PREFACE. 



The main facts here presented, together with the numerical results of the field 

 reductions, were communicatod to the National Academy of Sciences at their 

 session of January, 1867. 



The definite reductions were made during the spring and summer of 1867, Mr. 

 A. T. Mosman assisting in them until May, when he was ordered elsewhere, and 

 Mr. S. C. Chandler, jr., then taking the principal part in them, until they were 

 essentially completed in the following October. 



This report, in its present form, was prepared during the year 1867, except the 

 final chapter upon the velocity of the signals, which has chiefly been written 

 during the present month, since receiving from the Superintendent of the Survey 

 permission to print the report. 



B. A. GOULD. 



Cambridge, Massachusetts, 

 February, 1869. 



(V) 



