OBSERVATIONS ON THE MAGNETIC DIP AND DECLINATION. 345 
Magnetic dip at Park’s Hill, by a mean of forty-eight observations, on needles 
Nos. 3 and 4, made on the 27th, 28th, and 29th of November, 1840, . . 77° 02/5 
The observations were made under a tent for the protection of the instrument; the 
snow nearly a foot deep upon the ground. 
There was a slight fall of snow during the observations, on the 27th and 28th of 
November. 
On the 29th it was cloudy. Needle No.3 was more strongly charged with magnetism 
on the 29th than on the 27th. On the 29th pains were taken to charge it fully to 
saturation, and it vibrated more briskly than on the 27th, 
A. continuation of Observations on the Dip of the Magnetic Needle; embracing thirty-eight 
additional points in the United States, New Brunswick, and Canada, between Wash- 
ington and the head waters of the River St. John. Made during the years 1841, 1843, 
and 1844, By James D. Graham, Major, United States Corps of Topographical 
Engineers. Read January 17, 1845. 
The instrument with which the following observations were made, was constructed by 
Gambey, of Paris, and is probably one of the best that has come from the hands of that 
ingenious and skilful artist. The close coincidence which has uniformly been shown in 
the results of its two needles, and the consistency in the repeated readings requisite for 
the many sets of observations that have been made with it, seem fully to justify this 
remark. 
It is constructed of gun-metal, except the cover for the dip-circle. This is a square 
box open at the base, for passing it over the dip-circle, and has a door opening and shut- 
ting on hinges, for convenience in placing and adjusting the needles, and is made chiefly 
of wood, and fine plate glass. 
The vertical or dip-circle is ten inches in diameter, and is divided, as usual, into four 
quadrants of ninety degrees each. ‘The degrees are divided each into six equal parts, or 
directly to ten minutes. These smaller divisions are easily subdivided by estimation, 
after a little practice, aided by the magnifying lenses that are attached to the instrument, 
to the nearest one or two minutes. 
It has two needles, each nine and a half inches long, and nine-sixteenths of an inch wide 
in the middle where pierced by the axle. ‘They are here gently rounded upon both narrow 
edges, and taper thence to a point at cach extremity. They play entirely within the 
interior ring of the dip-cirele, whose diameter is one-twentieth of an inch greater than 
the length of the needles, leaving one-fortieth of an inch clear space at each end of the 
needles, By this arrangement, all parallax in reading the angle of the dip is avoided. 
The pivots of the steel axles rest, and roll freely when the needles vibrate, upon two 
polished agates, whose upper surfaces are of cylindrical form. 
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