334 HENEY AND THE TELEGRAPH. 



■whicli I constructed in 1831 ; and wliicli easily siipports nearly one ton (more than a 

 thousand l^Lilograrames) when submitted to the current of a strong battery of 24 pairs. 

 It consists of two horse-shoes opposed to each other, formed of round bars from three 

 to four inclies (8 to 10 centimetres) in diameter, and from two to two and a half feet 

 (60 to 80 centimetres) in total length. The two arms of each horse-shoe are enveloped 

 with about 1,100 yards (one thousand metres) of copper wire 26 thousandths of an 

 inch (two-thirds of a millimetre) in thickness. The same current traverses succes- 

 sively the 2, 200 yards (two thousand metres) of wire ; but the helices are so disposed as to 

 bring their opposite poles together." And the author repeats that as soon as the cur- 

 rent is established, the lower free magnet is attracted to the upper fixed maguet with 

 such force as to lift "an enormous weight, often exceeding a thousand kilogrammes."* 



The remark just made applies equally to this example of the Henry "intensity" 

 magnet, that bj- the substitution of the multiple coil and the "quantity" battery, it 

 should have equalled the Henry Yale-College magnet, if not his Princeton magnet. 



There is however in this latter account, an evident error of date, which should be 

 noted. The differences of detail (in every particular) between the two magnets re- 

 ferred to, preclude any suggestion of the latter being an inaccurately remembered 

 account of the former. The systematic excess of the latter magnet in every element 

 of construction aud performance equally excludes the possibility of its having been 

 devised by its author prior to his notice before the "Soci6t6 Philomatiqne" (on the 

 23d of June, 1832) of his success in developing a magnetic power of 900 pounds. And 

 the fact that Pouillot, in the second edition of his Elements of Experimental Physics 

 published iji 1832 ("revue, corrig^e et augmentee"), makes no allusion whatever to 

 such a magnet, may be taken as conclusive evidence that no such magnet (nor any 

 other) was constructed by him in 1831. t 



The error of statement, in his third edition of the Elements of Physics is easily ex- 

 plained as a simple inadvertence in trusting to memory for a precise date.t 



It may be accepted with tolerable certainty that Pouillet's later and larger magnet 

 could not have been made earlier than the latter part of 1832. And yet this inadver- 

 tent antedating by one year (wholly unimportant though it be) has been very precisely 

 reproduced in the fourth edition of Pouillet's Physics, published in 1844, in the fifth 

 edition published in 1847, in the sixth edition published in 1853, in the seventh edi- 

 tion published in 1856, and presumably in every subsequent edition, as well as iu the 

 numerous translations of this popular work. The earliest date of publication of Pou- 

 illet's 900-pound magnet is August, 1832; of his second, or 2200-pouud magnet, is 1837. 



NOTEE. (From p. 289.) 

 henry's early telegraphic experiments. 



The following are some of the testimonials of living eye-witnesses to the operation 

 of Henry's early electro-magnetic telegraph, during the years 1831 aud 1832. 



The Hon. Alexander W. Bradford, a former pupil of the Albany Academy under 

 Henry's professorship in 1831, and who left the academy in 1832, thus recalls his aca- 

 demic experiences, a third of a century later : ' 'And there was another professor, whose 

 life has been spared, who rose with the sun to instruct his pupil eager after knowledge; 

 who giving his heart and soul to the duties of the school, had yet time for exx^loring 



* Elements de Physique Expermientalc, etc. par M. Pouillet, third edition, 2 vols. 8vo. 

 Paris, 1837, liv. iii, sec. iv, chap. 5, art. 277. Vol. i, p. 572. 



\EUments de Physique Expcrimentale, etc. jiar M. Pouillet, second edition, 4 vols. 8vo. 

 Paris, 1832. 



X As if to magnify this accidental error, Dr. Lardner, iu a popular text-book on the 

 telegraiih, makes the off-hand statement : "In 1830, an electro-magnet of extraordinary 

 power was constructed under the superintendence of M. Pouillet at Paris. 

 With a current of moderate intensity the apparatus is callable of supporting a weight 

 oi'Stvcral ions." (Lardner and Bright's Electric Telegraph, 12mo., London, 1867, chap, 

 ii, «cc. 39, p. 22.) 



