194 Dr. Tyndall o?i the Progress of the Physical Sciences : 



The average difference is therefore 0'124 (almost exactly the 

 same as in the case of the positive waves) ; or leaving out the 

 9th and 18th examples, which, from an inspection of the one 

 above and below in each case, are evidently anomalous, the ave- 

 rage sum is 0*065, or f of an inch per second. 



Hence we have for the velocity of waves of translation gene- 

 rally the expression 



V a 



{a±2k)ff, 



the positive or negative signs being taken according as the wav« 

 is positive or negative. 



XXVII. Reports on the Progress of the Physical Sciences. 

 By John Tyndall, Ph.D., Marburg. 



Recent Researches on Electro-magnetism. 



1. Ueher die Anker der Electromagnete, by Dr. J. Dub. Poggendorff's 



Annalen, No. 8, 1848, Sept. 6, 1848. 



2. Ueber die Magnetisirung von Eisenstaben diirch den galvanischen 



Strom, by J. Miiller. Ibid. No. 3, 1850, April 12, 1850. 



3. Ueber die Magnetisirung von Eisenstaben diirch den galvanischen 



Strom, by BiifF and Zamminer. Annalen der Chemie und Phar- 

 macie for July 1850. 



TEN years ago Lenz and Jacobi of St. Petersburg published 

 their celebrated investigation on the action of electro- 

 magnets. The thoroughness of that investigation seemed to 

 leave little hope that future experimenters could add anything to 

 its results, and this consideration appears to have repelled phy- 

 sicists from engaging in the further working of what might be 

 considered an exhausted mine. The subject, however, has lat- 

 terly attracted considerable attention; and various facts have 

 been observed, the tendency of which is to unsettle some of the 

 most important laws supposed to have been established by the 

 two experimenters above mentioned. In the following paper an 

 attempt is made to place before the readers of the Philosophical 

 Magazine a brief sketch of the rise and progress of these diver- 

 gences. 



About seven years ago a very cui'ious series of experiments 

 was made by Professor Hessel of this University, which demon- 

 strate in a veiy striking manner the influence of mere shape on 

 the lifting power of a magnet. It was found that when an 

 iron nail of a certain construction was brought in contact with 

 two poles of unequal power, on pulling the magnets more widely 

 asunder the nail did not cling uniformly to the stronger pole, 

 but that the presentation of its point to the weaker pole was suf- 

 ficient to give to the latter a complete predominance ; so that 

 the cham formed by the two magnets and the nail always gave 



