=Forecast.= A prediction of some future event. A forecast may be founded on fancy or on fact. A horoscope of a person is a forecast based on the fancy of an astrologer. A weather forecast is founded on the action of a barometer, and even then the forecast is uncertain enough. Forecasts of the time of eclipses, the return of comets and the like are founded on cold scientific facts and hence come true at exactly the calculated time. =Frequency.= How often an event occurs. =Gas.= That form of matter which is like air. Some gases remain in the gaseous state at ordinary temperatures. Gases have a tendency to expand without limit. See Vapor. =Generate.= To produce. To set up; as a battery generates a current; the Sun generates power. =Gravitation.= The force which attracts all bodies near the Earth to it and all bodies to each other. =Heavens.= The sky. The space as far as the eye can see about the Earth. The space in which the stars and their planetary systems move. =Horizontal.= See Fig. 185. =Horoscope.= A forecast of the future of a person made by an astrologer who pretends to read the future from the aspect or position of the planets and stars. =Hyperbola.= See Fig. 129. =Hypothesis.= See Idea. =Idea.= A notion of a plan or scheme which may be more or less vague. =Supposition.= A clearer conception of a plan or scheme which is based on such facts as may be thought of. =Hypothesis.= A plan or scheme assumed to be true and carefully reasoned out from all the facts obtainable. =Theory.= A plan or scheme which has been proved true by experiment, examination or comparison. =Illusion.= An image which deceives the eye. The other senses can also be deceived by illusions. =Impact.= Coming together of two objects. =Impress.= To form on, or to affect, as light waves impress a dry plate. =Indicate.= To show. To point out. =Indicator.= That which points out or shows something. =Limb.= The edge of the Sun, Moon or Planets. =Lore.= Learning on any subject. =Lunar.= Of, or having to do with the Moon. =Magnetic lines of force.= The direction of the force of magnetism acting in and around a magnet. =Magnetic storm.= The Earth is a great magnet and sometimes when sun spots of unusual size or numbers appear, the magnetic lines of force of the Earth are violently affected. This is called a magnetic storm. =Mass.= (1) Amount of matter. (2) A lot of molecules or particles of matter brought and held together. =Matter.= The substance of which a thing is formed. A molecule of matter is made up of atoms, and a mass is a lot of molecules held together by some attractive force.