14 



OBSERVATIONS ON 



Dip. 



Epoch of 



Duration 



, 



Calculated 



commencing 



of 500 



|5 



duration 



vibration. 



vibra- 



at 60°. 



hrs. min. sec. 



tions. 



B g 





Square of tbe 



Horizon- 



Total in- 



preceding. 



tal inten- 



tensity : 





sity. 



hor. being 

 1000. 



Total inten- 

 sity : that at 

 Cincinnati 

 = 1000. 



171. Fort Delaware, Pea-pateh Island.— Lat. 39° 88' 16" N. Long. 75° 36' 52" W. June 

 14, 1846. 



71° 33' 42" 

 7ar. 3 13 09 W. 



7 35 28.0 A 



8 40 32.4 



9 21 02.4 



1152.0 

 1402.8 

 1396.0 



1150.80 

 1399.90 

 1393.56 



13243406400 



19597200100 



19420094736 



Mean 



930.55 

 928.00 

 930.56 

 929.70 



2939.45 



986.04 



REMARKS. 

 171. Fort Delaware, Pea-patch Island.— Geology : Alluvial mud, at least 70 feet deep. Water diked 

 Gilt, and the experiments made at a point lower than high-water mark. 



Remarlis on a New MetJiod of ascertaining the Magnetic Inclination.- — It occurred to 

 me at this place, as the needles had performed consistently at White Hill and at 

 Girard College, where the dip is about 72°, that, at all places where the dip is less 

 than 72°, the dip-circle might be turned in azimuth so much that the readings should 

 always be near to 72°, and then use might be made of such points of the needle-pivots 

 as had been proved to be good. I accordingly calculated a table, by which, when 

 the approximate dip, by direct experiment in the magnetic meridian, had been 

 ascertained, the azimuth in which the dip would be 72°, was given. This was after- 

 wards applied in the following manner: Having ascertained the approximate dip 

 by direct observation, the azimuth was set off as above, say eastwardly, and a read- 

 ing noted; the dip-circle was then turned in azimuth to the magnetic meridian, 

 where a second reading was noted ; and, finally, it was turned in azimuth west- 

 wardly, equal to the first eastward azimuth, where a third reading was noted. This 

 was repeated through all of the usual eight reversals and readings, thus producing 

 three columns in the field-book, two in equal azimuth east and west, and one in 

 the meridian. The mean of the readings in azimuth was then reduced by the ratio — 

 COS. az. : E : : cot. dip in az. : cot. dip in meridian. 



The following is an example : — 



Dip at Port Norris. 





Needle No 2 







Needle No. 1 





11° Az. E. 



0° Az. 



11° Az. W. 



11° Az. B. 



0°Az. 



11° Az. W. 



A. North, E.E. 72°2r00" 



72° 00' 00" 



72° 20' 00" 



71° 09' 30" 



71° 05' 00" 



71° 09' 00" 



E.W. 71 39 00 



71 28 80 



71 39 00 



71 05 80 



70 51 00 



71 05 00 



W.E. 72 15 00 



71 57 00 



72 11 00 



71 04 30 



70 45 30 



71 05 30 



W.W. 71 05 00 



71 48 30 



72 04 30 



71 00 30 



70 48 30 



71 04 30 



B. North, W.W. 71 53 00 



71 40 00 



71 57 30 



72 41 00 



72 16 30 



72 38 80 



W.E. 71 55 30 



71 45 00 



71 55 30 



72 33 00 



72 02 00 



72 34 00 



E.W. 72 04 30 



71 41 30 



71 58 30 



72 46 00 



72 29 80 



72 44 30 



E.E. 72 04 30 



71 45 30 



72 05 80 



72 44 30 



72 28 80 



72 42 30 



Mean 72 02 04 



71 40 45 



72 01 02 

 72 02 04 



71 53 04 



71 37 34 



71 53 00 

 71 53 04 



Mean of readings in Az. 72 01 33hyNo.2. Mean of reading in Az. 71 53 02byNo.l. 



