OF THE UNITED STATES. 355 



to the telescope. Tlie glass cover rests on three points, and 

 is held to them by the spring of a brass ring above. 



The needle .r.r hangs edgeways, has a jewelled cap mount- 

 ed in brass, which can be screwed in the centre from both 

 sides, to verify and compensate the parallelism of the mag- 

 netic line of the needle with the middle line of its iigure, 

 which serves for the readings. 



Four pieces are adapted outside of the needle box. pro. 

 Jecting a little above it, to receive a large spirit level across 

 the bo.\. in two situations, at right angles to each otiicr, one 

 parallel to the telescope, the other parallel to the axis, — by 

 which the instrument is levelled. 



The needle box has below two strong pieces b, b diame- 

 trically opposite to each other. These form the sockets of 

 the horizontal axis //, c of the telescope, bearing at its thicker 

 side c the piece //, in which a tube of four inches long is fas- 

 tened. Through this tube passes a telescope, in all rcsi)eets 

 exactly equal to that of the |)lane tal)le alhidadc, so that they 

 might be interchanged in case of accident. 

 . This telescope describes a complete vertical circle, to 

 which it is of course adjustable by the motion of the wires, 

 and it was not found pioper to give it any other adjustment. 

 Its verticality is best verified by the reflection in a mercury 

 horizon of the pole star, or any other object seen under a 

 large vertical angle, when the instrument is adjusted by the 

 level. The coirection is of course to be made, if necessary, 

 half by the wires, and half by the supports of the level. The 

 needle itself is then equilil)rated for this adjustment by the 

 brass counterpoise / 'I'he adjustments are so sim[»le, easy, 

 and apparent, as to need no description. 



The needle is prevented from moving by a stop, when not 

 in use. 



The needle case with all the above rests upon six pillars d, 

 d, by which it is made fast to the plate n, n which is the Ih'st 

 of the stand part of the instrument. All the lower parts being 

 exactly equal to those of the plane table, I shall refer to the 

 description of that instrument for further details. 



