376 OBSERVATIONS ON THE MAGNETIC DIP AND DECLINATION, 
From the foregoing table we have the following exhibit of the magnetic dip at Rouse’s 
Point: 
| Needle No, 1. | Needle No. 2. of both Needies. 
By thirty-two observations made in the plane of the 
| magnetic meridian, . . 76° 40! 
[By sixty-four observations made in the plane of the a 
magnetic meridian, . 76° 40'.84 
By two hundred and fifty-si -six observations made at: every 
| 10° of azimuth, from 0° to 80° inclusive, and reduced 
| to the plane of the magnetic meridian,. . . . . |76°42'.45/76°39'.52) 76°40'.98 
Showing scarcely a sensible difference between the dip observed by both needles in the 
plane of the magnetic meridian and that deduced from the observations on both needles, 
in various azimuths, from 10° to 80°, inclusive. 
The results shown in column ten of table A, seem exceedingly satisfactory for both 
needles, especially when it is considered that an error of 1’, in reading the inclination at 
the azimuth of 70°, would cause an error in the computed dip of 2’.6, and a like error at 
the azimuth of 80° would cause an error in the computed dip of 5’.5. 
A very slight defect in the instrument, indeed, either in the form of the axle of the 
needle observed on, or the want of perfect freedom from magnetism in the metal of which 
the dip-circle is composed, would operate as sufficient cause to produce a much greater 
error than 1’ in the observed inclination at those high azimuths, even supposing no 
errors to be made in the actual reading of the needles. 
But we consider the quantities in column nine as the more correct indices of the per- 
fection of the form of the axles of the needles, and of the freedom of the dip-circle from 
magnetism and from ferruginous matter. 
In regard to needle No. 1, the quantities in that column show that the greatest differ- 
ence between the observed inclination in any azimuth, and the inclination computed 
from the data afforded by the observed azimuthal angle, and the observed dip in the 
plane of the magnetic meridian, is only 1’.73. 
In regard to needle No. 2, the greatest difference is 2/.28, but the next greatest is 
only 1/.33. 
The character of this instrument and the form of the axles of its needles have been 
still farther tested, and in a manner equally satisfactory, by the following highly inte- 
resting series of observations, made at the astronomical station on the east shore of Lake 
Memphremagog, in Stanstead, Canada East, by Lieutenant A. W. Whipple, between 
the Ist and 7th of November, 1845. 
These observations and the several results are exhibited in the table marked B, as 
rendered by Lieutenant Whipple. The mode of observation adopted by him was, first, 
to ascertain the dip by both needles from careful observations made with the dip-circle, 
in the plane of the magnetic meridian; he then observed at every 15° of azimuth from 
0° to 90°, turning the plane of the dip-circle round from north towards the west, and 
then in the several planes, at right angles to the former ones. 
