Davis's manual of magnetism : including galvanism, magnetism, electro-magnetism, electro-dynamics, magneto-electricity, and thermo-electricity . f oscillations made by it in agiven time. When examined by means of apparatusconstructed with great delicacy, the needle is foundto be seldom at rest. 215. The instrument represented in Fig. 82 is intended to illustrate the magnetism of the earth, on the supposition stated in 212. The compound bar „. „^ magnet, n s, is placed Fig 82. , • • r m the magnetic axis oi the earth, not coinciding exactly with the axis of rotation, N. S. A small magnetic need
Davis's manual of magnetism : including galvanism, magnetism, electro-magnetism, electro-dynamics, magneto-electricity, and thermo-electricity . f oscillations made by it in agiven time. When examined by means of apparatusconstructed with great delicacy, the needle is foundto be seldom at rest. 215. The instrument represented in Fig. 82 is intended to illustrate the magnetism of the earth, on the supposition stated in 212. The compound bar „. „^ magnet, n s, is placed Fig 82. , • • r m the magnetic axis oi the earth, not coinciding exactly with the axis of rotation, N. S. A small magnetic needle, placed at B, on the magnetic meridian, will point both to the magnetic pole 5, and to the north pole, N, both being in the same line. But, if the needle be placed at A, or any where except on the magnetic meridian, it will point to the magnetic pole alone, the two poles not being in the same direction. The several magnets represented at 7i s are not fastened together, but only fixed on one axis. This allows their poles to be separated a little, to imitate more closely the distribution of terrestrial magnetism; the. TERRESTRIAL MAGNETISM. 135 earth really having four magnetic poles, two strongand two weak: the strongest north pole is in Ameri-ca, the weakest in Asia. The line of no variationon the earths surface differs considerably from themagnetic meridian, and the lines of equal variationand equal dip are not exactly meridians and parallelsof latitude to the magnetic pole. The action of themagnetism of the earth at its surface is, therefore,irregular. 216. The variation of the needle at any place isfound by observing the magnetic bearing of anyheavenly body whose true position at the time isknown. It is immediately obtained by comparingthe direction of the needle with the north star whenit crosses the meridian, or by calculation when thenorth star is at its greatest elongation. An obser-vation of the sun, however, is usually preferred. The r,. an Jatltudc of a pla
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Keywords: ., bookcentury1800, bookdecade1850, booksubjectmagnetism, bookyear18
