APRIL 6, 1870

In a book that contained the letters of Elise Waernskjoid I found a letter in which she says my ancestor would leave for America on April 6, 1870. I have no idea what port they left from but could anyone let me know the names of the ships that left Apr 6th from Norway or where I could get a list. Also there is no record of them leaving in the police records. Wouldn't they have to be if they embarked from Norway? What reasons would they not be listed? Thanks.