POPULAR MISCELLANY. 



713 



Science Ladders: Lowest Forms of Water 

 Animals. N. D'Anvers. New York: G. P. Put- 

 nam's Sons. Pp. 59. 50 cents. 



First Year Manual and Text-Book of Arith- 

 metic. James H. House. Syracuse, New York : 

 C. W. Bardeeu. Pp. 15(5. 50 cents. 



A Practical Arithmetic. G. A. Wentworth 

 and Rev. Thomas Hill, D. D., LL.D. Boston: 

 Ginn, Heath & Co. Pp. 351. 



Notes on Ingersoll. Rev. L. A. Lamhert. 

 Buffalo, New York: Buffalo Catholic Publica- 

 tion Company. Pp. 184. 50 cents. 



Herbert Spencer on American Nervousness. 

 George M. Beard, M. D. New York : G. P. Put- 

 nam's Sons. Pp. 17. 50 cents. 



Political Economy. Francis A. Walker. New 

 York : Henry Holt & Co. Pp. 490. $3.25. 



Introduction to the Study of Organic Chemis- 

 try. Adolph Pinner. Translated and revised 

 by Peter T. Austen. New York : John Wiley & 

 Sons. Pp.403. f2.55. 



Catalogue and Index of the Publications of 

 the Smithsonian Institution. William J. Bases, 

 Washington, D. C. Smithsonian Institution. 

 Pp. 328. 



Slight Ailments : Their Nature and Treat- 

 ment. Lionel S.Beale. Philadelphia : P. Blakis- 

 ton, Son & Co. Pp. 283. $1.25. 



The Cause of Variation. M. M. Curtis. 

 Marshall, Minnesota : Published by the author. 

 Pp. 115. 



Report of the Chief Signal-Officer, War De- 

 partment, 1880, pp. 1,120, with 119 Charts. The 

 same, 1881, pp. 1,290, with 59 Charts. Washing- 

 ton: Government Printiug-Office. 



POPULAR MISCELLANY. 



Experimental Demonstration of Ohm's 

 Law. An interesting experimental demon- 

 stration of the truth of Ohm's law was re- 

 cently given by Professor Alfred M. Mayer, 

 of the Stevens Institute of Technology, be- 

 fore the New York Electrical Society. This 

 law, as is well known, affirms that in any 

 electrical circuit the current flowing varies 

 directly as the electro-motive force and in- 

 versely as the resistance, or, in symbols, 



E 



C = , where C is the current, E the 

 R' 



electro-motive force, and R the resistance of 

 the entire circuit, including that of the gener- 

 ator. To demonstrate the truth of this law, it 

 is only necessary to show that, the resistance 

 remaining constant, the current increases in 

 the same ratio as the electro-motive force 

 when this is augmented ; or that, the elec- 

 tro-motive force being mainMned constant 

 the current varies in the same ratio as the 

 resistance as this latter is raised. In Pro- 

 fessor Mayer's experiments the current was 

 measured by means of a Thompson reflect- 

 ing galvanometer a delicate instrument in 

 which the deflections of the needle are mul- 



tiplied to any desired extent by means of a 

 beam of light, reflected from a small mirror 

 attached to the needle, which is received 

 upon a screen. The novel feature of Pro- 

 fessor Mayer's demonstration consisted in 

 his mode of obtaining the current so that the 

 electro-motive force could be known with 

 great accuracy, and readily varied. This 

 consisted in generating it by means of the 

 movement of a coil of wire along a bar-mag- 

 net. The electro-motive force of the cur- 

 rent so produced depends upon the number 

 of lines of magnetic force cut by the moving 

 coil in a unit of time, so that this can be 

 varied by varying the speed with which a 

 given coil is moved, or, the speed remaining 

 the same, by varying the number of coils. 

 Professor Mayer resorted to the latter meas- 

 ure, his apparatus consisting simply of an 

 upright bar-magnet over the end of which 

 a loop of wire could be slipped, the distance 

 which this could slide being limited by a 

 stop. The movable coils consisted of the 

 same lengths of copper wire, in which there 

 were taken one, two, or more loops, the re- 

 sistance of each of these pieces being the 

 same, so as to maintain that of the complete 

 circuit constant. The coils were placed over 

 the upper end of the magnet, and carried 

 down until they rested upon the stop, the 

 needle of the galvanometer brought to the 

 zero of the scale, and then the coil pulled 

 off the magnet with a quick motion. The 

 deflection of the needle indicated a variation 

 in the current in proportion to the number 

 of loops of wire used, and when the resist- 

 ance was varied in proportion to the amount 

 of this variation. A better form of the 

 apparatus is one in which the coil, instead of 

 being moved by hand, is drawn up quickly 

 by a spring, when it is released by the pulling 

 of a trigger. With this, Professor Mayer is 

 at present studying the development of mag- 

 netism in eloctro-magnets. 



More abont the Lignified Snake. 



Doubts are expressed, in a paper recently 

 read by Professor C. V. Riley before the 

 Biological Society of Washington, as to 

 the so-called " lignified serpent " of Matto 

 Grosso, Brazil, which was described and il- 

 lustrated in the November number of the 

 " Monthly," being really a serpent at all ; 

 Professor Riley rather believes the forma- 



