186 Reports on Special Researches 



Terrestrial Magnetism, in accordance with plans submitted by the Director. These plans 

 made provision for the following: 



(1) A site of somewhat over 7 acres, admirably located amidst rural surroundings, 

 in the District of Columbia, in close proximity to the extensive National Rock Creek Park, 

 and sufficiently removed from industrial distm-bing influences (Fig. 1). 



(2) A conunodious fireproof building, solidly built, of reinforced concrete and brick 

 construction, containing the Director's headquarters and working rooms for the staff, 

 library and archives, physical laboratory, instrument shop, testing-rooms, and certain 

 observational facilities. This main building consists of a sub-basement, a basement, two 

 floors, a spacious and well-lighted attic, and a specially constructed roof with observation- 

 deck. It is 102 feet long, 52 feet wide, 49 feet from the ground to the roof, and 62 feet from 

 the foundations of the sub-basement to the roof. Its erection was begun in May 1913 and 

 it was completed on February 14, 1914. (Plates 1 and 8.) 



(3) A one-story non-magnetic building (standardizing magnetic observatory), about 

 28 feet wide and 58 feet long, erected during 1914, to provide the necessary facilities for 

 tests and researches requiring a non-magnetic structure (Plate S). This observatory is 

 sufficiently removed from the main building to be practically beyond its disturbing 

 influence. 



(4) Two or three smaller or accessory structures for sjaecial investigations in atmos- 

 pheric electricity and allied subjects, to be erected in 1915. 



Thus, the Dei:artment of Terrestrial Magnetism now possesses exceptional facilities 

 for its varied work, both in the field and in the laboratory, in magnetism, electricitj^, and, 

 to a certain extent also, in gravity. These three broad subjects appear more or less inter- 

 related, and it is exceedingly difficult, even if that were desirable or worth while, to draw 

 sharp lines of demarcation between them, or between distinctive laboratory problems and 

 those of Nature at large. 



SITE. 



The site which most nearly met the requirements, and which, after considerable 

 search, was finally selected, is a tract of land of about 7.4 acres at 36th Street and Broad 

 Branch Road, Washington, D. C, in close proximity to the extensive National Rock Creek 

 Park. It is on about the center line of the northwest portion of the District of Columbia, 

 approximately a mile southeast of the northwest boundary line of the District, about 3 

 miles almost due north of the United States Naval Observatory, and about 1 mile north 

 of the United States Bureau of Standards and the Geophysical Laboratory of the Carnegie 

 Institution of Washington. The nearest electric car-line is about 2,100 feet to the west 

 of the site. While this distance would be insufficient were it the intention, which it is not, 

 to register at the site the variations of the Earth's magnetism, tests have shown that the 

 distance is ample for the proposed testing and standardizing of magnetic instruments. 

 Furthermore, this car-line will ultimately be converted into a sub-surface insulated system 

 and hence the limiting distance of disturbance effects will be still further reduced. As will 

 be seen by reference to the map (Fig. 1), the site is so surrounded by streets, gullies, and 

 hills as to give reasonable assurance that it will answer the purposes for some time to come. 

 The general distribution of the Earth's magnetism in Washington and vicinity is generally 

 found to be somewhat irregular, but, for the site selected, it is practically uniform. 



The area of the tract, as has been said, is about 7.4 acres; its elevation above mean 

 sea-level varies with the topography from about 207 to 280 feet. It has a commanding 

 view in all directions over a rolling country. The geographical position is 38 57' 24" north 

 latitude and 77 03' 52" west longitude. 



The puichase of the main tract was concluded on April 15, 1913, and in 1914 a small 

 additional tract was purchased. Two dwelling-houses, which were on the site, have been 



