APPENDIX 287 INDEX 293 USEFUL MEMORANDA. One statute mile is 5280 feet. One nautical mile is 6086 feet = 1¹⁄₆ statute mile. A knot is a speed of 1 nautical mile per hour. Hence the following rules:— To convert Knots to miles per hour—multiply by 1¹⁄₆. Miles per hour to knots—multiply by ⁶⁄₇. Feet per second to miles per hour—multiply by ²⁄₃. ⁄ Feet per second to knots—multiply by ⁶⁄₁₀. Knots to feet per minute—multiply by 100. WAVES AND RIPPLES IN WATER, AIR, AND ÆTHER. —⋄— CHAPTER I. WATER WAVES AND WATER RIPPLES. We have all stood many times by the seashore, watching the _waves_, crested with white foam, roll in and break upon the rocks or beach. Every one has more than once cast a stone upon still water in a lake or pond, and noticed the expanding rings of _ripples_; and some have voyaged over stormy seas, whereon great ships are tossed by mighty _billows_ with no more seeming effort than the rocking of a cradle. In all these things we have been spectators of a _wave-motion_, as it is called, taking place upon a water surface. Perhaps it did not occur to us at the time that the _sound_ of the splash or thunder of these breaking waves was conveyed to our ears as a wave-motion of another sort in the air we breathe, nay, even that the _light_ by which we see these beautiful objects is also a wave-motion of a more recondite description, produced in a medium called the _æther_, which fills all space.