THE HISTORY OF BUCKS COUNTY, PENNSYLVANIA: DAVIS, 1876: VARIATION OF THE COMPASS NEEDLE, BUCKS COUNTY - Appendix 0 from the discovery of the Delaware to the present time by W. W. H. Davis, A.M., Democrat Book and Job Office Print., Doylestown, PA, 1876. Editorial Note: For those who desire to read this appendix it was thought that including the complete remarks of both editions (and times) would be of interest. COMPASS. 1875 Edition VARIATION OF THE COMPASS NEEDLE, BUCKS COUNTY, PENNSYLVANIA. Magnetic declination, or as it is commonly called the variation of the compass needle, may be known for any period during the last 200 years in Bucks county, with as much precision as the instruments used by early observers would allow. Determinations of the westerly declination before and about the year 1700 were made at Hatboro, and also at Philadelphia city, and repeated at irregular intervals down to the present time. These give for the city westerly magnetic variation at 14 intervening periods, and for at least as many periods at the first-named place, but the instruments used in early days were rough in comparison with such as are now employed for like purposes. But, taking the two series here mentioned, and applying an adjustment of special utility in such calculations, the average derived is known to be less in error than the single series of observations actually made at either of the two places. The following average values for westerly deviation of the compass needle, as between Philadelphia and Hatboro, were furnished by Superintendent C. P. Patterson, from the office of the United States coast-survey, where computations from similar records are made by Charles A. Schott, chief of the Computing Division: (See Table 1) In accordance with the law inferred from computations, the variation of the compass needle can be predicted for a few years beyond the present time, as appears by the table. Supposing a proportionate distribution of magnetism at Philadelphia, Hatboro, and Doylestown, the variation of the needle at Doylestown would be about 17' (say a quarter of a degree) greater than at Philadelphia, and about 10' (one-sixth of a degree) greater than at Hatboro for any period within the last two centuries. If the distribution has been, as supposed, the variations for Doylestown are readily found by adding 0 degree 22 to each of the values given in the preceding table; we have therefore the following: (See Table 2) The present yearly increase is eight-hundredths of a degree, or four-eighths minutes of arc. To find the variation at any other place in Bucks county, for any year between 1680 and 1880, with near approach to precision, the magnetic declination (at any time in 1876, 1877, 1878, 1879, or 1880,) must be actually observed at the particular place. The difference between the variation found there, and the variation given by average in the Doylestown table for the same year, if applied to any former date, will give the variation at the place for that earlier date. In the year 1800, or when the variation was least, the north end of the needle pointed only 2 degrees 10' to the west of north at Doylestown, but in 1680 it pointed fully 9 degrees west of the north. This difference of about 7 degrees measures nearly the whole of the secular swing in magnetic variation. About 1802 the annual change, having decreased for many years, was nothing. Increase then commenced, and in 1870 the annual rate of increase in variation was 4.'7. The rate of annual change is probably again decreasing, but still it is carrying the north end of the compass needle further and further westward, and that deviation will continue until a time beyond the close of the present century. 1. The figures in this table are exactly applicable also at Allentown, Millport, Penn Haven, Wilkesbarre, and Towanda; at all intervening places in the same direction; and equally applicable at Mount Holly, New Jersey, and other places in that direction, all, however, under the supposition of a normal distribution of magnetism. COMPASS. 1905 Edition. VARIATION OF THE COMPASS NEEDLE, BUCKS COUNTY, PENNSYLVANIA (2) (2) This chapter has been specially prepared for this book [1905] by the Division of Terrestrial Magnetism of the U. S. Coast and Geodetic Survey. The magnetic declination, or as it is commonly called by the surveyor and the mariner, the variation of the compass needle, is the angle between the true north and south line and the magnetic north and south line as defined by the compass needle. Now it has been found by observation that the direction of the compass needle is constantly changing and that in general it is different at different places at the same time. Thus at Philadelphia a century ago the declination was about 2 degrees West, while it is now nearly 8 degrees West, showing a change of almost 6 degrees interval. At the present time the declination in the United States varies from 20 degrees West in Maine to 23 degrees East in Washington. As until quite recently all land surveys were traced by means of the compass, it is imperative that the surveyor should know what changes have been taking place in the direction of the needle in order to successfully retrace the line of old surveys. The numerous fluctuations to which the compass needle is subject have been discussed in various publications of the Coast and Geodetic Survey, the material having become available as the result of special or other magnetic observations. The reader is referred to those publications for a more detailed treatment of the subject of Terrestrial Magnetism. (3) (3)See e. g., Appendix I, Report for 1895 and Appendix I, Report for 1896. Copies of these can be had by application to the "Superintendent of the Coast and Geodetic Survey." The fluctuation of the most importance to the surveyor are known as the solar-diurnal variation and the secular variation. The diurnal variation is periodical and is completed in 24 hours. At about 8 A. M. The north end of the needle reaches its extreme easterly position for the day and from that time moves to the west until about 1 P. M., when the westerly extreme is reached; then the motion is reversed and the needle moves slowly to the east until approximately the same position is reached the next morning from which it started. Between 10 and 11 A. M. And between 6 and 7 P. M., the needle reaches its average position for the entire day of 24 hours. The following table derived from five years of continuous observations at Philadelphia, ending June 30, 1845, shows for each hour from 6 A. M. To 6 P. M., and for each month of the year the amount by which the needle differs from its average position for the day. A plus sign indicates that the needle is to the East, a minus sign to the West, of its average position for the day (24 hours). (See Table 3.) This table applies equally well to any part of Bucks County and may be used for correcting observations made at any hour of the day to the mean value for the day. It will be seen that the daily range is about twice as large in summer as it is in winter. The secular variation of the magnetic declination is a change extending over a long term of years. Whether it is periodic, like the diurnal variation cannot be said at present, as our accurate information does not cover sufficient length of time to determine whether the needle actually does retrace its path after a long interval. Assistant Charles A. Schott of the Coast & Geodetic Survey has made a special study of this subject and the results of his latest investigations are published at Appendix No. 1 of the Report for 1895. The secular variation of declination in Bucks county during the past two centuries may be derived from observations at South Bethlehem between 1742 and 1897, Jamesburg, N. J., between 1761 and 1887, Harrisburg between 1795 and 1895, Hatboro between 1680 and 1850 and Philadelphia between 1701 and 1895, discussed in the above Appendix. These observations indicate that the declination was probably at its maximum sometime during the last half of the Seventeenth century from which time it decreased steadily until the minimum was reached in the first decade of the present century. Since then it has been increasing again, at first slowly and then with greater rapidity, until the annual change amounted to about 4.'7 per year. It is still (in 1900) increasing though at a reduce rate, about 3 minutes per year, and it is expected that a maximum value will again be reached in about 25 years. The following table may be used for any part of Bucks county, for finding, by differences, the change in magnetic declination or variation of the compass between any two dates since 1680. (See Table 4.) If, for example, it is desired to know the change of the compass direction between July 1, 1757, and August 15, 1900, we find from the table that the compass has changed from three degrees, fifty-five minutes W., at the early date to seven degrees, twenty-nine minutes at the later date, or has moved from 1757 to 1900 three degrees and thirty-four minutes to the West. The few observations available indicate that the distribution of magnetism in Bucks county is quite irregular and this conclusion is borne out by geological formation, as shown in a paper by B. D. Lyman, in the "Journal of the Franklin Institute" for October, 1897. The general direction of the lines of equal magnetic declination (isogonic lines) is north and south and the declination increases from west to east. The following results of observations at Doylestown are taken from the Annual Report of the Secretary of Internal Affairs of Pennsylvania for 1887. (See Table 5.) The differences in the last column show that on the average the computed table gives directly the declination for Doylestown. It should be borne in mind that every compass has an error of its own which may amount to half a degree in modern instruments and even more in those in use in the last century. For this reason the use of the compass in making surveys should be avoided wherever possible. END OF APPENDIX "COMPASS."