Properties most evident are studied first. Then those hidden from cursory view. Radio-activity revealed by the electrician. A property which may be universal, and of the highest import. Its study brings us near to ultimate explanations. Faraday’s prophetic views 197 XVI. MEASUREMENT Methods beginning in rule-of-thumb proceed to the utmost refinement. Standards old and new. The foot and cubit. The metric system. Refined measurement as a means of discovery. The interferometer measures 1/5,000,000 inch. A light-wave as an unvarying unit of length 208 XVII. MEASUREMENT CONTINUED Weight, Time, Heat, Light, Electricity, measured with new precision. Exact measurement means interchangeable designs, and points the way to utmost economies. The Bureau of Standards at Washington. Measurement in expert planning and reform 219 XVIII. NATURE AS TEACHER Forces take paths of least resistance. Accessibility decides where cities shall arise. Plants display engineering principles in structure. Lessons from the human heart, eyes, bones, muscles, and nerves. What nature has done, art may imitate,--in the separation of oxygen from air, in flight, in producing light, in converting heat into work: Lessons from lower animals. A hammer- using wasp 245 XIX. QUALIFICATIONS OF INVENTORS AND DISCOVERERS Knowledge as sought by disinterested inquirers. A plenteous harvest with few reapers. Germany leads in original research. The Carnegie Institution at Washington 267 XX. OBSERVATION What to look for. The eye may not see what it does not expect to see. Lenses reveal worlds great and small otherwise unseen. Observers of the heavens and of seashore life. Collections aid discovery. Happy accidents applied to profit. Popular beliefs may be based on truth. An engineer taught by a bank swallow 279 XXI. EXPERIMENT Newton, Watt, Ericsson, Rowland, as boys were constructive. The passion for making new things. Aid from imagination and trained dexterity. Edison tells how the phonograph was born. Telephonic messages recorded. Handwriting transmitted by electricity. How machines imitate hands. Originality in attack 299 XXII. AUTOMATICITY AND INITIATION Self-acting devices abridge labor. Trigger effects in the laboratory, the studio and the workshop. Automatic telephones. Equilibrium of the atmosphere may be easily upset 329 XXIII. SIMPLIFICATION Simplicity always desirable, except when it costs too dear. Taking direct instead of roundabout paths. Omissions may be gainful. Classification and signaling simpler than ever before 340 XXIV. THEORIES HOW REACHED AND USED