PROFESSOR FORBES'S EXPERIMENTS ON 





MAGNETIC INTENSITY. 1 



Place, Greenhill, near Edinburgh. 



Ther. 

 Reauni 



Date 



, 7th May 1833. Beginning, 



5h3m41s.6 18°.0 



Needle, Flat. End, . . 



25 i 



J4.5 17°.3 



can, 17.65 



Arc. 



Sec. 



Arc. 



Sec. 



Arc. 



Sec. 



Sec. 



300 Vibrations. 







20 



41.6 





47.8 





52.0 



56.7 



m s 



IB 15.1 





18.3 





24.0 





28.3 



32.5 



14.2 





54.7 



10 



0.4 





5.3 



9.2 



14.5 





31.0 





36.5 





40.8 



45.7 



14.7 





8.6 





14.0 



5 



17.7 



22.0 



13.5 





45.0 





49.8 





54.2 



58.2 



13.2 





21.7 

 68.3 





26.0 

 3.3 





30.7 



7.7 



34.5 



12.8 





m s 





35.0 





39.0 





43.6 



Jlean, 18.14.00 





11.1 





15.8 





19.8 



= 1094.00 



Rate 4- » 



9. My method of observing was to keep the chronometer at the ear till the 

 instant that the termination of a vibration was observed, then to count five beats 

 of the balance (corresponding to two seconds), which affords time to bring the 

 dial-plate into view, and the seconds entered in the Table are those read off at 

 that time, namely two seconds later than the absolute times. Thus the imprac- 

 ticable attempt to observe two things at once by the eye is avoided. For this and 

 some other suggestions, I am indebted to my friend Captain P. P. King, R. N. 

 The observations are registered in lithographed forms bound into volumes. 



10. In the choice of stations I have been extremely particular, often at great 

 personal inconvenience. * Places remote from any trace of habitation have most 

 usually been selected, and in no case have intensity observations been made in a 

 house. The specialties of the sites will be noticed in the following Tables. I have 

 invariably removed all masses of iron from my person ; and in my later experi- 

 ments even took the precaution of carrying thin shoes, in order that the heavily 

 nailed ones which I usually wear, might be removed to a distance. The chrono- 

 meter, too, has generally been held at some distance from the apparatus. But 

 some direct experiments lead me to believe that the influence of the two last 

 mentioned sources of error is insensible. 



* None but those who have been engaged in observations of the very same description, where 

 the eye, the ear, and the memory are all actively employed, can have an idea of the difficulty of always 

 finding sites free from the interruptions of curiosity, or natural obstacles. 



