342 THE CRYSTAL FALLS IRON-BEARING DISTRICT. 



uatecl horn- circle. It is therefore a portable sundial. The gnomon is a 

 thread, which is attached at one end to the center of graduation of the hour 

 circle near the rear sight and at the other to a point in the forward sight so 

 taken that the angle made hj the thread with the plane of the hour circle 

 is equal to the latitude of the place. When this instrument is leveled and 

 set up in the meridian on a sunny day, the thread will cast a shadow on the 

 hour circle at the correct apparent solar time, from which mean time may 

 be determined by applying the equation of time. Conversely, if it is so set 

 up that the shadow of the thread falls on the correct apparent time, the 

 sights of the instrument are in .the true meridian. In this position the 

 declination of the' horizontal needle may be read off from the graduated 

 circle. At work, this instrument is mounted on a light Jacob's staff, or it 

 may be held in the hand. The Jacob's staff, although often inconvenient 

 to carry, is preferable, as with it the readings are all taken at the same 

 height above the ground and the leveling is more exact and steady. In 

 a correctly constructed instrument, with good time, the readings may be 

 made to half a degree. Correctness in the time, however, is indispensable 

 to good work, and this is best secured by keeping a standard watch in camp 

 and referring the working watches to it daily. 



The dip needle needs no description. In geological work that form 

 known as the Norwegian, in which the needle is pivoted on a universal 

 joint, is not so useful as the type in which the needle is rigidly confined to 

 one plane. In taking the readings, this plane in which the needle is free 

 to swing is made to coincide Avith the vertical plane determined by the 

 pointing of the horizontal needle. The circle is graduated to single 

 degrees, and with skillful work the readings are reliable to one or two 

 degrees. It may be added that the south end is weighted, in order, either 

 partly or completely, to balance the vertical component of the earth's force. 

 It was found better not to balance it completely, but only to such an extent 

 that the north end of the needle would dip about 10° (the graduation zero 

 being horizontal) in an area removed from local disturbances. It is no 

 doubt desirable that all the dip needles used in the same work should be 

 brought to approximately the same index error, in order that the readings 

 may be more directl)^ comparable. In practice, however, it was found quite 

 impossible to keep our three needles in unison, on account of the rough 

 usage to which they were unavoidably subjected. As, however, the form 



