=Mercury.= (1) The name of the planet nearest the Sun. (2) A silver white liquid metal. =Molecule.= The smallest part of matter that can exist separately without the substance it forms being destroyed. =Morning Star.= A planet which can be seen in the east just before the Sun rises. =Morse code.= A telegraph code of dots and dashes invented by Samuel F. B. Morse. The Morse Code is used for telegraph and heliograph signaling. =Motion.= Anything that changes position. There are two kinds of motion (a) simple motion and (b) compound motion. _Simple motion_ can be either _translated_ or _rotary_, while _compound motion_ is both _translated_ and _rotary_, so that while a body is being translated, or moved through space, it is also rotating on its axis. The Sun and planets, then, have compound motions. =Nature.= Everything contained in space that has not been shaped by human hands. =Nautical almanac.= The American Nautical Almanac is a book of the stars published by the Bureau of Navigation of the United States. It is published three years in advance and is sold by the Superintendent of Documents, Washington, D. C., at 30 cents per copy. This is designed chiefly for the use of navigators of ships. It gives the exact positions of the Sun, Moon and planets and much other astronomical information. It is based upon the calculations made by the United States Naval Observatory; these are printed in the Nautical Almanac, and on this all other almanacs are based. =Northern lights.= See Aurora Borealis. =Obscured.= Hidden from sight. =Official Handbook of the Boy Scouts.= A book published by the organization of Boy Scouts and which contains the rules and regulations of that organization and the requirements they must meet in order to win merit badges. =Offset.= To equal; to balance. =Opposition.= See Appendix. =Optic Nerve.= The nerve that carries the impression of light received by the eye to the brain. =Orbit.= The path followed by a body. Atoms have orbits as well as the stars. The paths followed by planets and comets round the Sun. =Parabola.= See Fig. 129. =Parallel.= See Appendix B. =Particle.= A bit of matter. A particle may be a bit of matter which can be seen or it may mean an amount so small that it cannot be seen. =Pencil.= A group of rays from the same source of light. See Beam. See Ray. =Pendulum.= A body suspended from a fixed point and free to swing.