1837.] 



Dip and Intensity at Madras, 



221 



Reversed the Poles. 



No. 



A. 



B. 



No. 





A. 



B. 



9 



7° 



28' 



7° 22' 



1 0 

 1 u 



o 





6° 



38' 



11 



7 



12 



7 7 



12 



6 



16 



6 



21 



13 



7 



16 



7 24 



14 



6 



24 



6 



28 



15 



7 



26 



7 23 



91 



6 



27 



6 



33 



Mean, 



7* 



21' 0" 



7° 18' 37" 





6° 



18' 37" 



6° 



21' 30 







Needle marked No. 2. 











1 



7 



31 



7 20 



2 



7 



2 



6 



38 



3 



7 



31 



7 42 



4 



7 



8 



6 



58 



5 Inverted the axis 



,7 



42 



7 25 



6 



6 



55 



6 



14 



7 



7 



50 



7 30 



8 



6 



45 



6 



55 







Reversed the Poles. 











9 



7 



24 



7 6 



2 





n 



6 



21 



11 



7 



26 



7 4 



4 



6 



10 



6 



28 



13 



6 



34 



6 44 



6 



6 



15 



6 



0 



15 



6 



34 



6 43 



8 



6 



23 



6 



4 



Mean, 



7 



19 0 



7 11 45 





6 



34 45 



6 34 45 



And taking the general mean, we get the true Dip 

 with Needle No. 1 6 49 56 No. 

 ditto ditto „ 2 6 55 4 



Mean, 6 52 30 



N. B. The numbers 1, 2, 3, &c. exhibit the order in which the ob- 

 servations were made c During the present century, I cannot find that 

 any observations for Dip have been made at Madras, but there is one 

 result on record dated 1775, when Abercrombie found it to be 5©15'N.j 

 if this result can be trusted, it would appear that the Dip is on the in- 

 crease at the rate of 1' 34" in a year. 



With regard to the needles employed for the magnetic intensity, it 

 may be necessary to state, that they are constructed after the model of 

 that of Professor Hansteen. The needles are cylinders, 2\ inches 

 long and .3 inch in diameter, save that the ends are abruptly sharpened 

 to a point; these needles are freely suspended on their centres by a few 

 filaments of unspun silk, which are hooked on to a brass stirrup, move- 

 able upon the needle ; by which means a perfect adjustment to horizon- 

 tality can be effected; the needle thus suspended is enclosed in a rec- 

 tangular glass box immediately over a divided circle, from which the 

 arc of vibration can be read off and the number of oscillations counted. 

 The zero of measure here employed, is the time of performing 100 

 vibrations at a temperature of 60°, commencing with an arc of 20° and 

 ending at from 2° to 4°.— If these measures could be observed to ulti- 



