264 



HORIZONTAL MAGNETIC INTENSITY, ETC. 



was then deduced by Hansteen's formula. Similar experiments were made 

 with the other needles, as below. 



Min. Sec. 



March 22(1, 1840. Needle No. 1. Temperature 38°. 500 vibrations in 25 34.8 



Temperature 91.4. " " 25 44.8 



Coefficient deduced, 0.000117 



March 25, Mean temperature 37° 36'. Needle No. 1. 500 vibrations in 25 33.8 

 Mean temperature 91 5 " " " " 25 44 



Coefficient deduced, 0.000122 



March 24. Mean temperature 50. 



86.12 



Mean for No. 1, 



Needle No. 2. 500 vibrations in 



Coefficient deduced. 



March 25. Mean Temperature 48. Needle No 3. 500 vibrations in 



90 " " " " 



Coefficient deduced. 



0.000125 



25 18.8 



25 26.8 



0.000145 



20 34.6 



20 37.6 



0.000058 



This last needle, the flat one with lozenge points before referred to, seems 

 very little affected by temperature. 



It may, perhaps, excite surprise, and, possibly, some doubt as to the accu- 

 racy of my observations, that the dip given above for Cincinnati is so much 

 less than that which I obtained in November, 1837, the latter being 70° 45'.75, 

 and the former 70° 25'.5312; and that the dip at Louisville, August 31, 1839, 

 should have been 70° 08', and on March 11, 1840, only 69° 54'.4375, or, as a 

 mean, 69° 54'. 8750. It is probable, from an experiment recorded in Major 

 Long's expedition, making the dip at St. Louis, in 1819, 70° 30', and, by my 

 own observations, in 1839, making it, at the same place, only 69° 31'.4, the in- 

 terval being twenty years, that the dip at this part of the earth is diminishing 

 at the rate of about 3' per annum. This will account for part of the change 

 observed at Cincinnati. The remainder is probably produced by the seasons, 

 which Hansteen allows to have an influence to the extent of 15'. Allowing 

 7' for the annual diminution of the dip since November, 1837, we have for the 

 change produced by the seasons, 



70° 25'.5312 + 7' + 5: = 70° 45'.75, and 

 X = 70° 45'.75 — 70° 25'.5312 — 7' or 

 X = 13' 288 = the above named change. 



