I found out why I've never been able to find my grandmother's family in the 1900 U.S. Federal Census - it's not that they were simply out of town, they weren't in the country to be enumerated!
The following passenger list is from Ancestry.com's UK Incoming Passenger Lists, 1878-1960.
Mrs. Chas. C. Adsit, age 32 is listed with a "Profession, Occupation or Calling" of "Wife." (However, at the far right, under the "foreigner" column, there is a check mark under "Male" so perhaps it's my great-grandfather, Charles C. Adsit traveling with his children?) Son, Charles is 7 years old, and daughter Eliz[abeth] is listed with an age of 2 or 3; it's hard to tell. She turned three years old four days after arrival, on June 18, 1900. Last is Sally Wither, the 25-year-old Scotch maid who worked for the Adsit family for many years; you can see her in the family's census listing in 1910 and 1920.
The Majestic sailed from New York City, N.Y. and arrived at Liverpool, England, on June 14, 1900. They would probably have had to leave their home in Chicago before the end of May, and the 1900 U.S. Census was an official count as of June 1, and the census workers would have visited homes throughout the month of June.
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