58 



EIGHTH REPORT — 1838. 



Table IV. contains observations made at Tortington on the 

 17th of October, 1837, with Captain Fitz Roy's Gambey, and 

 its needle No. 2. The dip is here successively deduced from 

 the angles of inclination observed in azimuths 90° apart from 

 each other. In such case, cot^ B = cot^ i + cot^ i', S being the 

 true dip, and i and i' the angles of inclination in any azimuths 

 90° apart. In the first example in the table, i is the angleof 

 inclination shown by the needle when the plane of the circle is 

 removed 10° from the magnetic meridian ; that is, when it is in 

 the direction of N. 10° E., and S. 10° W; therefore includes, 

 and is the mean of observation with the poles direct and re- 

 versed, and with the index of the azimuth circle at 10° and 190°; 

 e" is in like manner a mean of the angles of inclination with the 

 poles direct and reversed, when the index of the circle is at 

 (10 + 90^=) 100°, and at (100 + 180° = ) 280°: here cot2^■ + cot2 

 l'=cot^69° 13'-5 + cot-86° 15'-2 = cot28; whence 8 = 68°56''64. 

 In the next deduction, the values of i and i' are obtained with 

 the index of the azimuth circle at 20° and 200°, (20 + 90°=) 1 10° 

 and 290°, and so forth. 



Table IV. 



Tortington, Oct. 17, 1837, with Captain Fitz Roy's Gambey, 



Needle 2. Observer, Major Sabine. 



10 

 190 

 100 

 280 



20 

 200 

 110 

 290 



30 

 210 

 120 

 300 



40 

 220 

 130 

 310 



Poles 

 direct. 



69 15 



69 07 

 12 



86 25 



70 04-5 



70 00-2 

 82 36-2 

 82 45-5 



7139 



71 26-2 

 79 09-7 

 79 20-5 



73 37-7 

 73 23-8 

 76 07 

 76 16-5 



Poles re- 

 versed 



69 03 

 69 29 



86 28-7 

 85 5.5 



69 50 



70 16 

 82 50-7 

 82 23-5 



71 16-2 

 7140-3 

 79 26 

 78 56 



73 18-3 

 73 40-2 

 76 23-2 

 75 55 



^6913-5 

 I 86 15-2 



70 02-7 

 82 39 



} 

 } 



1 71 30-4 

 170 13 



73 30 

 76 10-4 



Dip 

 deduced. 



68 56-64 



68 55-64 



68 56-96 



68 56-26 



50 

 230 

 140 

 320 



60 

 240 

 150 

 330 



70 

 250 

 160 

 340 



80 

 260 

 170 

 350 





 180 



Poles 

 direct. 



Poles re 

 versed. 



76 06-5 75 50-7 

 75 53-2,76 13 

 73 34-2 73 49-2 

 73 44-3 73 24-3 



79 08-7 

 78 52-8 

 7134 

 71 39-2 



82 31-7 



8218-8 

 70 05-7 

 70 07-5 



86 11 

 85 56 

 69 09-2 

 69 13 



55-5 



68 52 



78 48 



79 16-5 

 71 50-2 

 71 19-5 



8214 

 82 36-5 

 70 22-5 

 69 52-5 



85 44 



86 14-7 

 69 30-5 

 69 02-3 



68 46 



69 12-2 



V 76 00-9 

 1 73 38 



1 79 01-6 

 } 71 35-7 



1 82 25-2 

 I7OO7 



-86 01-5 

 .6913-9 



• 68 55-4 



Dip 



deduced. 



.68 56-40 



68 56-05 



6855-55 



68 54-90 



58 56-4 



