lie. 



angle that the zeros of the graduations or extensions (supposed to be in coincidence 

 with the geometric axis) makes with the magnetic axis. As a rule, needles of the latter 

 type always do have such an index error in addition to those arising from eccentricity 

 of graduation, and the difficulty encountered by the maker to determine the magnetic 

 axis. 



While the methods of reversion for eliminating errors of eccentricity and non- 

 coincidence of the principal axes just described are good for scientific research, they 

 are ill adapted for the work of the surveyor. 



A surveyor's compass is ordinarily graduated to single degrees, and a fine instru- 

 ment is seldom divided to read to less than one-half degree directly or from six to ten 

 minutes by estimation ; therefore it is useless to try to arrive at results closer than 

 these. A needle will serve its purpose well if the two principal axes coincide within 

 the degree of accuracy obtainable with its length and with the compass as a whole. 



The following question may now properly present itself : What is the best form 

 for a magnetic needle for the surveyor in order that it shall be so nearly free from 

 errors due to the above causes as not to require reversion and yet to come within the 

 degree of accuracy obtainable in practice? If now we have recourse to Pig. 1 

 and such modifications of the shape there shown as is delineated in Figs. 2, 3 

 and 4, we shall see that since the greatest superficial area lies in the azimuthal 

 plane, deviations of the two axes cannot be infrequent, and, therefore, these 

 shapes are not well adapted for surveying instruments. Fig. 4, in particular, 

 presents the curious combination of a large surface at the ends and a small one at the 

 hub. This increased spread of surface by the arrow-shaped ends renders the needle 

 especially liable to this defect without so much as compensating for it in greater 

 magnetic intensity as was probably the supposition, since the arms connecting it with 

 the central hub are quite narrow and limit the magnetic intensity. This faulty design 

 is probably based on the supposition that since in a well hardened and well constructed 

 needle the magnetic intensity is greatest at or near the extreme ends, gradually weaken- 

 ing towards the center where it becomes quite indifferent, that if the ends are made big 

 the needle will the more readily assume the magnetic direction. 



On the other hand, the edge bar needle as made by C. L. Berger & Sons and shown in 

 Fig. 7, with its greatest superficial area in the vertical plane, reduces the chances of a 

 non-coincidence of the two principal axes in the azimuthal plane to a minimum. In 

 order to add strength and stability to this needle it is made thicker at the middle than 

 at the ends, which are quite thin. 



If in the manufacture of a needle of this form the arms are bent so as to be 

 symmetrical to the axis .passing through the center of the needle cap and at right angles 

 to the line connecting the ends of the needle, as shown in the exaggerated diagram, 

 Fig. 8, the magnetic axis will be contained in a plane parallel to the vertical plane pass- 

 ing through the ends of the needle and a little removed from it. 



The error in the reading introduced by this small distance will be very small com. 

 pared with the error due to the eccentricity caused by the above mentioned bending. 

 But, supposing the case that in a badly constructed needle of this class one arm is bent 

 more irregularly than the other, so as to unsymmetrically distribute the mass of the 

 metal of the needle, then the resultant polarity consequent upon the irregular distribu- 

 toin of masses of the metal may be slightly at an angle to the line passing through the 

 ends of the needle, causing an error due to the divergence still very small as compared 

 with the error due to the eccentricity such a needle would have. 



So far.in the foregoing explanation we have dealt with the magnetic needle alone. 

 It will now be necessary to treat this task in relation to other functional parts that go 

 to make up a surveying instrument, since there are other causes, which, singly or com- 

 bined, may conduce to produce the differences mentioned in the reading of different 

 needles. 



However, before dealing with this subject in its further complexity in relation to 



of all 



the compass and instrument, we first of all take it for granted that there be no 

 attraction in the instrument itself or by any iron concealed on the person of the 

 observer.* 



*To determine whether an instrument itself has any iron in it to disturb the needle, it is a good plan, after 

 setting the transit so that both compass needle and the vernier read zero, to go around the circle, setting the 

 vernier ahead ten degrees each time, and noting whether the compass needle also describes an arc of precisely 

 ten degrees. If it does not, there is some local attraction. Before making this test it will be well to first test 

 the needle as to sharpness of pivot mentioned later on and to breathe on the glass cover of the compas$ 

 and on the rubber frame of the reading glass so as to remove any electricity which may be present. Hoth ot 

 these articles being insulatorsare very easily electrified by the process of cleaning in a dry atmosphere, thereby 

 affecting temporarily the reading of the needle. 



