. Carnegie Institution of Washington publication. 366 Special Reports abrasion, or otherwise. Examination of pivots under a high-power microscope often shows rust scars or pits of considerable magnitude; extended field experience shows that such oxidation takes place readily, especially in the tropics, despite utmost care. In the cases which have been analyzed, these large deviations in the correction-curves are nearly always such as would be produced by an elevation rather than by a depression on the pivot, that is, the maximum precedes and the minimum follows as the inclination in- creases.
. Carnegie Institution of Washington publication. 366 Special Reports abrasion, or otherwise. Examination of pivots under a high-power microscope often shows rust scars or pits of considerable magnitude; extended field experience shows that such oxidation takes place readily, especially in the tropics, despite utmost care. In the cases which have been analyzed, these large deviations in the correction-curves are nearly always such as would be produced by an elevation rather than by a depression on the pivot, that is, the maximum precedes and the minimum follows as the inclination in- creases. As the pivot rolls upon such an elevation with increasing northerly inclination, the needle is at first restrained, giving too small a reading and requiring a positive cor- rection, and then pitches forward over the obstruction, giving too large a reading and requiring a negative correction. Following the usual convention as to sign of southerly inclination the order of maximum and minimum would be reversed in the southern hemisphere (compare Fig. 5). So far no clearly defined case such as would be produced by a simple depression has been revealed; the case of needle No. 1 of circle No. 177, which at first seemed to be of that type, was, as indicated, a combination of other causes not evident from the available data. It is not apparent why curves of the one type should predominate since the microscope shows that pits or scars are often present. It may be that, since the pits are a later development, the behavior of the needle has become so bad by the time they appear as to defy analysis or to cause the rejection of the Fio. 7.—Showing Time-Change in Correction-Curve for Needle No. 1 of Circle No. 177. To determine whether a rust particle adhering to the axle could produce undulations in the correction-curves of the dimensions observed, assume the particle to be of such form that tangents drawn from its apex to the circumference of the pivot do not touch it at any oth
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