346 OBSERVATIONS ON THE MAGNETIC DIP AND DECLINATION. 
The usual arrangement is provided, of a pair of brass Y’s, which can be raised 
and depressed at pleasure, to lift the needles from the agates, and set them down again, 
for the purpose of giving a new bearing of the axles, or to shorten the vibration of the 
needles. 
The azimuth circle is an open ring eight inches in diameter, connected by six radial 
bars with the socket at the centre. The quantity of metal round about the needles is 
thus diminished without any sacrifice of strength or steadiness. ‘This circle is divided 
into ‘degrees, numbered round from 0 to 360°, and these again into half degrees, and 
the readings are to single minutes by means of a vernier. 
For bringing the plane of the dip-circle to coincide with the magnetic meridian, 
there is a horizontal needle nine and a quarter inches long, whose socket at the centre, 
rests upon the point of a pin projecting from the middle of a steel axle, whose pivots 
rest in the Y’s of the dip-circle. The pin which supports the horizontal needle is kept 
vertical, by the gravity of a brass bob, diametrically opposite to it, also attached to the 
axle, and which serves as a handle in adjusting the needle concentric with the dip-circle. 
This part of the apparatus is, of course, removed when the plane of the dip circle is 
brought to coincide with that of the magnetic meridian. It was attached to the instru- 
ment after it was imported into this country, by Mr. James Green, mathematical instru- 
ment maker, in Baltimore. 
A very delicate spirit-level is provided, for levelling in azimuth, by means of three foot- 
screws. A pair of bar magnets and a good thermometer with Fahrenheit’s scale, are 
appendages to the instrument, and are conveniently packed in the travelling case. 
The needles are designated by the figures 1 and 2 respectively, marked upon the flat 
sides. 
The method of observing has been the same as was described in a preceding portion 
of this paper, read before the Society on the 16th of August, 1844. That is, the needles 
were always read at both ends, in four different positions, with the poles direct, and in 
the same number again with the poles reversed. ‘T'wo of these four changes were, in 
each case, made by reversing the face of the dip circle, and the two others by reversing 
the ends of the axles upon the agates. 
When the instrument was first observed upon, the needle was read four times suc- 
cessively in each of its positions with respect to the instrument, in order to ascertain the 
degree of coincidence of these several readings, the needle being lifted from the agates 
and allowed to assume its rest anew after each reading. ‘These several readings were 
found to differ so little in the same relative position of the needle, seldom amounting to 
as much as three minutes, and usually not exceeding one or two minutes, that it was 
considered unnecessary to read more than twice in the same position in order to elimi- 
nate errors of reading or of position of the axles upon the agates. Hence each needle 
has been usually read only twice at each end, in the same position, though at several 
stations they have been read as many as three times at each end, in each position. 
The needles were always raised from the agates and allowed to assume their positions 
anew, after each reading. 
In order to show more clearly the mode of observing, and, what is not less important, 
the character of the instrument, the observations made at Washington on the Ist, 4th, 
