268 ON THE MAGNETIC DIP AT SEVERAIi 



state of our knowledge of the subject would authorize us to expect. 

 1 furnished myself with a dipping apparatus, and two six inch needles, 

 adapted to it, made also by Robinson ; a chronometer made by Moly- 

 neux & Sons, and a declination or variation apparatus made by Trough- 

 ton & Simms. I am under obligations to Professor Bache, for the 

 kind manner in which he communicated to me his mode of manipu- 

 lation, and for the opportunity of witnessing his experiments, both at 

 Westbourn Green and at the observatory of Paris. It would seem to 

 be a very simple operation to count the vibrations of a freely suspended 

 magnetic needle, and note their time by a chronometer ; to perform 

 the various reversals with the dipping apparatus, &c., &c. ; yet, although 

 not destitute of mechanical skill and experience, it was not until I had 

 had considerable practice, that I could proceed with confidence and 

 certainty. It had been my intention to make a series of observations 

 at or near to London, so often repeated as to be able to refer my obser- 

 vations on intensity especially, to the intensity of that place as unity. 

 But the delay of workmen to finish my instruments, and the pressure 

 of other business, permitted me to make only a single series. 



The needles which I used for determining horizontal intensity were 

 three in number; two of them, Nos. 1 and 2, were of the Hansteenian 

 model, cylindrical, terminating in cones, one-eighth of an inch in dia- 

 meter, two and a half inches long, and weighing, with thin, light brass 

 stirrups for suspension, about sixty-five grains each. The third. No. 

 3, was a flat needle, three inches long, one-fourth of an inch wide, and 

 about one-fortieth of an inch thick, terminating in an angle or point 

 of about sixty degrees, at each end, and weighing forty-four grains. 

 Through the politeness of Mr Airy, the astronomer royal, I was ena- 

 bled to vibrate these needles contiguous to the observatory at Green- 

 wich, and on the site lately laid off for a magnetical observatory. 

 From the vibration of these needles at Greenwich, August 26, 1837, 

 and at Cincinnati, January 17, 1838, in both cases in a medium so 

 rarified as to support only half an inch of mercury, after proper reduc- 

 tion for temperature, &c., I obtained indications of the ratio of horizon- 

 tal intensity at the former place, to that of the latter, as follows. 



By needle No. 1, 1 to M624; by needle No. 2, 1 to 1 -1639 ; by 

 No. 3, I to 1*2037. I attribute the disagreement of the results ob- 

 tained by Nos. 1 and 2, and that obtained by No. 3, to a probable di- 



