60 WRITINGS OF JOSEPH HENRY. [1832 



in the 17th volume of the American Journal of Science. 

 One of these, it will be recollected, formerly belonged to 

 Prof. Hansteen, of Norway, and the other to Capt. Sabine. 

 They were suspended, according to the method of Hansteen, 

 in a small mahogany box, by a single fibre of raw silk. The 

 box was furnished with a glass cover, and had a graduated 

 arc of ivory on the bottom to mark the amplitude of the 

 vibrations. It had also two small circular windows, diamet- 

 rically opposite to each other, through which the oscillations 

 of the needle could be seen. 



In using this apparatus, the time of three hundred vibra- 

 tions was noted by a quarter second watch, well regulated to 

 mean time; a register being made at the end of every tenth 

 vibration, and a mean deduced from the whole, taken as the 

 true time of the three hundred vibrations. Experiments 

 carefully made with this apparatus, were found susceptible 

 of considerable accuracy; as the individual observations, 

 after a small correction for temperature, give a result, ex- 

 cept in a few instances, differing from the mean of a number 

 made under similar circumstances, by a quantity not greater 

 than one part in nearly a thousand. 



The observations were repeated daily, when the weather 

 would permit, from the latter part of September to the last 

 of November, either at the hours of 12 noon, or between 

 5 and 6 p. m.* I was always assisted in making them by 

 the same person, — my relative, Mr. Stephen Alexander, — to 

 whose skill and experience I am much indebted for any 

 accuracy they may possess. 



In April, 1831, a new series was commenced, to determine 

 if the needles still indicated the same degree of magnetic in- 

 tensity. No material difference was observed, except in the 

 following instance, when a remarkable anomaly was exhib- 

 ited. 



On the 19th of April, at 12 o'clock noon, an observation 

 was made with the Hansteen needle, the result of which dif- 

 fered only the fractional part of a second from the usual 



* These times were chosen only on account of being most convenient. 



