afraid of the word explosion and shudder with apprehension at the mention of dynamite. The latter, particularly, conjures up visions of anarchists, bombs, and all manner of wickedness. Yet the time seems to be coming when every farmer will regard explosives, of the general type known to the public as dynamite, as among his most trusty implements. It is so already in some places. In the United States explosives have been used for years, owing to the exertions of the Du Pont Powder Company, while Messrs Curtiss' and Harvey, and Messrs Nobels, the great explosive manufacturers, are busy introducing them in Great Britain. It will perhaps be interesting first of all to see what this terror-striking compound is. One essential feature is the harmless gas which constitutes the bulk of our atmosphere, nitrogen. Ordinarily one of the most lazy, inactive, inert of substances, this gas will, under certain circumstances, enter into combination with others, and when it does so it becomes in some cases the very reverse of its usual peaceful, lethargic self. It is as if it entered reluctantly into these compounds and so introduced an element of instability into them. It is like a dissatisfied partner in a business, ready to break up the whole combination on very slight provocation.