From this table, which was prepared with the aid of the long series of observations at Chambersburg, Washington and Baltimore, we find that the needle veered about 2° 26'.5 to the west between 1750 and the middle of the present year. Let us see what values we shall get from the table of the changes for the county-seats. Watersville is about midway between Rockville and Westminster; likewise about midway between Frederick and Ellicott. Again, we notice from the table that the needle apparently remained west of the true meridian throughout the interval 1750 to 1900. If it ever was "true to the pole " during that interval this occurred not far from the year 1800. The needle may have pointed east for a brief period after this time, but if it did so, the easterly deflection was of small amount. No absolutely correct value of the declination at Watersville for the present year could be furnished, since no observations have, as yet, been made there and because of the prevalence of local disturbances in this part of Maryland. Properly grouping and combining the observations made at the stations surrounding Watersville, viz., Frederick, Damascus, Unity, Westminster, Ellicott City, Baltimore and Cockeysville, the following values were obtained: THE DISTRIBUTION OF THE MAGNETIC DECLINATION IN MARYLAND. EXPLANATORY REMARKS. At no two periods of the earth's history is the distribution of magnetism within the earth's crust precisely the same. Any graphical representation of that distribution will therefore apply only to a particular time. In the present instance the isogonic chart or chart of the lines of equal magnetic declination (variation) for Maryland has been constructed for the year 1900. These lines, as is well known, connect all the places in Maryland where the angle of deviation of the compass direction from the true north and south direction is the They should not be confounded with the lines which would be traced if we were to start out, for example, from some point in southern Maryland and always follow the direction indicated by the north end of the compass. Such lines have been termed magnetic meridians. same. Future reports will contain likewise the charts showing the distribution of the magnetic inclination and the intensity of the magnetic force. In order to supply the practical demands of the surveyor it was necessary to publish first the isogonic chart. It has been termed "preliminary," as in its construction only the 1896 results of the magnetic survey could be utilized. Still, though it is regarded as preliminary, it is nevertheless far superior to any chart embracing the same region hitherto published. The values of the declination. given in the table below may undergo slight modifications in future reports, when the more precise reductions to the epoch of the chart are known. These changes, however, will be of such a nature as not to affect the practical use which the land surveyor may make of the present values. Besides the data resulting from my last year's work and given in Table XIII, I have made use of all the material observed and collected by the Coast and Geodetic Survey in Maryland and vicinity. This information is contained in Table XIV. The total number of observa Taking the total number of observations in Maryland, 56, the stations averaged one to every 218 square miles (564 square kilometers) of the total area (land and water 12,210 square miles); or, for every area 15 miles square there was, on the average, one station. The preliminary reductions of my observations to the mean of day (24 hours), and the corrections on account of magnetic disturbances, were made as fully explained elsewhere. The reasons have also been given for referring the chart to the year 1900. The corrections necessary to reduce the 1896 observations to January 1st, 1900, were made assuming for the present that the secular change is at the rate of 3′ per annum over the entire state. From the auxiliary table XIA we find that the average annual change between 1895 and 1900 is as follows: The geographical positions which I assign to my stations are for the time being taken from the following sources: Stations 1, 1A, 2, 3, 4, 5, 6, 19, 20, 20A, 26, 28, 31, 34, 32A, 40, 41, 42, from the topographic sheets of the U. S. Geological Survey. Stations 7, 8, 9, 10, 11, 13, 14, 15, 16, 17, 17A, 18, 21, 22, 24, 27, 29, 33, 35, 36, 37, 38, 43, from Martenet's map (34 miles to the inch) for 1886. Stations 12, 23, 32, 39, from Coast and Geodetic Survey data. Station 30 from data supplied by Superintendent of Naval Academy. Station 25 from Rand & McNally's map of 1895. The question of more precise geographical positions may be given attention in a future report. As already explained in another part, some of the declinations obtained may require corrections amounting from 1'-3' by reason of appreciable errors in the geographical positions which at present had to be adopted. Such corrections, however, are of no special concern to the surveyor. The Coast and Geodetic Survey table is given in precisely the same form as furnished by them. The stations in italics are secular variation stations; west declination is marked by a plus sign; D stands for declination and AD is the correction which was necessary to obtain the reduced value, D1900 for January 1st, 1900. be noticed, D is given in decimals of a degree. refer to the mean of day (24 hours). In this table, it will TABLE XIII. Summary of the magnetic declinations observed by Maryland Geological Survey in 1896. Massey. 10 Ridgely. 2 3 4 5 6 Linden, B. S..... Montgomery.. 39 00.5 77 03.1 July-Dec. 3 27.4 W 1 A Linden, A. S... . . Montgomery.. 39 00.777 03.1 La Plata 3 Nov. 25 3 39.0 66 3 37 W 348 5.09 10 4 36.8 10 Mean of 14 days. At 3:38 At 9:49, 10:34, 3:58 Dip observed only. Dorchester.. 38 19.975 13.3 66 23 5 25.3 5 35 At 9:55, 10:23 2 The times given in this column are the local mean times when the observations were made. 3 Observations made in 1897 when reduced to 1900.0 give same value, 4° 46′. * Artificial local disturbance at the 1896 station. The value of declination as given was corrected with the aid of the 1897 observations on the Fair Grounds. 5 Observations made when establishing surveyor's meridian line in 1897, on the Court House lot, reduced to 1900.0 give 6° 02′. Sept. 17 was a day of large disturbance. 6 Artificial local disturbance suspected at this station. A day of large disturbance. 8 The value given for 1900 has been obtained by making use also of the work of 1897. "This station in a greatly disturbed region. The observations for 1896 will be presented in connection with the 1897 work. 7 At 11:27 7 5 24.5 5 34 At 3:08 66 13 14 5 36.0 545 14 4 02.8 4 12 At 3:31, 3:55 66 25 3 37.0 3 46 At 10:20, 10:56 7 4 44.4 W 454 Snow storm; I, obs'd. At 10:09, 10:37 |