ADDRESS OF PROF. S. NEWCOMB. 44. r » 



number of experiments illustrative of the theory are described. 

 With a battery having a single plate of zinc, of half a square foot 

 ct' surface, lie made a magnet lilt a weight of 750 pounds, — more 

 than thirty-five times the weight of the magnet. 



In the same year, 1831, lie describes a little machine tin- produ- 

 cing continuous mechanical motion by magnetic attraction and 

 repulsion. He considered the apparatus to lie merely a philosophical 

 toy involving a principle which at some future time might he applied 

 to a useful purpose. 



In 1830, at the request of Professor Renwick, he commenced a 

 series of observations to determine the magnetic intensity at Albany. 

 This gave him occasion to investigate a subject of which the evi- 

 dences had before Keen very conflicting, namely, the effect of the 

 aurora upon the magnetism of the earth. 



In 1831, April 1!', at (i p. m., a remarkable phenomenon was 

 noticed, namely, an extraordinary increase in the number of vibra- 

 tions of the needle, and in the consequent magnetic intensity of the 

 earth. Every precaution was taken that no local influence should 

 affect the magnet, l>ut the result was the same. About 9 o'clock in 

 tin 1 evening a brilliant aurora commenced. The idea now occurred to 

 him that it might he connected with the magnetic disturbance, and 

 another observation of the magnet was therefore made. The result 

 was the opposite of what had been anticipated, for instead of show- 

 ing a continuous increase the intensity was now far below the aver- 

 age. An extended discussion of other results of the same sort is 

 given, followed by an inquiry into the origin of the aurora. 



The next important investigation in which Professor Henry 

 appears is that which led to his being an independent discoverer of 

 magneto-electricity. In the early experiments in this direction we 

 have an interesting example of how a discovery may he long re- 

 tarded through the want of correct theoretical notions. The idea 

 entertained by the early experimenters of the present century seems 

 to have been that since a galvanic current passing around a core of' 

 soft iron renders it magnetic, it may he expected that a magnet placed 

 inside of a coil of wire will cause a current of electricity t«> pass 

 through it. Accordingly, endeavors were made to produce this 

 current by using powerful magnets. But since a continuous gal- 



