Heat Transference; conduction, uses of good and poor conductors, convection, in nature, heating and ventilating systems, radiation, 3 peculiarities, value of sun's radiation. Heat and Moisture; relative humidity, dew point, formation of dew, fog, rain, snow, etc., evaporation, effects, conditions. Heat Measurement; specific heat, heat of fusion, of vaporization, combustion. Vaporization; Boiling point, laws of boiling, distillation, artificial cooling. Heat Engines; steam, gas.--construction, action, efficiency, mechanical equivalent of heat. Heat equivalent of fuels. CHAPTER IX MAGNETISM (1) GENERAL PROPERTIES OF MAGNETS =199. Magnets.=--Since the times of the early Greek philosophers men have known of certain stones that have the property of attracting to themselves objects of iron and steel. Such stones are called _natural magnets_. It is thought by many that the name magnet is derived from Magnesia in Asia Minor, where these stones are abundant, though this is but tradition. It was also learned long ago that iron and steel objects when rubbed with natural magnets become magnetized, that is, acquire the properties of magnets. These are said to be _artificial magnets_. [Illustration: FIG. 169.--A bar magnet.] [Illustration: FIG. 170.--A horseshoe magnet.] Some 800 years ago it was discovered that magnets, natural or artificial, when suspended so as to turn freely, always come to rest in a definite position pointing approximately north or south. This is especially noticeable when the magnet is long and narrow. Because of this property of indicating direction, natural magnets were given the name of _lodestone_ (lode-leading). Artificial magnets are made by rubbing steel bars with a magnet or by placing the steel bar in a coil of wire through which a current of electricity is flowing. The magnetized steel bars may have any form, usually they are either straight or bent into a "U" shape. These forms are known as _bar_ and _horseshoe_ magnets. (See Figs. 169 and 170.) Magnets retain their strength best when provided with soft-iron "_keepers_," as in Fig. 171. [Illustration: FIG. 171.--Bar magnets with keepers.] =200. Magnetic Poles.=--If a magnet is placed in iron filings and removed, the filings will be found to cling strongly at places near the ends of the magnet, but for a portion of its length near the middle no attraction is found. (See Fig. 172.) These places of greatest attraction on a magnet are called _poles_. If a bar magnet is suspended so as to swing freely about a vertical axis the magnetic pole at the end pointing north is called the _north-seeking_ pole; at the other end, is the _south-seeking_ pole. In most places the needle does not point to the true north, but somewhat to the east or west of north. The direction taken by a magnetic needle is parallel to the _magnetic meridian_. [Illustration: FIG. 172.--Iron filings attracted to the poles of a magnet.]