EEPORT OP THE SECRETARY. 101 



System of magnetic shielding. — Three magnetic shields are employed. The outer is 

 a square iron box without top or bottom and of aljout 2 tons weight. Within this, 

 and supported on the swinging iron cage, are two concentric iron cylinders, of which 

 the inner is only 3 inches in diameter and immediately incloses the galvanometer. 

 No side openings are made in the shields, the galvanometer being read by means of 

 a spot of light reflected vertically through a glass in the capstone of the pier. 



It will be noticed from this description that all observations will be conducted in 

 the upper room, while the galvanometer itself is below the level of the ground in a 

 room of very constant temperature. It is lioped shortly to continue work on this 

 apparatus, and to provide the galvanometer with a needle system of great sensitive- 

 ness, which it is thought from the exjieriments already made may be used without 

 prejudicial tremors in a vacuum and at ten seconds (single) vibration. If these 

 expectations should be realized the useful or working sensitiveness of the new 

 galvanometer will be several hundred times that customarily employed in mapping 

 the infra-red solar spectrum, and a large new field of work will be practicable. 



(3) Ohservation of the solar eclipse. — Considering the near approach of the path of 

 totality of the total eclipse of May 28, 1900, you deemed it desirable for this Observa- 

 tory to take part in the observation of this important astrophysical phenomenon, and 

 this view having recommended itself to Congress, an appropriation of $4,000 was 

 made immediately available for the purpose in February. The use of the special 

 ajiparatus already belonging to the Ol)servatory, tlie very generous offer by Prof. 

 E. C. Pickering, the director of the Harvard College Observatory, of the loan of the 

 new 12-inch lens of 135 feet focus belonging to the Harvard College Observatory, and 

 of several other valuable pieces of optical ai)i)aratus, together with loans of a 5-inch 

 lens of 38 feet focus by Princeton University, and of a 5-inch equatorial by the Naval 

 Observatory, enabled the expedition to take larger proportions than this modest 

 appropriation might otherwise have justified. 



This api^aratus with many other adjuncts was temporarily installed in Washington 

 in the Smithsonian grounds, and placed in the position each piece would occupy in 

 the actual eclipse. This was with the view of familiarizing the observers with them, 

 by successive rehearsals, which went oil with assiduous practice during two months 

 before all was taken down for shipment to the proposed site. 



Oioice of site. — Three successive years of special observations had enabled the 

 Weather Bureau to determine the relative chances of cloudiness at A^arious points 

 along the eclipse track, and from these results it appeared that the interior of the 

 country was more favorably situated in this respect than those parts of the j^ath lying 

 near the coasts. Certain towns appeared more favorable than others, owing to local 

 conditions, although such special results were less trustworthy than the broad indica- 

 tions already cited. However, as the Eastern stations were to have both a higher 

 solar altitude and a longer duration of totality, and as personal inspection of the 

 ground by me, together with a careful consideration of the Weather Bureau results, 

 seemed to justify the selection, the town of Wadesboro, N. C, was approved by you 

 in preference to stations more remote from Washington. The actual grounds 

 occupied formed a nearly level plat of several acres extent, sheltered from the wind 

 by knolls, Iniildings, and trees, but being almost the highest land thereabouts, and 

 indeed about 600 feet above sea level. These grounds were freely offered by John 

 Leak, esq. , of Wadesboro. A shed and the necessary piers for instruments were 

 erected in the latter part of April. 



Plate No. VIII gives a ground plan of the Smithsonian camp, showing the canvas 

 covers of the two 135-foot tubes, the sheds containing the bolometric apparatus and 

 its accessories, and the photographic cameras for the 40-inch and 11-foot lenses, with 

 numerous other pieces; and also on the left a portion of that of the Yerkes Observa- 

 tory. 



Plate No. IX gives a general view of the camp as finally occupied. The equipment of 

 the Smithsonian expedition is shown at the right of the wagon track, while on the 



