OBSERVATIONS ON THE MAGNETIC DIP AND DECLINATION. 369 
the 4th. And the dip obtained by Mr. Nicollet with his instrument, by Robinson of 
London, from a mean of one hundred and twenty observations, made on the 5th of June, 
1841, was 71° 15.4. Giving value to the observations of that period in proportion to the 
number of observations made with each instrument, we may state the dip at Washington, 
June Ist to 5th, 1841, to be, '71° 15.7, and at the same place, on July 8th, 1844, '71° 10/5. 
Decrease, in three and one-tenth years, 5’.2, or at the rate of 1.67 per annum. 
XLII. 1844, July 15th.—Baltimore. 
At the second Baltimore station, in the series of the year 1841; namely, one hundred 
and sixty-one feet on a magnetic course of 8. 47° W. from the curb-stone which marks 
the south-west corner of Calvert and Reed Streets. 
NeeEp.e No. 1. 
Polarity of No. of Time of 
Marked end, Readings. Observation. ‘remperature. Dip observed, 
N 16 2h. 12m., P.M. 88°.5 71°.29'.6 
Ss 16 Pie eee, 89 71° 35'.9 
Dip, by thirty-two observations on needle No.1, . ... . . . 71° 82'.7 
NEEDLE No. 2. 
N 16 3h. 43m., P.M. 89° 71° 35'.84 
Ss 16 4 23 . 88 .5 71° 28'.93 
Dip, by thirty-two observations on needle No.2, . . . . . . 71° 32'.38 
Dip, by a mean of sixty-four observations on needles Nos. 1 and2,. . . . 71° 82.54 
Weather clear. A gentle southerly breeze. 
I obtained, for the dip at this station, by sixty-four observations on the same needles, 
with the same instrument, on June 10th, 1841, 71° 32’. See No. XIV. (page 354.) 
Both sets of observations were quite satisfactory, and all the circumstances under 
which the two sets were made were equally favourable, and very similar. The tempe- 
rature was nearly the same, so that no sensible correction is due, on that ground, to 
either set, for comparing it with the other. The difference in the dip obtained after a 
lapse of three years and. thirty-six days, is only half a minute at this station, and the 
more recent observations give the greater dip. This may seem anomalous, but I have 
no reason to doubt the accuracy of either of the results. No buildings have been erected 
near the station since the first observations were made, nor is there any visible change of 
local circumstance whatever, that could have affected the dip. 
XLIV. 1844, July 16th.—Baltimore. 
Observatory of St. Mary’s College, being the same station as No. XVII. Latitude 
39° 17' 55”, N.; Longitude 76° 37’ 30”, W. 
- Neepue No. J. 
Ss 16 8h. 12m., A.M. 83° 41 42°76 
N 16 oS ae " 85 i060 OL 
Dip, by thirty-two observations on needle No. 1,.. . . . . ~~. =71° 89'68 
