298 OBSERVATIONS ON THE MAGNETIC DIP, 
And as 29.8940 1.4755891 Total intensity, August 18, 1840, horizontal being 10. 
To 29.8513 1.4749632 Total intensity, May 8, &c. 
Sois 10. ly Horizontal intensity at Cincinnati, August 18, 1840, assumed as unity. 
i is: 99.856 0.9993741 Total intensity, May 8, that on August 18, 1840, being 10. 
Thus it appears that the magnetical intensity at Cincinnati, May 8th, 1841, was ex- 
tremely near that at the same place in August 18, 1840, which has been used as the unit 
or standard of comparison for the Iowa and Wiskonsin observations, made in 1839, the 
difference being only 1.44 parts in 1000. It appears, by applying the equation C = *P 
in which C = cosine of dip, R = radius, H = horizontal, and 'T’ = total intensity, that 
an increase of 1/45’ of the estimated dip would complete the equality. 
If it be asked why I have not included more than two needles in obtaining the above 
mean result of the horizontal intensity, I answer, that No. 1, since being used, August 
18, 1840, had been connected, by iron keepers, with No. 2, and had evidently changed 
its magnetism. Nos. 4, 5, and 6, had not been used in August, and, indeed, were for the 
first time used in this series. 
Having made and explained the above comparisons, I shall proceed to use the squares 
of the times of five hundred vibrations of each of the six needles, as given above, for 
May 8th, 1841, as quotients or standards of comparison, for obtaining the values of in- 
tensities, as observed at other times and places referred to in the above, as unity. I 
had intended to have used the results obtained at Philadelphia as the standard of unity ; 
but I have found it more convenient to refer all my observations to a locality to which I 
could have more frequent access, and at which I could, from time to time, observe the 
secular changes and anomalous fluctuations of the magnetical elements. I am rather 
surprised to find the total intensity at Cincinnati not only near to that at Philadelphia, 
but a little above it. 
40. Total intensity at Cincinnati, May 8, 1841, horizontal intensity, at the same time and place, being 1, 10, or 
As cos. 70° 26' 11” 9.5248544 
To R. 10. 
Sois 10 L, Assumed horizontal intensity at Cincinnati. 
To 29.8638 1.4751456 ‘Total intensity at Cincinnati, May 8, 1841. 
41. Philadelphia, Latitude 39° 57’, N.; Longitude 75° 10’, W.; March 30, 1841. 
Duration | Calculated Total intensity, 
Dip. No of | Epoch of commencing |__ of 500 Tempe- duration | Square of the Horizontal) Total intensity, |that at Cincin- 
ae Vibrations. Vibrations.|rature. at 60°. preceding. intensity.| Hor. being 1000.|nati being 1000. 
i a 1 1h.29m.01s.2 1604. 43° |1607.272583316,.8529)| 917.57 
2 2°06 O12 16821 44 2:1585.62 2514190.7844| 918.57 
3 2 40 03 6 {1290.4 | 50 1291.15 1667068.3225 916.19 
| 4 4 04 02 4 /|1156.8 | 44 51157.99)1340940.8401 919.26 
| 5 4 33 00 1408.8 | 44 (1409.98 1988043.6104 917.43 
1 6 5 08 04 8 (1403.6 ! 43 ‘1405.7 (1975992.49 | 918.01 
Mean,| 917.84| 2970.80 994.783 
Particular locality, in the yard of the Magnetical Observatory, near the Girard College. 
Gcology, probably primitive rocks, covered by many feet of gravel and drifted materials. 
