vi INTRODUCTION. 



The declinations were determined with a new Gambey declinometer belonging 

 to the Girard College, the astronomical observations were made with a sextant and 

 vertical circle and chronometer (Grant, No. 3861). The dip was determined with 

 a portable circle by Robinson, the total intensity with Lloyd needles by llobinson, 

 and the horizontal intensity by a magnetic liar and cylinder according to the method 

 described by me in the American Phil. Trans., Vol. V, 1837, in which the vibrations 

 are made in a raritied medium. 



