Glossary 627 



Gravitation: The attractive force by which all bodies of matter in the 



universe are drawn toward each other. 

 Gravity: The attractive force which tends to draw all bodies toward the 



center of the earth. 

 Gray matter: The gray nerve tissue, consisting mainly of nerve cells, 



forming a thin outside covering of the cerebrum and cerebellum and 



the inside portion of the medulla and the spinal cord. 

 Gullet: The food passage extending from the mouth cavity to the 



stomach ; the food passage of the paramecium. 

 Gyro-compass (jl'ro) : An application of the gyroscope used in place 



of the magnetic compass on ships and aircraft. 

 Gyroscope (ji'ro-skop) : A heavy metal wheel, mounted in a metal 



frame. When spinning rapidly, a gyroscope tends to keep its original 



position and to resist any force to change this position. 

 Gyrostabilizer (ji-re-sta'bi-llz-er) : An application of the gyroscope by 



which ships and airplanes may be kept on an even keel. 



H 



Hail : Ice particles formed by the freezing of raindrops. 



Heart: A hollow, conical, muscular organ which pumps blood through 



the circulatory system. 



Heat: A form of energy resulting from the motion of molecules. 

 Heat equator: The belt of equatorial calms. 

 Heating systems- 

 Hot air system: The method of heating a building by means of cur- 

 rents of warm air conducted from a furnace to the various rooms 

 through metal tubes. 



Hot water system: The method of heating a building by means of 

 hot water conducted from a boiler through pipes to radiators in the 

 various rooms. 



Steam heating system: The method of heating a building by means 

 of steam conducted from a boiler through pipes into radiators in the 

 various rooms. 

 Heavens: The sky. 



Helium (he'li-wm) : A light, non-inflammable gas used in airships. 

 Helix (he'liks) : A coil of insulated wire wound around the iron core of 



an electromagnet. 

 Hemoglobin (he-mp-glo'bln) : Coloring matter of the red corpuscles of 



the blood ; contains iron which attracts oxygen. 

 Hemorrhage (hem'o-raj) : Discharge of blood from a blood vessel; 



bleeding. 



Heredity (he-red'i-ti) : Transmission of qualities from parent to child. 

 Heroin (he-ro'm) : A narcotic drug made from opium and from coal tar. 

 Hertzian waves (hert'si-an) : Waves in ether caused by the vibratory 



motion of an electrical discharge. Radio waves are Hertzian waves. 

 Hibernation (hi-ber-na'shwn) : The act of passing the winter in a state 



of sleep or inactivity, as do certain animals and plants. 

 "High": On a weather map, "high" indicates the centers of highest 



barometric pressure. It usually means fair weather. 

 Hilum (hi'lm) : Scar on a seed at the point where it was attached to 



the parent plant. 



Horizon: The line where the earth and sky seem to meet. 

 Hormones (hor'monz) : Chemical secretions of the ductless glands, 



which arouse other organs to greater activity. 



