TERRESTRIAL MAGNETISM, 



Dip. 



Epoch of 

 commencing 

 vibration, 

 hrs. min. sec. 



Duration 





Calculated 



of 500 



ll 



duration 



vibra- 



at 60°. 



tions. 



H " 





Square of the 



Horizon- 



Total in- 



preceding. 



tal inten- 



tensity : 





sity. 



hor. be- 

 ing 1000. 



Total inte: 



sity : that at 



Cincinnati 



= 1000. 



156. St. Mary's, Ohio.— Lat. 40° 32' N. Long. 84° 19' W. September 5, 1845. 



72° 00' 20" 

 Var. 3 04 00 E. 



1 50 00.0 P 



1153.0 



78.0 



1151.06 



2 28 03.0 



1400.8 



77.0 



1399.03 



3 02 00.2 



1396.2 



78.0 



1394.00 



13249391236 



19592849409 



19432360000 



Mean 



92.3.14 

 926.50 

 923.14 

 925.45 



3002.6 



1005.6 



157. Carrolton, Ohio.— Lat. 39° 38' N. Long. 84° 09' W. September 8, 1845. 



71° 10' 00" 

 Var. 4 45 27 E. 



4 40 02.4 



1130.8 



75.25 



1129.68 



12761769024 



966.25 



2993.2 



1002.4 



REMARKS. 



156. St. Mary's, Ohio. — Geology: deep diluvium of clay, sand, and gravel, superimposed on the cliff 

 limestone, or upper silurian rocks. 



157. Carrolton, Ohio. — Geology: blue limestone of Cincinnati; lower silurian. 



IX. Series for 1846. — Made in the Service of the United States Coast Survey. 



The following observations have been reported by me to Prof. A. D. Bache, the 

 Superintendent of the Coast Survey, and they are the property of that department 

 of our government. But, by the special permission of the Secretary, procured by 

 the kindness of Prof Bache, I am permitted to publish them here, in order to com- 

 plete my series. The following tables do not, however, include all of the observa- 

 tions made by me for that survey, the results of the experiments with Lieut. 

 Riddell's apparatus to determine absolute horizontal intensity of magnetic force 

 being omitted. These results wiU appear in the course of the publications of the 

 Coast Survey. 



The latitudes and longittides are in part those which have been determined by 

 the Coast Survey. In other cases, they have been fixed by approximate admea- 

 surements from the survey stations, and are probably correct within a small fraction 

 of a mile. Those which are most doubtful, I have marked with the sign (?) . 



The dips or inclinations were determined by the same dip-circle heretofore used 

 by me, but the needles had been recently repaired, and, unfortunately, gave un- 

 satisfactory results. An expedient was, however, resorted to, which, especially in 

 the last part of the series, enabled me to obtain a very close approximation to the 

 true inclination. A particular account of this expedient will be given in this paper. 

 (See article "Fort Delaware.") 



The declination or "variation" was in all cases determined with the utmost care, 

 as being of the greatest practical importance in forming the charts of the Coast 

 Survey. The apparatus used for this purpose was the portable declinometer 

 described by Lieut. Riddell, consisting of a tubular magnet, mounted and suspended 

 as a collimator, and read by a theodolite telescope. The instruments at each station 

 3 



