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According to his death record, he died in Boston, was a resident of Brooklyn..

Massachusetts, Deaths, 1841-1915
Name: William H. Deforest
Event: Death
Event Date: 09 Aug 1860
Event Place: Boston, Suffolk, Mass.
Residence: Brooklyn, N.Y.
Gender: Male
Age: 53
Marital Status: Married
Occupation: Merchant
Birthplace: Woodstock, N.Y.
Estimated Birth Year: 1807
Father: Stephen