460 R. W. Willson — Magnetic Field in 



since many of the points observed are so near the piers as to 

 be unduly affected by their presence. As the magnetic effect 

 of the mass of the building upon the Horizontal Intensity at 

 any point is too complex to admit of estimation, I have con- 

 fined myself principally to an investigation of the effect of the 

 piers. 



To facilitate these observations a board 28 feet long, and 8 

 inches wide was carefully smoothed and brought to a straight 

 edge and wooden horses, put together with copper nails, were 

 prepared of just the height of the piers ; a sliding piece 15 

 inches long with a guide bearing upon the edge of the board, 

 carried the variometer with its needle in a plane seven inches 

 above the top of the piers and 3 feet 3 inches above the floor. 



The long board being brought into position, supported by 

 the piers or horses, the variation at different points could be 

 rapidly observed without any sacrifice in point of accuracy, the 

 instrument being slid from one point to another with very 

 slight derangement in level and azimuth, so that the latter 

 adjustment was made once for all while the former was very 

 quickly accomplished. The changes of temperature are also 

 less troublesome factors than when the observations extend 

 over considerable periods of time. Variations of the horizontal 

 intensity during the observations have usually been determined 

 and corrected for by occasional observations at the same refer- 

 ence point. 



The piers consist of rectangular columns of brickwork laid 

 in Portland cement. They are 3 feet by 2 feet at the base, 

 tapering to 2 feet square at the first floor, above which they 

 rise 2 feet 4 inches, and are capped by slabs of blue stone 3 

 feet square and 4 inches thick. The foundations are 5 feet 

 below the basement floor and the total height of the column is 

 about 18 feet. As the building faces within 2^ degrees of the 

 magnetic South, and as the sides of the piers are parallel to the 

 walls of the building, it will be sufficiently accurate in describ- 

 ing their magnetic effect to speak of either as if exactly 

 oriented. 



If the brick of which the piers are made contains magnetic 

 material we should expect to find indications of a magnetic 

 distribution depending upon the inductive action of the earth 

 and the form and position of the piers, independent of any 

 permanent magnetism existing in the bricks previous to their 

 being placed in their present places, and which, on account of 

 its random distribution, in so large a number of bricks, would 

 have no effect except in causing small very local disturbances 

 easily recognizable as such. 



The effect of the earth's induction upon the piers would 

 produce, on the whole, free south magnetism on the upper 



