Dip and Variation of the Magnetic Needle. 99 
factory, yet I think we shall not be very far from the truth if we 
assume that the dip in the eastern states decreases 0/.5 per year, 
and is stationary in the western states. In the subsequent dis- 
cussion, I have in part evaded the difficulty, by assuming Jan. 1, 
1840, as the common epoch, which is not far touche mean time 
of the observations. 
. In discussing the observations, I inv divides siateiieates fouie 
groups according to longitude. The following table contains all 
the observations between longitudes 71° and. 78° W. which I 
have been able to obtain. Where several observations have been 
made at the same locality, I have endeavored to deduce the most 
probable mean result. 
TaBre I.— Observations of the Magnetic Dip from 71° to 78° west longitude. 
| staan. 
Dip 
Date. ; Authority. Jan. 1,) Diff. 
1840. 
Lat. |Lon.| Dip. 
Wantreal. "711835. Nov. 25|Capt. Back and — barre 76 40.7 <148 
Iswego, 26/76 36.75, 11,3|1839. Sept. dpret Loomis, i751 4 
Utica, 3 7\ 5.1374 67:211839, Sept. 3 74 57.0|+ 3.8 
Syracuse, _ [43 076 14:74 50.9)1839. <p 2 “ {74 50.7} 64 
Schenectada, : 36.1)1839. S 74 35.9|— 4.1 
ibany, 3 tape 4} 1835. Mee: ‘- Profs. Henry, Bache, and Loomis, |74 45.4|+125 
Cambridge, 20.1]1839. Sept. 17 Pars ate cag 419.9|=— 40 
. Do heater, hoes 0}: _ 17418.8)--2.9 
Worcester, 1839, Sept. 16/P of. yas s, ‘174 20 4)+ 27 
Springfie) 8 ee eb. 26) Profs: Bache and Loomis, 74. 7.4)— 0,3) 
Longmeadow é Sept. 14 “Se i vg W745 0.7] 
Rene . Peb, 27|Profs. Bache.and Loomis,. 9,8)+ 3.8) 
Ww Pol One 3. C id Loomis, a one iD 
est Point, :. Ock: Profs. Courtenay an ia, 73 28.0) — 1.0) 
ew Haven, it. 11/Prof. Loomis, 73 26.5)— 0.5 
New York, [40.43/74 2)72 55.3/1840. Jan,. 3) } MESS" Sabine, Fmakl lin, Back, Hie 55. 
Princeton, 22/74 40/72 47. 1/1839. Sept. 21/P: of. Loomis, {724 
Philadelphia, {39 57/75.11)71 56.1)18 3 Me a Bache'and Loomis, 
Balt weg 76 3 35 6184 i 
ng ES 
tude 41° 51 N., ong 73° 52 Ww. we obtain twenty-equa- 
tions of condition of the usual form. From these eq 
we obtain 6 =2.8772, z=+.01389, y=+. 00077, and the ‘direc: 
tion of the isoclinal Be is from N. 86° 50’ W. to S. 86° 50’ EB. 
"Phe increase of dip in the normal direction is 50’.077 to sixty 
geographical miles. Computing from these data the dip at the 
several, stations, we obtain the differences given in the last column 
ove. Four of these differences exceed ten minutes. ‘The as- 
sumed ip at Montreal i is the mean of ae two haute © ‘obser 
} —— 
tre wee T° 64 1833. ‘April 19. Geet ase 
ee) ss 1838. 
