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VII. Notices respecting New Books. 



The Earth a Great Magnet. By Alfred Marshall Mayer, Ph.D., 

 Professor of Physics in the Stevens Institute of Technology. London : 

 Triibner and Co. 



r I THIS is the report of a lecture delivered before the Yale Scientific 

 -*- Club on February 14, 1872, in which the lecturer proposed to 

 present to his audience " one prominent truth in simple and striking 

 experiments." The truth which is kept steadily before the mind 

 throughout the lecture is, " that the earth is a great magnet ;" 

 and this truth is developed, step by step, by experiments of the 

 most conclusive kind, each having been rendered distinctly visible 

 to the audience by means of the vertical lantern, so that the pro- 

 cesses of magnetizing and demagnetizing, with all the interesting 

 motions of the needles, were seen projected on a luminous screen 

 of eighteen feet diameter. 



The lecture itself is a masterly production, and exhibits the re- 

 sult of much close reading as well as experimental research. 

 Quotations are given from earlier writers on magnetism, illustra- 

 tive of the sound knowledge which they possessed ; and as each 

 experiment illustrative of the lecture is described as well as the 

 apparatus employed in manipulation, the reader is conducted from 

 a consideration of the most ordinary magnetic phenomena pre- 

 sented by bar and electro magnets, to that of the same pheno- 

 mena evolved from terrestrial magnetism. A paragraph selected 

 from the closing portion of the lecture will fully substantiate this 

 statement. 



" Now we have finished our experiments ; and what have they 

 shown ? I have temporarily magnetized a bar of soft iron, by 

 pointing it towards a pole of our large magnet. I did the same 

 with the bar and the earth. I permanently magnetized an iron 

 bar, by directing its length towards the pole of the magnet, and 

 vibrating it with a blow of a hammer. I did the same with a 

 bar, struck when pointed towards the earth's magnetic pole. I 

 have shown you the action of a small magnetic disk on iron 

 filings placed above and around it. You saw that the earth pro- 

 duced the same action on the beams of the aurora. I showed 

 you the action of this disk on a freely suspended magnetic needle, 

 and pointed out to you the earth's similar action on a dipping-needle 

 carried over its surface. I have evolved a current of electricity 

 from a magnet, by cutting with a closed conductor across those 

 lines in which a magnetic needle freely suspended places its length. 

 I did the same with the earth by cutting across those lines which 

 are marked out by the pointing of the dipping-needle. There- 

 fore, what am I authorized to infer ? When the effects are the 

 same, the causes must be the same ; for according to all the princi- 

 ples of philosophy, and conformably to that universal experience 

 which we call common sense, like causes produce like effects." 



To those who are desirous of possessing in a compressed form the 

 Phil. Mag. S. 4. Vol. 45. No. 297. Jan. 1873. F 



