236 Instructions for the Scientific Expedition 



In addition to these parts, the apparatus is provided with a brass 

 bar of the same length as the magnet, (furnished, like it, with cross 

 wires at the extremities, and knife-edge bearings,) for the purpose of 

 determining the zero points of the micrometers ; a brass scale, divided 

 to 10', used in adjusting the value of their divisions ; and a hori- 

 zontal needle, to be employed in determining the azimuth of the 

 vertical plane in which the needle moves. 



Adjustments. — The following are the adjustments required in this 

 instrument : 



1 . The instrument being placed on its support, in a suitable posi- 

 tion with respect to the other two instruments, the azimuth of the 

 plane in which the needle is to move may be adjusted in the follow- 

 ing manner. The plane is made to coincide, in the first instance, 

 with the magnetic meridian, by means of the horizontal needle which 

 moves upon a pivot fixed to the top of the scale. A small theodolite 

 (or other instrument for measuring horizontal angles) is then placed 

 on the base ; and its telescope brought to bear on a distant mark. 

 The telescope should then be moved through a horizontal angle equal 

 to the intended azimuth of the instrument, but in an opposite direc- 

 tion. The base of the instrument is next to be turned, without dis- 

 turbing the theodolite, until the mark is again bisected by the wires 

 of the telescope : it is then in the required azimuth. The base 

 should then be levelled, and permanently fixed. 



2. The microscopes should now be adjusted, 1. to bring the image 

 of the cross wires of the needle to coincide with the wires of the 

 microscopes; and 2. to make the arc value of the interval of the 

 wires, corresponding to one revolution of the micrometer head, ex- 

 actly equal to five minutes*. These arrangements have been nearly 

 eflfected in the first construction of the instrument ; for the purpose 

 of completing the adjustment, the microscopes are capable of a double 

 motion, one of the entire body of the instrument, and the other of 

 the object glass alone. It is manifest that these two movements are 

 sufficient to effect both adjustments. The former is attained when 

 the cross of wires is seen distinctly (and without parallax) at the 

 same time that the microscope wires are exactly in the focus of the 

 eye-piece : the latter is accomplished when the moveable wire of the 

 microscope is made to pass over a given number of divisions of the 

 scale, by double the number of complete revolutions of the micrometer 

 head. 



3. The fixed wires of the microscopes are then to be adjusted to 

 the same horizontal line. This is effected by means of the brass 

 needle. This needle being placed upon the agate planes, by its 

 knife edges, and allowed to come to rest, it is manifest that the line 

 joining the cross wires will be horizontal, provided it be perpendi- 

 cular to the line joining the centre of gravity and the axis. To 

 effect this latter adjustment, the needle (a great part of whose weight 

 is disposed below the knife-edge) is furnished also with a small 



* This adjustment is by no means a necessary one. It is sufficient for all pur- 

 poses if the arc value coiTesponding to one revolution of the micrometer be accu- 

 rately kuown. 



