United States passport applications between 1795 and 1925 are available on several different sites for those whose ancestors may have temporarily left the United States. Keep in mind that many people who traveled before World War I did not obtain passports.  However these applications can contain significant genealogical clues, particularly for those whose applications are from the 1890s and after. They can be found (with indexes) at: free on FamilySearch –in progress fee-based at Ancestry.com fee-based at Fold3 They are on microfilm, but the microfilmed copies are arranged chronologically making it difficult to find the person of interest if you do not know when they applied.
Get the Genealogy Tip of the Day Book
Archives