miles of land area-such a variety of geological formations as Maryland. To quote from the report of the State Geologist: "The most ancient rocks which make up the earth's crust as well as those still in the process of deposition are here found, while between these wide limits there is hardly an important geological epoch which is not represented." The investigation of the local disturbances, which are quite marked in certain regions, and their correlation with geological structure will, therefore, be of peculiar interest and value in this state. GENERAL ACCOUNT OF THE MAGNETIC SURVEY OF MARYLAND. INTRODUCTORY REMARKS. In the summer of 1896, while spending a vacation at a little village, Linden, nine miles north of Washington City, I made some magnetic observations which clearly showed that the distribution of the earth's magnetism is quite irregular in the vicinity of Washington-a fact already noticed to some extent by the observers of the Coast and Geodetic Survey. It seemed to me most desirable to extend the investigations over a larger area, if possible, and, accordingly, I laid my plans for a detailed magnetic survey of Maryland before Professor William Bullock Clark, in charge of the recently organized Geological Survey of Maryland. Professor Clark heartily endorsed the idea of a detailed magnetic survey and authorized me in a letter dated July 25, 1896, to proceed as soon as possible with the survey, putting a sum of money at my disposal which, with careful management, it was hoped would suffice to make magnetic observations at one or two stations in each of the twenty-three counties comprising the state. With the aid of additional funds, obtained as already stated in the Introduction, the number of stations at which observations were made during 1896 could finally be increased to 46, thus averaging one station to every 265 square miles (687 square kilometres) of the total area' of Maryland. Application was next made to the Honorable John G. Carlisle, then Secretary of the United States Treasury, under which department the Coast and Geodetic Survey is placed, for the loan to the State of Maryland of a complete Coast and Geodetic Survey magnetic outfit. In return for the loan of the instruments it was agreed that the Coast and Geodetic Survey should be furnished with the results of the magnetic survey. This application was favorably acted upon and the instruments were turned over to me by the Coast and Geodetic Survey, at the request of the State Geologist. After some preliminary investigations conducted at Linden, it was possible to begin active operations early in September. The instruments furnished by the Coast and Geodetic Survey were: Mean Time Chronometer Bond No. 195. Tent No. 25 "A" and poles. 50-foot Steel Tape No. 86. On October 23d, the above-mentioned chronometer, which proved to be not in the very best condition, as it stopped at frequent intervals, it was possible to exchange for Mean Time Chronometer Dent No. 2256. The C. & G. S. tent was found too heavy (weighing with poles easily 100 lbs. and possibly more) and took too much time and labor to put up and take down and pack for transportation. I therefore purchased one of Copeland's small, light lawn tents supported in the centre by a single pole. This tent gave every satisfaction and proved a great convenience. About five minutes were required to erect it. or take it down and wrap for transportation. It was 7 x 7 feet square at the base and about 7 feet high in the centre. The tent might 1 Total area of Maryland is 12,210 square miles (31,624 square kilometres), of which 9860 square miles (25,538 square kilometres) consist of land area and 2350 square miles (6086 square kilometres) of water area. advantageously have been a trifle larger, about 8 x 8 feet. A few alterations had to be made, such as the substitution of all the iron parts by copper, brass or wood. For the iron frame spreading out the tent at the base of the pyramid on top, I substituted two light pieces of wood fastened together in the centre by a brass screw, about which the two pieces could turn and be spread out in the form of an X. They were held in place by small brass hooks in each corner of the base of the pyramid. The pine rod (2 inches in diameter) was somewhat over 8 feet long and consisted of two pieces which, when joined together, were held in place by a piece of brass tubing. The rod terminated in a copper tip which passed through the brass ring of the tent on top. About the copper tip could be passed guy ropes to hold the tent in time of strong winds or for the purpose of allowing the centre pole to be placed at an angle from the vertical and thus to be shifted away from the centre of the tent. With the aid of these guy ropes the whole tent could be easily tilted back far enough to allow making azimuth observations on the sun. seven light pegs (2 x x 8 inches). corner-were sufficient. The tent was held down by Very often four-one at each The conditions under which the magnetic survey had to be commenced were such that it was highly essential that the observer should be as independent and as free in his movements as possible. As stated, the funds had to be carefully managed, it was not known how long the Coast and Geodetic Survey could spare its instruments, and the time which the observer could devote to the survey was restricted. As a first step, the amount of baggage to be transported from place to place was reduced as much as possible and disposed in such a way that no package would weigh over 30 pounds, and hence could be easily handled. After a few trials, the instruments and personal effects were put up in three packages, each weighing 25 to 30 pounds. The chronometer was always in the special care of the observer. It fitted snugly in a leather case to which was attached a handle for holding or carrying it. In traveling on the railroad the packages could generally be safely entrusted to the baggage-master when specially cautioned. The special Theodolite No. 163 the observer did not carry along with him, as there was an alt-azimuth instrument in connection with the magnetometer. After a few stations had been occupied, the stand for the dip circle was also discarded, an extra head or top having been prepared instead, which when fitted on the magnetometer tripod permitted the placing of the dip circle on it. Thus but one tripod was needed for the making of the entire set of observations. BASE STATION. Linden was selected as the base station of the survey. In addition to being the home of the observer, it was nearly in the centre of the area over which the observations were to be made and was moreover only within a few miles of the Washington Magnetic Observatory and the Coast and Geodetic Survey office. After some preliminary investigations had been made at the base station, and the magnetic elements well determined, I was ready to start out on magnetic trips, returning to the base station at various times during the period of the survey. ITINERARY OF MAGNETIC TRIPS. In the table below D stands for magnetic declination, I for magnetic inclination, H for horizontal component of earth's magnetic force, and TS for time signals transmitted telegraphically at noon by the Naval Observatory at Washington over the Western Union telegraph wires. These time signals were used to obtain the rate of the chronometer. 1 From station to station in a straight line; for example, La Plata is 23 miles from Upper Marlboro. 19 22 Massey, D, I, H 24 Ridgely, D, I, H TS received at Silver Spring. Day fine. 66 66 Traveled 18 miles in buggy over Fine weather. Rain next day. While waiting for train to Princess Weather and site splendid; light Fine, clear day. Station slightly af- In the afternoon; weather good. Weather fair, moderate wind. 8068 25 38 Westminster, 56 Cumberland, 38 Oakland, D, I, D, I, H 46 13 100 Point of Rocks, I, H By this time at least one station had been located in every county of Maryland in accordance with the State Geologist's instructions. Hereafter the observations were made either to fill out large gaps between some of the previous stations or to verify the marked irregularities in the distribution of magnetism in central Maryland which had made itself very apparent by this time. |