260 C. C. Hutchins — New Declination Instrument. 



Art. XXVII. — A JYeiv Declination Instrument ; by C. C. 



Hutchins. 



The declination is the only magnetic element that the 

 engineer and navigator wish to know, and it is, hoped that the 

 apparatus to be here described will be found as useful and 

 much more simple than commonly employed in finding it. 

 Reference to the figure will make the following details plain. 



We have a telescope of 1-inch diameter and 6-inch focus. 

 Upon the telescope tube are two carefully turned bronze rings 

 upon which the telescope rests in a light cradle C-D. The 

 cradle is suspended with floss silk from a torsion-head H. 



The telescope hangs in a box upon which are mounted two 

 level phials. One side of the box is of glass and is removed 

 by a convenient knob K. v 



A rod runs the length of the box and bears two hook-like 

 supports A-B. When the rod is revolved by means of a 

 lever at the eyepiece end, and the lever slipped over a catch 

 at O, the supports then lift the telescope from its cradle and 

 support it. The eyepiece of the telescope projects through 

 one end of the box, where it is covered with a cap to protect 

 it from air currents. Before the objective is a plate-glass 

 window. 



The whole is mounted upon a small divided circle reading 

 to minutes. The socket of the circle may be made to fit the 

 ordinary engineer's tripod. 



The peculiarity of the apparatus lies in the telescope, whose 

 tube is made of steel and is magnetized. It was found that a 

 very satisfactory tube could be made by case-hardening a piece 

 of ordinary bicycle tubing, and magnetizing. The eyepiece of 

 the telescope has a single wire at its focus. 



The operation of the instrument is as follows : — 



It is set up on a meridian line. About 200 feet away, and 

 in the approximate direction of the magnetic meridian, is 

 placed a horizontal scale divided to inches, with bold marks. 



The telescope being removed from the box, a brass tube of 

 the same weight is substituted, and the torsion head is rotated 

 until the suspension is free from torsion. The telescope is 

 replaced and the box being rotated until it points to the dis- 

 tant scale, the excursions of the vertical wire are observed, 

 and its point of rest found in the usual way. This observation 

 is repeated after rotating the telescope 180° in its cradle. 



This eliminates the effects of parallax and the lack of coin- 

 cidence of the magnetic and telescopic axes. 



The mean of the two points of rest is found ; the telescope 



