96 Dip and Variation of the Magnetic Needle. 
? Observations. Equations of condition. | Differences. |Difterences. | 
1822. Dec., 73° 0.5)0+ .9537=60/5 |—15’.96|10..58 | 
1825. March, 73 27.0] 3+ 3.2004=87 .0 |+-15.02 
1833. April 1, 72 49.3 | 5+11.2464=49 .3|- 6.63 
1834. Aug. 7, 72 51.7 |6+12.5974=51.7|— 1.54;—1.92 
1839. Sept. 9, 72 52.2 |6+17.6874=52 .2|+ * ye Se 
These. equations furnish us 9=78’.365, 4=— 1.9947, from 
which data we obtain the differences in the third column above.. 
But three of these observations, viz. the first, fourth, and fifth, 
were made at the same locality, and are much better suited for 
comparison. 'These give 5=60'.434, d= —0.5411, and the dif- 
ferences computed are given in the last column above. J regard 
this as the most probable value of 4 afforded by the New York 
observations. In determining the absolute value of the dip, I 
know no reason for rejecting the other two observations, and as- 
suming the last determined value of 4, the most probable value 
of 9 is 65.084, which gives for Jan. 1, 1840, 72° 55.34, It will 
be noticed I have gone upon the supposition that the motion of 
the needle has been uniform throughout the entire period; yet 
the last three observations might lead us to infer that since 1833 
the dip had been sensibly stationary. 
I know of no other observations made in the United States 
before the year 1833. In the years 1833 and ’34, the. dip was 
observed at several places by Prof. Bache and others; and in the 
year 1839 I observed at the same stations. The observations 
were made with different instruments, and seldom at exactly the 
same localities ; nevertheless it may be instructive to institute a 
comparison. ‘The following table exhibits the two series of ob- 
servations, with the interval elapsed and the change of dip. 
Int rval Change 
| Station. | Date. | vip. | Observer. Station.| Date. | Dip. | thea in yrs..jof a'p- of dip. 
UJ 
roe Bee ry, JAlb‘ny, (Sept. 4, 1839174 51/3] Prof.Loomis,| 5,728 | +57 
ro ache, 
‘pring.,| “ 14, 1839|74 6.9) “ 096 | -38 
1834|74 2.8) te Pave “ rt 1839|73 59. “ 5.115 | -3.2 
1833|73 25.8 Pestana Wee Ptah Be Ta Riay FE. 5,819 | -41 
834|73 37.2) “to 
72 0.2|Prof, Bache, _}Philad., 1838)71 43.9| Prof. Bache, 5.069 | 54 
ee 23; 1839 7.1|/Prof. Loomis 
* 13, 1840/71.53.3IProf. Bache. , an 
The entire oe of dip, divided by the number of years, 
gives — 0’.4026 for the annual motion, which result accords very 
well with the last value of 4 before determined. 
